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The Electrochemical Produc-
tion of Colloidal Copper
A THESIS
PRESENTED TO THE FACULTY OF THE GRADUATE
SCHOOL OF CORNELL UNIVERSITY FOR THE
DEGREE OF DOCTOR OF PHILOSOPHY
By THOMAS ROLAND BRIGGS
Reprinted from the Journal of Physical Chemistry, 17, 281 (1913).
THE ELECTROCHEMICAL PRODUCTION OF
COLLOIDAL COPPER
BY T. ROLAND BRIGGS
PART I–SCHÚTZENBERGER'S ALLOTROPIC COPPER
In the year 1878, Schützenberger' announced to the
French Academy a new and allotropic modification of copper,
which he had prepared by the electrolysis of a neutral aqueous
solution of cupric acetate. He was greatly impressed with
the peculiar properties of the new form of copper and pro-
ceeded at once to its further investigation. I quote from his
first paper:
“The following facts establish the existence of an allo-
tropic modification of copper, distinct in its physical and
chemical properties. The electrolyte employed is a ten
percent, aqueous solution of copper acetate, which has been
boiled for several minutes. . . . . . There are employed two Bunsen
or three Daniell cells of moderate size and all rise in
temperature during the electrolysis is to be avoided. The
negative platinum plate is placed [opposite and] parallel to
the sheet of copper serving as the soluble positive electrode,
at a distance of three or four centimeters. Its size should
be a little less than that of the positive electrode. Under
these conditions the face of the negative platinum electrode
becomes covered with a deposit of allotropic copper.
“Physical Properties.—The allotropic copper occurs
in layers having a metallic lustre . . . . . . and is less red than
Ordinary copper, resembling certain bronzes. It is brittle,
absolutely lacks malleability and can be reduced to an impal-
pable powder. . . . . . 5 y
Schützenberger continues by pointing out that the density
of the new form of copper varies between 8.0 and 8.2, which
is decidedly less than that of ordinary sheet copper. He
continues: - -
* Comptes rendus, 86, 1265 (1878).
277983

282 - T. Roland Briggs
“Chemical Properties.—The layers, washed with previously
boiled water and exposed while moist to the air, are super-
ficially oxidized with great rapidity; they become beautifully
iridescent and in a few moments take on a deep indigo-blue
color. In warm water, from 50 to 60 degrees in temperature,
and in slightly basic copper acetate solution, the Oxidation
is instantaneous. In the beginning, the electrolysis of a bath
of basic acetate gives only deposits rich in copper acetate and
it is only after a certain time, when the bath has become more
nearly neutral, that the operation goes properly.
“The allotropic copper, exposed to the air as a dry powder
and at the ordinary temperature, blackens after a short time.
and changes to copper oxide. -
“The way in which it acts with pure nitric acid, cold
and diluted with ten times its weight of water, is character-
istic; its surface is quickly cleaned [of oxide) and the metal
is attacked with the immediate evolution of nearly pure
nitrous oxide, at the same time becoming covered with an
olive-black layer of unknown nature. Ordinary copper is
with difficulty attacked by acid of this strength, while with
more concentrated acid it gives off mainly nitric oxide, and
this without any blackening. Sometimes samples [of allo-
tropic copper] are obtained which evolve mixtures of nitrous
and nitric oxides when dissolved in nitric acid but it is easy
to show that this [mixture] is formed from the presence of
both forms of copper. This effect often occurs when the bath
becomes warm during the electrolysis or if it contains acid.
“The allotropic copper is converted into ordinary copper
by the action of heat or by prolonged contact with a dilute
solution of sulphuric acid.
“Heated to IOO’ Centigrade in a vacuum or in carbon
dioxide the modified copper does not disengage hydrogen.
The peculiarities of its physical and chemical properties should
not be attributed to the presence of a hydride of copper nor
to occluded hydrogen, and can be explained only by the ex-
istence of a special form of copper, susceptible to oxidation
and dissolving in nitric acid with an evolution of nitrous oxide
Electrochemical Production of Colloidal Copper 283
even in the cold. It is quite probable that this modification
corresponds to the copper of the cupric salts. In changing to
Ordinary copper it polymerizes, evolves heat and gives by oxi-
dation cuprous oxide before passing to the final state of cupric
oxide.” -
In a second paper' Schützenberger described further
experiments upon allotropic copper and also mentioned the
preparation of an allotropic form of lead. From lead acetate
Solutions that had been decomposed with an excesss of caustic
potash he obtained by electrolysis a form of metallic lead
that very easily became oxidized in the air with the production
of the yellow crystalline oxide.
Certain of Schützenberger's results and his conclusions
did not long remain unchallenged. Wiedemann” was unwilling
to admit the existence of an allotropic modification of copper
and brought to the French chemist's attention a paper pub-
lished by himself as early as the year 1856. In this early
communication,” Wiedemann described the electrolysis of
neutral copper acetate solutions and the production of a pe-
culiar bronze-like deposit at the cathode. The following
interesting paragraph occurs in this article:
The copper deposited at the negative pole takes with it
a quantity of copper oxide from the solution, the deposit then
becoming very brittle and dark brown. The amount of copper
oxide held by this deposit is dependent upon the concentra-
tion of the solution.” +
At that time (in 1856) Wiedemann did not further in-
vestigate the nature of the copper deposit but after the an-
nouncement of Schützenberger's work he again took up his
former study, obtaining somewhat different results from those
of the latter chemist. Wiedemann's cathode films were in-
variably impure and contained, besides oxide of copper, very
appreciable amounts of acetic acid.
I Comptes rendus, 86, 1397 (1878).
* Wied. Ann., 6, 81 (1879).
* Pogg. Ann., 99, 193 (1856).
284 T. Roland Briggs
Wiedemann performed several quantitative experiments
in order to ascertain the relationship between the amount
of current consumed and the mass of the substance or substances
deposited by this current upon the cathode, as well as to de-
termine the amount of the copper actually present in the de-
posit. He arranged three electrolyzing dishes in series, to
wit—a silver coulometer, a saturated aqueous solution of
cupric acetate and a weaker solution of the same salt. Plat-
inum cathodes were used in all three cells. After determining
the weight of the substance deposited from the acetate solu-
tions by a given current (as indicated by the coulometer),
each deposit was dissolved in pure nitric acid and its copper
content determined. The data follow, with the calculated
results:
(1) Number of the experiment VI VII
(2) Copper in Solution in grams per cc O. 273 O. I36
(3) Cathode deposit in grams O. 422 O. 363
(4) Cathode deposit calculated in grams O. 263 O. 263
(5) CuO equivalent to cathode deposit O. 473 O. 422
(6) CuO calculated from (4) O. 329 O. 329
(7) Excess of CuO in deposit O. I44. O. O.93
(8) Percent CuO adsorbed 35. 3 25.4
Under (2) are given the concentrations of copper in the
two acetate Solutions. Under (3) are the actual weights
of each cathode deposit; while (4) and (6) give the weights of
copper and copper oxide required by Faraday's law, assuming
all the copper to be in the divalent condition. (5) gives the
amount of copper as Oxide found by analysis in each cathode
deposit. (7), obtained by subtracting (6) from (5), gives
the excess of copper in the deposit, assuming it to be present
as cupric Oxide. -
The numbers in this table indicate that there was de-
posited on the platinum cathodes decidedly more copper than
was required by Faraday's Law. Assuming that the excess
of copper was present as cupric oxide, it must have been ex-
tracted (adsorbed) from the electrolyte. The presence of
cupric oxide in the deposit, noticed by Wiedemann in 1856,
was thus confirmed.
Electrochemical Production of Colloidal Copper 285
As a result of his own experiments, Wiedemann was un-
able to accept the views of Schützenberger as to the allotropy
of copper and explained the observed phenomena as being
due to the presence of admixed cupric oxide in the deposit.
He concludes with the following statement:
“Therefore the copper precipitated from the acetate
solutions contains very definite amounts of cupric oxide and
the more concentrated is the solution employed, the greater is
the amount of copper oxide which the primarily deposited
copper finds in its company.”
Schützenberger was not long silent. In reply' to Wiede-
mann he stoutly defended all his former statements and con-
clusions.
“Having with great care studied the chemical phenomena
which accompany this electrolysis [of cupric acetate], I am
convinced that the deposited copper enjoys special and charac-
teristic properties which cannot be explained unless there is
admitted the existence of a new and allotropic state of this
metal.”
Wiedemann's deposits of copper were invariably rich in
oxide but concerning this observation, Schützenberger wrote
the following:
* “At the beginning of my experiments, I obtained, as
did M. Wiedemann, deposits rich in the oxide, but in follow-
ing out my researches as I have specified to the Academy,
I was able to reduce the proportions of the oxide to below
five percent. . . . . . . .
“The density is very much less than that which would
result from a mixture of copper and copper oxide in the pro-
portions furnished by his [Wiedemann's] analyses. Finally
. . . . all the properties mentioned disappear spontaneously
when the allotropic copper is exposed to an atmosphere of
oxygen or more rapidly, when heated to IOO or 150 degrees
centigrade. The existence of an allotropic modification of
lead and a red modification of silver. . . . gives more weight to
my conclusions.” -
* Bull. Soc. chim. Paris, [2] 31, 291 (1879).
286 T. Roland Briggs
At this point the controversy ended and the subject was
not again considered until in 1881, Mackintosh' disagreed with
the conclusions of Schützenberger. He prepared the so-
called allotropic copper and by analysis found that it contained
appreciable amounts of carbon and hydrogen as well as of
oxide and, as a result of his work and that of Wiedemann,
concluded that Schützenberger's assumption of allotropy was
unnecessary. He thus explained the peculiarities of the new
form of copper: “The cause of the rapid oxidation . . . . Seems
to be that the deposit is very porous.”
For many years this question of the allotropy of copper
was allowed to remain unsolved until it was again taken up
in great detail by Carl Benedicks.” Benedicks checked
Schützenberger's results and, in general, succeeded in corrobo-
rating them. He employed a rotating cathode and acidified
cupric acetate solutions, so obtaining “acetate-copper”
almost free from oxide. He thus described the deposits
which he obtained.
“This acetate-copper,’ as I have already shown, pos-
sessed all the properties of the allotropic copper, as for ex-
ample, its marked tendency toward oxidation (blue tarnish
when moistened), rapid evolution of nitrous oxide with nitric
acid (dilute), low specific gravity, bronze-like color, etc.
Hence the properties cannot be explained by the assumption
of an admixture of cuprous or cupric oxide.”
Benedicks also found that this “acetate-copper” invariably
contained oxygen, carbon and hydrogen. The following
are his analytical results: -
Percent C H2 Acetic acid Fxcess O,
Loss in H2 Percent Percent Percent Percent
I. 58 O. 248 O. O52 O. 62 O. 96
2.86 O. 441 O. O75 I . [O I 76
2.84 o. 688 O. I O7 I . 72 I . I 2
* Am. Chem. Jour., 3, 354; Chem. News, 44, 279 (1881).
* Metallurgie, 4, 5, 33 (1907).
Electrochemical Production of Colloidal Copper 287
The above data were obtained by the analysis of de-
posits resulting from the electrolysis of approximately O.7
normal copper acetate solutions containing from 1.2 to 2.4
percent of acetic acid.
As a result of his experiments, Benedicks advanced the
theory that in the case of acetate-copper one was dealing
with a solid solution of acetic acid in ordinary copper and not
with an allotropic form of the metal. As an alternative he
suggested that the acid might be present as a dispersed phase
(colloidal suspension) in the solid copper, in other words,
that acetate-copper was an “acetic acid cuprosol.” In support
of his idea of solid solutions he submitted the following ex-
perimental observations: - -
(1) Acetic acid is present in the deposit but the actual
inclusions cannot be seen even with the most powerful micro-
Scopes.
(2) The specific volume of acetate-copper is less than that
required by a mechanical mixture of acetic acid and copper
in the proportions determined by analysis.
(3) A high electrical resistance, in one case eight times
as great as that of pure copper, indicates wide-spread internal
change. The conductivity is partially restored by heat.
The data illustrating this point follow: -
Resistance (micro-ohms per cm") of the acetate
copper before heating was . . . . . . . . . . . . . . . . . . I4 - 4
After heating to 200° C. . . . . . . . . . . . . . . . . . . . . . 3. O
Resistance of pure copper. . . . . . . . . . . . . . . . . . . . I. 7
(4) The hardness of the deposit exceeds that of ordinary
copper. •.
(5) A condition of internal strain causes spontaneous
cracking and contraction, this being accompanied by the ex-
pulsion (sweating out) of Small amounts of acetic acid.
(6) The formation of nitrous oxide by the interaction
of dilute nitric acid on acetate-copper indicates a deep-seated
internal change which cannot be due either to acetic acid
or to copper oxide mechanically held by the deposit of copper.
288 T. Roland Briggs
Discussion of Previous Work
Benedicks' conclusions are decidedly vague. While the
possibility of the existence of a solid solution of acetic acid
in metallic copper is, per se, perfectly plausible, nevertheless
the proofs adduced in its support are weak and inconclusive.
The invisibility of acetic acid in the test portions of acetate-
copper does not prove that the acid is in solution in the metal
any more than the invisibility under, the microscope of the
individual particles of certain gold hydrosols proves that the
latter are one-phase systems, i. e., true solutions. The al-
ternative of the cuprosol of acetic acid is interesting but one
must admit that it is straining the point a great deal more
than the facts necessitate. -
The determination of the specific volume and the con-
traction so discovered is admittedly rendered of doubtful
value by the experimental difficulties and need not be con-
sidered as highly important. The “sweating out” of small
amounts of acetic acid and accompanying cracking and con-
traction of the deposit can be explained better by the theory
that will be proposed than by postulating a very instable
solid solution of acetic acid in copper. The formation of
nitrous and not nitric acid might be a property of such a solid
solution, but this behavior can hardly be brought forward
as an indication of its actual existence.
The increased hardness and the decreased electrical con-
ductivity remain as perhaps favorable to Benedicks' idea.
But it must be remembered that the deposits studied by the
latter were obtained under different conditions from those of
Schützenberger. Benedicks' electrolyzed copper acetate con-
taining from 1.2 to 2.4 percent of acetic acid; the other dealt
with neutral solutions.
Wiedemann's objections to Schützenberger's statements
were overcome by the latter's repeated investigations and the
results of Benedicks. There remains no doubt but that de-
posits almost free from oxide may be obtained under certain
conditions (rotating cathode, acidity, moderate concentration),
Electrochemical Production of Colloidal Copper 289
which exhibit all the properties described by the French.
Scientist.
The major objection to Schützenberger's idea of allo-
tropy lies in the fact that the copper deposit is not pure but
invariably has been found to contain carbon, hydrogen and
Oxygen (probably as acetic acid) and small amounts of oxide.
Allotropy postulates an internal change in a pure substance
and, while, in the case under discussion, the copper in itself
may or may not be in an unusual form (crystalline or amor-
phous, etc.) the assumption of allotropy as the explanation
of the behavior of acetate-copper is unnecessary and meaning-
less.
- J. Maxwell Garnett, in an interesting paper' upon the
“Colors of Metal Glasses and Thin Films,” quotes Roberts-
Austen as thus defining allotropy. “The occurrence of metals
in allotropic states. . . . means that the atoms are differently
arranged in the molecules.” Schützenberger considered that
the copper in the new form of the metal was presentin a peculiar
molecular form.
By heating or by contact with acids this instable molecule
of copper was caused to polymerize with the evolution of con-
siderable heat, and the polymer so formed corresponded to
the ordinary form of the metal. Concerning this idea, Mack-
intosh” writes as follows: -
“To account for the phenomenon of the spontaneous
charige of allotropic to ordinary copper in Oxygen he frames
the theory that allotropic copper corresponds to the copper
of the cuprous [?] salts and that the molecular change is
accompanied by development of heat. But we can see how
liable to spontaneous oxidation such a large quantity of finely
divided copper would be when it contained both carbon and
hydrogen in a loose state of combination, and it is unnecessary
to suppose the existence of another form of metallic copper
to account for the phenomenon.”
As I shall proceed to show that Schützenberger's allo-
* Phil. Trans., 205, 237 (1906).
* Loc. cit.
29O T. Roland Briggs
tropic copper was made up of particles of normal copper in
the colloidal condition and hence in a state of extreme sub-
division, it becomes at once evident that, Mackintosh was on
the right path. He recognized the fact that chemical reac-
tivity is enormously influenced by surface phenomena and
that in a substance of “great porosity’’ these become of very
great magnitude, owing to the enormous surface development.
Thus iron, released from its amalgam by the distillation of .
the mercury, readily burns in the air because of the great
surface thus exposed to oxidation.
Mackintosh was also of the opinion that much of the in-
creased activity of the copper was due to its holding in loose
chemical combination both carbon and hydrogen. Such a
combination does not exist and the mechanism by which these
elements are held in the copper is governed by physical rather
than by chemical forces (adsorption).
Mendeléeff" has described the results and conclusions of
Schützenberger very inaccurately and was of the opinion that
the modified form of copper was not allotropic but that it
owed its unusual properties to occluded hydrogen or to the
formation of a hydride. We will not discuss this statement
further than to point out that Schützenberger himself showed
fairly conclusively that such cannot be the case and that the
Small amounts of hydrogen present do not play an important
rôle.
Schützenberger's allotropic copper shows a striking re-
semblance in nearly all its physical properties to the allo-
tropic silvers prepared by Carey Lea.” By the reduction of
silver tartrate with ferrous tartrate or of silver citrate with
ferrous citrate, dextrine and other reducing agents, silvers
were prepared which possessed very remarkable properties
and existed apparently in many different and allotropic forms.
By the first mentioned process in neutral or acid solutions,
Lea obtained a form of silver" that “exactly resembled me-
* “Principles of Chemistry,” 2, 404. e
* Am. Jour. Sci., [3] 37, 38 (1899); 41, 42 (1891); etc.
* Ibid., [3] 41, 179 (1891).
Electrochemical Production of Colloidal Copper 291
tallic gold.” A brief résumé of the more striking properties
of the gold-colored silver will be given. *
The deposit of metallic silver, after filtering and washing,
was of a golden color and by further washing with water con-
taining one percent of Rochelle salts changed to a reddish
or coppery gold. The dried lumps of metal were lustrous
and mirrors formed upon glass were, when dry, optically con-
tinuous. The allotropic metal was exceedingly brittle and
pulverulent and, ground in a mortar, gave a fine powder of
ordinary silver. *
The yellow silver was readily transformed by the action
of heat into the ordinary white form. Contact with mineral
acids and with certain salt solutions caused a like transforma-
tion which was also readily induced by shock and the high
tension spark discharge from a Leyden jar. The density was
very much less than that of ordinary silver. The yellow
allotropic silver invariably contained from two to five percent
of iron, carbon and hydrogen and other impurities, depending
upon the substances present during its reduction.
The extraordinary similarity between the yellow “allo-
tropic” silver of Lea and the copper of Schützenberger will
be made more evident by consideration of the physical prop-
erties of the latter substance. Its color was that of certain
bronzes and it was exceedingly brittle and pulverulent. By
contact with dilute mineral acids or with strong acetic acid it
was slowly changed to the ordinary form of copper and this
same transformation could be brought about by subjecting
the substance to moderately high temperatures. The de-
posits of acetate-copper contained carbon, hydrogen and copper
oxide as irremovable impurities. The density was abnormally
low.
Lea's yellow silver was more active chemically than the
ordinary form of the metal. In dilute hydrochloric acid the
so-called “gold-colored allotropic silver’’ was changed to a
mixture of normal “white” silver and silver chloride—a re-
action which takes place only to a very slight extent with
292 T. Roland Briggs
fine filings of the normal metal. E. A. Schneider" entirely
confirmed Lea’s observation.
Benedicks' acetate-copper possessed a very low electrical
conductivity, which could be increased very greatly by sub-
jecting the metal films to heat, as we have seen on a previous
page. But it was shown by Faraday” in the case of mirrors
of colloidal gold, that such films of colloidal metals, though
optically quite homogeneous, nevertheless offered extraordi-
narily high resistance to the passage of an electric current
and hence must have been composed of innumerable small
particles of metal in more or less imperfect contact. Barus
and Schneider” measured the resistance of films of Lea's
silver and found them non-conducting, or nearly so, because
of the great contact resistance between the particles. The same
phenomenon was studied by Lüdtke" who found that the elec-
trical conductivity increased as the mirrors were allowed to
age, a very strong indication that in the course of time, the
particles coalesce and reduce the contact resistance.
On heating gold-colored allotropic silver to 180° Centigrade
Carey Lea" found that it was no longer easily pulverizable
in a mortar. Heat has the same effect upon acetate-copper
and the change is to be ascribed to the coalescence of the parti-
cles under the influence of heat.
A summary of certain of the points of similarity between
the so-called allotropic forms of silver and copper will be given
at this point.
(1) The color of the allotropic forms is distinctly different
from that of the normal metal. The color of Lea’s silvers
may be almost anything depending upon conditions—yellow,
golden or coppery colors predominate. Schützenberger's
copper was brownish or bronze-like.
* Ber. chem. Ges. Berlin, 25, 1440 (1892).
- * Phil. Mag., [4] I4, 407, 512 (1857).
* Zeit. phys. Chem., 8, 278 (1891).
* Wied. Ann., 50, 678 (1893).
* Am. Jour. Sci., [3] 41, 179 (1891).
Electrochemical Production of Colloidal Copper 293
(2) The apparent specific volume is in both cases greater
than that of the normal metal; that is, the density is less.
(3) The electrical conductivity of the modified form is
very much less than that of the ordinary form and is increased
in both cases with age and heat.
(4) Both modified forms are easily reduced to an impalpa-
ble powder by grinding in a mortar.
(5) In both cases, the allotropic modification is con-
verted more or less easily into the normal form by a variety
of agencies, such as heat, light, ageing, contact with dilute
acids and salt solutions and so forth.
(6) The chemical activity of both allotropic forms is
very much greater than that of the ordinary metal.
It is now universally conceded that Lea's so-called allo-
tropic silver owed its properties to its finely divided, colloidal
condition and that the gold-colored form consisted, not of an
allotropic modification, but of ordinary silver in the form of
a hydrogel. In other words, no molecular rearrangement had
occurred within the silver molecule—such a rearrangement being
the only premise on which to base a condition of allotropy.
Prange,” Barus and Schneider,” and later, Garnett,” all came
to the same conclusion, substantially as outlined above.
“Allotropic” silver being colloidal and showing, in all
respects, a perfect similarity to Schützenberger's allotropic
copper, there can be but one conclusion concerning this latter
Substance. Allotropic or acetate-copper is an electrically
deposited hydrogel of normal copper. We shall now proceed
to further proofs in support of this hypothesis and to an elab-
oration of the idea in a somewhat unique manner.
It now becomes necessary to account for the formation
of colloidal copper by the electrolysis of acetate solutions.
An aqueous Solution of copper acetate on standing or, more
rapidly, when heated (cf. Schützenberger's directions) be-
* Rec. Trav. chim. Pays-Bas, 9, 126 (1890).
* Zeit. phys. Chem., 8, 278 (1891).
* Phil. Trans., 205, 237 (1906).
2.94. T. Roland Briggs
comes partially hydrolyzed with the production of an insoluble
basic acetate or of cupric hydroxide.
A portion of this may be precipitated; the remainder
will stay in suspension. On electrolyzing such a solution,
if it contains a sufficient quantity of free acetic acid, the sus-
pended hydroxide of copper will assume a positive charge,
wander by cataphoresis to the cathode and there concentrate,
it having been shown by Perrin' and confirmed by (as yet un-
published) work in this laboratory that oxides or hydroxides of
the heavy metals suspended in acid solutions assume a posi-
tive charge and wander with the current to the cathode. Many
of the organic emulsion-colloids behave in a similar manner
and become positively charged in weakly acid suspension.
Müller and Bahntje.* have confirmed these statements
in a most conclusive manner by certain experiments upon
the effect of organic colloids (emulsion-colloids) in the electrol-
ysis of copper-sulphate solutions. They advanced the idea
that copper, while being deposited at the cathode during
electrolysis, is at first in a colloidal condition and, if a suitable
protecting colloid be present, this condition will be maintained
to a greater or less degree, depending upon the colloid employed
in the solution. Thus copper, from sulphate solutions con-
taining gelatine, has a structure indicative of very finely crys-
talline structure and has a low specific gravity because of the
organic matter carried down with it during its formation.
Finally, unless the organic colloid is present at the cathode
during the separation of the copper, no change in the appear-
ance and structure of the metal can be detected. .
The formation of colloidal copper by the electrolysis of
copper acetate is due to the action of the inorganic colloid
present in the solution and concentrating by cataphoresis
at the cathode. The gelatinous copper hydroxide or basic
acetate forms a “protecting” layer about each particle of
copper and, by preventing the coalescence of these particles,
maintains the metal in the form of a solid gel.
Jour. Chim. Phys., 2, 601 (1904).
* Zeit. Elektrochemie, 12, 317 (1906).
Electrochemical Production of Colloidal Copper 295
Silver, again, furnishes us with interesting comparisons.
Paal' prepared colloidal silver by the combined reducing and
protecting action of prot- and lys-albinic acids, the albuminous
substance forming the films about the metal particles. Han-
riot” found that the oxide of iron present in Lea’s golden silver
formed an integral part of the silver molecules; in other words,
present as the gelatinous hydrate, the iron plays a rôle ex-
actly analogous with that of the albuminoid bodies present
in Paal's silver. -
Copper is not unique in forming a colloidal deposit at
the cathode during the electrolysis of certain of its salts.
Vogel” obtained a form of silver deposited upon a platinum
cathode which tended to the formation of a colored powder
(compare Lea's silvers). By the electrolysis of silver nitrate
containing gelatine, Snowdon” obtained a yellow and a purple
deposit of colloidal silver on a rotating cathode and his experi-
ments have been repeated in the course of my investigations.
The same experimenter, in this laboratory, has studied”
the effect of gelatine upon the nature of the lead deposited
upon stationary and rotating cathodes from lead acetate solu-
tions and in every case the organic colloid if it concentrates
at the cathode, causes the metallic deposit to assume a more or
less finely crystalline or colloidal form.
* In Schützenberger's electrolysis, no organic substance
was added to the solution, the colloid, as has been stated,
being a product of the hydrolysis of the electrolyte. That
such a “protecting '' colloid actually exists in the solution
can be shown by the following interesting experiments.
It has been shown by Quincke," Bechold, Pickering,"
and others in the case of the well-known emulsions of oils
* Ber: chem. Ges. Berlin, 35, 22O6, 2224 (1902).
* Comptes rendus, I37, 122 (1903).
.* Pogg. Ann., II/7, 316 (1861).
*Jour. Phys. Chem., 9, 392 (1905).
“Ibid., Io, 500 (1906).
"Cf. Freundlich; “Kapillarchemie,” 459 (1909).
" Jour. Chem. Soc., 91, 2001 (1907); Zeit. Kolloidchemie, 7, 1 I (1910).
296 - - T. Roland Briggs
in Soapy water, that the oil is present as a highly dispersed
phase in drops suspended in the water containing the soap.
Each drop of oil is surrounded by a viscous film or membrane
of soap, the attraction between the drops is reduced to a mini-
mum” and a more or less stable and permanent emulsion is
formed. The soap functions as a protecting colloid and by
violently shaking an oil, such as kerosene or benzene, in water
containing sodium oleate as the Soap, emulsions may easily
be obtained consisting of 99 percent by volume of benzene
and I percent by volume of a one percent solution of sodium
Oleate.
With these results in view, we are justified in using
the emulsification test, in the case of a pure substance such
as benzene as an indication of the presence in a liquid of a
protecting colloid. A positive test indicates without doubt
that such a substance is present; a negative test, however,
is of little value for the reason that many colloids do not act
as emulsifying agents save under very special conditions.
The emulsification test for detecting the presence of a
protecting substance in suspension was first applied to an
aqueous solution of ferric acetate. Pure benzene was added
to an approximately 5 percent solution of ferric acetate
contained in a stoppered bottle and the whole vigorously
shaken until an emulsion was formed. By adding the benzene
in small quantities at a time and by thoroughly shaking after
each addition, a product was finally obtained which had the
consistency of blanc-mange, possessed an opaque brown
appearance and could be kept without marked change of
structure for at least a week. The product possessed all
the properties of the typical emulsions of benzene formed
in presence of soap (sodium Oleate).
Obviously, the conclusions to be drawn from the above
experiment are that an emulsifying substance is present in
the ferric acetate solution and that this substance must be
a product of the decomposition of the iron salt, for it is hard
* Cf. Donnan: Zeit, phys. Chem., 31, 42 (1899).
Electrochemical Production of Colloidal Copper 297
to see how the acetate itself could so function. It is a well-
known fact that the iron salts are strongly hydrolyzed in aque-
ous solution, with the production of a hydrated oxide of iron
which possesses a marked tendency to remain suspended in
the solution. By the dialysis of a freshly boiled and dilute
solution of ferric chloride or by similar treatment of a more
concentrated solution of ferric acetate, very characteristic,
red suspensions of ferric Oxide may be prepared with great
ease. It is this gelatinous substance, Suspended in the ferric
acetate solution, that forms the films about the drops of benz-
ene and gives rise to a typical benzene-in-water emulsion.
Having applied the emulsification test with such marked
success to the case of hydrolyzed ferric acetate solutions,
it was next employed as a measure of the “protecting '' action
of the hydrated copper oxide present in a copper acetate solu-
tion. As copper oxide shows a very much smaller tendency
to remain in suspension than does iron Oxide, it was expected
that the emulsification of benzene would be only partial; such
was the case when the experiment was performed. On shaking
benzene with a five percent cupric acetate solution, however,
a very distinct emulsion was formed, and the drops of benzene
did not completely coalesce until Several days had passed.
While the test was not nearly so striking as that with the iron
solution it was quite sufficient as an indication of the actual
existence of a gelatinous “protecting '' Substance such as was
postulated on a previous page. The results justify the state-
ment that a gelatinous, colloidal oxide (or hydroxide) of copper
exists in copper acetate solutions and that this undoubtedly
exerts a definite effect in the solution, similar in every respect
save that of magnitude, to the effect of iron oxide, sodium
oleate or gelatine. The extraordinary action of gelatine
furnishes the theme for the second part of this paper.
Since the mode of preparation and the great similarity
to other established colloidal forms of metals make it evident
that Schützenberger's acetate-copper was normal copper in
the condition of a hydrogel, it remains only to show wherein
some of the more peculiar properties of the substance serve
- 298 T. Roland Briggs
to further the above-mentioned hypothesis. Low specific
gravity, low electrical conductivity, great brittleness and
marked chemical reactivity are the usual accompaniment
of a state of colloidal subdivision. The effect of strong mineral
acids, very strong acetic acid and heat is to cause a coagula-
tion of the particles and, as a result, conversion to normal
“red” copper.
The presence of the very noticeable amounts of copper
oxide in Wiedemann's deposits can be explained by three
processes: (1) The oxidation of the deposit, (2) the adsorp-
tion of copper oxide from the solution, and (3) the presence
of copper hydrate films about the copper particles. More
will be said of this second important property in the second
part of this paper.
The presence of acetic acid in the copper is due no doubt
to its adsorption from the slightly acid solution and to its
presence as basic acetate which may serve as the insoluble
substance in the colloidal films. As the copper deposits age,
we should expect a certain amount of change from the colloidal
to the normal form of the metal, and this would certainly be
attended by a volume-shrinkage. This accounts very well
for the observations of Benedicks with regard to the cracking
of the deposit and resulting expulsion of small amounts of
acetic acid. * * * - -
Under certain conditions the colloidal gel of copper
is spontaneously changed to red copper with a considerable
evolution of heat. Colloidal silver evolves considerable
heat in changing to the normal white form."
The peculiar action of this form of colloidal copper upon
dilute nitric acid is a result of the extreme state of subdivision
in which the metal exists. The reactivity being greatly
intensified, it is but natural to find it combining with much
weaker acid solutions than are able to bring about chemical
reaction with Ordinary copper. Normal copper gives a mix-
ture of nitric and nitrous oxides (including nitrogen) when
* Prange: Rec. Trav. chim. Pays-Bas, 9, 12 I (1890).
Electrochemical Production of Colloidal Copper 299
dissolved in nitric acid, nitric oxide largely predominating.
But Sabatier and Senderens' have found that copper in the
finely divided state is readily oxidized by nitric oxide and
nitrous oxide is so formed. With ordinary sheet copper
this reduction of nitric oxide does not take place. When
“allotropic” copper was placed in dilute nitric acid a brown
or black discoloration was seen by Schützenberger, which
was probably cupric oxide produced by the reduction of the
nitric oxide, at first formed. When sheet copper dissolves
in the same acid, no brown layer is formed and nitric oxide
is the chief product. Thus the nitrous oxide is formed by
the reduction of nitric oxide by colloidal copper.
Summary . .
{ { 3 y
Schützenberger’s “allotropic" copper has been described
and the literature on the subject reviewed in detail.
The idea of “allotropy” held by Schützenberger and the
“solid-solution " theory of Benedicks have been criticized
and rejected.
By analogy with other known colloidal forms and by its
behavior and preparation, the so-called allotropic copper
has been shown to be a colloid hydrogel of the normal metal.
The formation of colloidal copper by the electrolysis of
copper acetate solutions has been explained by posttilating
the existence in such solutions of a gelatinous, colloidal sub-
stance, probably copper hydroxide, formed by hydrolytic
decomposition. -
That such a “protecting ” colloid actually exists has been
shown by emulsification experiments with benzene.
Every property of Schützenberger's allotropic copper
and Benedicks' acetate-copper can be explained on the basis
of the above theory.
PART II—BLUE GELATINE-COPPER
Copper and the copper alloys such as brass and the
bronzes lend themselves very readily to artistic decoration
* Comptes rendus, II4, 1429 (1892).
3OO T. Roland Briggs
by means of colored superficial films or “patinas.” Great
as is the variety of hues which may thus be imparted to copper,
it is a remarkable fact that a rich, permanent and true blue
patina for this metal is practically unknown. It was while
seeking such a blue surface film that the electrolysis of copper
acetate solutions containing gelatine was first performed.
One gram of gelatine was dissolved in about 325 cc of a one
percent solution of cupric acetate and this mixture electro-
lyzed between carefully cleaned and burnished electrodes of
sheet-copper. The electrolysis was allowed to proceed for
about five minutes with a cathode (and anode) current density
which varied from O. I5 to O.45 ampere per Square decimeter
in the different experiments. The deposition was carried
out at room temperature.
Great care was exercised in preparing the electrodes
employed throughout the work, in order that they might be
absolutely free from grease. The cleaning was done by first
“pickling” the electrodes in warm caustic alkali solution and
by following this treatment with a short immersion in nitric
acid diluted with its own volume of water. After being
thoroughly rinsed and dried, the electrodes were then burnished
with a rotating burnisher until a fairly good polish was ob-
tained.
The electrolysis performed, the cathode was found to
be covered on its inner surface with a thin, pale brown deposit,
which under the fingers possessed a peculiar slippery surface
due, no doubt, to the deposition of a certain amount of gelatine
with the metal. On prolonging the electrolysis for five min-
utes a noticeable amount of the gelatine was deposited at
the cathode; indeed the film was no longer adherent and could
be scraped off the surface of the electrode. No gas was given
off at either pole.
- In itself, this pale brown cathode deposit gave no indi-
cation of its peculiar properties, and it was by chance only
that these were discovered. An electrode, freshly coated with
a layer of the gelatine-copper, was by an oversight allowed to
remain in the solution of copper acetate from which the film
Electrochemical Production of Colloidal Copper 301
of metal had just been deposited and the current was turned
off. On removing the electrode from the solution, it was
noticed that the brown color originally possessed by the cathode
film had given place to a purplish blue of extraordinary bril-
liance and beauty. This led to further experiments.
A second electrode was then coated with gelatine-copper
and, after careful rinsing in cold water, immersed in a five
percent solution of cupric acetate. A most beautiful and
startling phenomenon was the result. For straightway there
ensued a remarkable series of color changes upon the surface
of the copper deposit; hues of marked evenness and color-
intensity followed each other in regular succession until the
electrode had acquired a magnificent deep blue coloration.
On continued immersion this blue color gradually faded to a
pale blue or to an olive-green, after which no further changes
were to be seen. This process we shall speak of as a “develop-
ment,” since it bears a certain resemblance to the develop-
ment of the silver image in photography. The color changes
occur in the following order:
Original pale brown
Golden brown
Reddish purple
Purple
Blue
Light blue
Final pale blue or olive-green
:
7.
This color-development in copper acetate is an exceedingly
beautiful phenomenon and can be stopped at any desired
point by means of careful manipulation. The speed of the
color-development increases rapidly with an increase in the
temperature of the developing solution and varies under the
influence of other and seemingly uncontrollable factors, of
which more will be said later.
Of the colors mentioned in the table given above the blue
is by far the most interesting. This color can be prepared
of great purity and brilliance but unfortunately is exceedingly
instable. In course of time when exposed to the air, the blue
3O2 T. Roland Briggs
patina loses its beauty and a pale blue-gray or a drab olive-
green effectively hides all vestiges of the original splendid
color. If the electrode, bearing its blue film, is heated in an
air-oven at 180° Centigrade or is gently and cautiously warmed
over a Bunsen flame the blue color is also destroyed and a
peculiar greenish gray film (doubtless copper oxide) is formed.
This latter film completely hides the normal copper color of
the metal and has remained unchanged for over a year.
A “water-white” collodion lacquer was then applied
to the blued copper with the hope that this would prove a
cure-all for these troubles and effectually protect the delicate
patina. But intense as is the blue reflection from the colored
film, nevertheless the lacquer robbed it of much of its brilliancy
and reduced it to a dull, paint-like coating of an indigo-blue
shade. The lacquer possessed the added disadvantage of
accentuating any red that might happen to be admixed with
the blue. e -
The electrodes that have been covered with the “devel-
oped '' film of gelatine-copper require the most careful
treatment at all times. Careless handling with moist or soiled
fingers is quite sufficient to ruin the beauty of the patina, and
immersion in practically any salt solution leads to similar
detrimental results. Dilute solutions of mineral acids and
bases instantly destroy the blue color of these films and gener-
ally displace the blue with a more or less even shade of brown.
(a) The Effect of Gelatine—the Protecting Colloid
A 1 percent solution of cupric acetate containing no
gelatine was electrolyzed under the conditions employed in
preparing gelatine-copper. The cathode deposit was dark
brown in color, possessed a granular structure and gave only
traces of blue color-development in copper acetate. The
deposit was identical with the “allotropic” copper of Schüt-
zenberger which we have discussed at length in the first part
of this paper. . - - -
Gelatine in the required amount was then added to this
acetate solution and the electrolysis repeated. The usual
Electrochemical Production of Colloidal Copper 3O3
pale brown cathode film was formed and the results on de-
velopment were completely satisfactory. Gelatine was the
determining factor in the process of the film formation.
The concentration of gelatine in the electrolyte was varied
between fairly wide limits without any very striking results;
there is, however, an optimum concentration of gelatine
which will give the best results. . The following summary of
results show this:
I. With little or no gelatine the cathode deposits were
identical with those of Schützenberger and were not develop-
able to an even blue color.
2. If the concentration of gelatine lay between O.25 and
o.66 percent the cathode films were even and translucent
and developed a beautiful blue color in copper acetate.
3. If the gelatine exceeded one percent in concentration,
the deposit was unsatisfactory and gave no development in
copper acetate solution. In this latter case the gelatine mi-
grated by cataphoresis to the cathode and was precipitated
with the copper to such an extent that there resulted a non-
adherent and sticky mixture of gelatine, copper oxide and
metallic copper.
The rôle of gelatine in the process of forming these de-
velopable films was shown in a second and more interesting
manner. Burnished copper and platinum electrodes were
dipped in a hot, concentrated gelatine solution, and then dried.
Thus treated, a portion of each electrode became coated with
a hard, thin and perfectly transparent film of gelatine and
the electrodes were then used as cathodes in a solution of cupric
acetate containing no gelatine. They were immersed to such
a depth that both the gelatine-coated surface and the clean,
uncoated metal came in contact with the solution. After
electrolysis, it was found that a brown, granular deposit of
copper had formed on the free surface of the electrode and
that this deposit was identical with the ordinary films obtained
from gelatine-free solutions. On the other hand, on that part
of the cathode which was covered with the gelatine layer,
the usual film of gelatine-copper had been deposited and this
N
3O4. T. Roland Briggs
deposit, immersed in copper acetate developer, soon showed
the usual blue color-reaction. The results, while not nearly
so perfect as those obtained in the usual manner, nevertheless
showed in a striking manner the importance of gelatine in
modifying the nature of the cathode film.
Having become convinced of the important part played
by gelatine in the process, we naturally expected a similar
result with other colloidal substances. Starch and gum arabic
were used but the results were negative; the copper film gave
little or no blue color in the developing bath of copper acetate.
In the case of soluble starch, a certain degree of development
actually was noticed but it was not in the least comparable
with that of gelatine-copper. Albumen, which might have
given better results, was flocculated by the copper acetate
electrolyte and could not be used.
It was thought that the failure of starch to give better
results was due to the fact observed by Müller and Bahntje'
that starch in copper sulphate migrated to the cathode only
in strongly acid solutions and hence only under these special
circumstances did starch modify the structure of their cathode
deposits. Copper acetate was then made strongly acid with
acetic acid, starch was added and the electrolysis performed.
In the case of soluble starch the acid had a slightly beneficial
effect, but the results were quite disappointing. Doubtless
the acid, while causing a migration of the starch to the cath-
ode, at the same time tended to coagulate the gel of colloidal
copper and so, by its action on the metal, neutralized the ad-
vantage gained by its effect on the starch.
(b) The Effect of Temperature Variations
As already stated, the best results were obtained in the
process by maintaining the electrolyte at room temperatures,
or between 20 and 25 degrees Centigrade. By prolonged
heating to temperatures above 60° C. the copper acetate
solutions are hydrolyzed to a large extent and deposit basic
* Zeit. Elektrochemie, 12, 317 (1906).
Electrochemical Production of Colloidal Copper 3O5
acetates and oxides of copper. When a freshly prepared
bath of acetate and gelatine wad electrolyzed at 50° C. and
with a current density (cathode) of about O.3 ampere per
square decimeter, the results were entirely different from
those obtained at the lower temperatures. At first, a golden-
yellow film spread out over the electrode but as the electrolysis
proceeded, this gave way to a bright red or red-brown layer
containing gelatine and cuprous Oxide. Under certain condi-
tions this layer could be peeled off from the body of the elec-
trode and, by examination, proved to be highly translucent.
After about ten minutes, the cathode was covered completely
with a bright red layer; on longer electrolysis the red was
displaced by brown. A second and freshly prepared acetate-
gelatine solution gave, after five minutes' electrolysis on a
water-bath at 85°C, a very beautiful salmon-pink film on a
cathode of copper.
A one percent solution containing one-half percent of
gelatine was then electrolyzed with the exceedingly small
current density of O. I ampere per square decimeter and at
57°-60° Centigrade. After ten minutes the cathode had
received a very beautiful shade of gold. At temperatures
above 60° C the gold became distinctly redder and if the
electrolysis was continued for an hour or so, a splendid red
coloration was the result. None of these gold, Salmon-pink
or red films are affected in the least by attempted development
of copper acetate solutions.
These results may be summed up as follows: Up to
35° C or thereabouts, electrolysis of copper acetate-gelatine
solutions gives a brownish film of copper at the cathode which
develops a blue surface coloration in copper acetate solutions.
Under the same conditions of time and current density, at
temperatures between 50° and 90° the cathode deposit is red
in color and does not lose this color by development in copper
acetate. With very low current densities and at temperatures
lying between 55° and 60°, a short electrolysis gives a golden-
colored film, while longer electrolysis leads to the red modifica-
tion. As the acetate is hydrolyzed and the gelatine decomposed
306 & T. Roland Briggs
by prolonged heating, the colors of the cathode film become
less beautiful and brown colors predominate.
These red deposits can be lacquered and when so pre-
served have kept their color unchanged for over a year. The
golden-colored sheets become brownish gold on standing and
then exhibit an iridescent appearance, the golden color being
perhaps the result of thin-film colorations (cf. Nobili's rings).
(c) The Nature of the Electrolyte
There now arose the question as to whether or not other
copper salts could be employed in this electrolysis in place
of the copper acetate that had been used up to this time.
Accordingly one, two and five percent copper sulphate
pentahydrate solutions were prepared and gelatine added in
varying proportions. The results upon electrolysis presented
nothing striking; the deposit was copper in its usual form
with a more or less burnished appearance (due to its finely
crystalline structure) and not the least trace of blue develop-
ment in either copper acetate or sulphate solutions was
noticeable.
A two percent Solution of copper formate containing
gelatine was next used and a light brown cathode deposit
resulted. This film underwent a fairly good development
in both copper formate and acetate, but the effects were less
satisfactory than those obtained with the acetate solutions.
Copper propionate was then prepared by decomposing
copper carbonate with hot agueous propionic acid, filtering
the blue liquid thus obtained so as to remove the excess of
carbonate, and allowing the copper propionate to crystallize
out on cooling and slow evaporation. Using one percent Solu-
tions of copper propionate with gelatine, a cathode deposit
was formed which was similar in every respect to the gelatine
copper obtained from the acetate solutions. All the films
developed a good, even, blue shade in copper propionate and
acetate solutions.
Copper glycollate was prepared from the carbonate and
glycollic acid but, owing to its slight solubility in water, nega-
tive results were obtained.
Electrochemical Production of Colloidal Copper 307
Copper chloride and nitrate gave results similar to those
with the sulphate; it is thus necessary to use the copper salts
of the lower fatty acids to obtain developable gelatine-copper
films.
(d) The Concentration of the Electrolyte
Returning once more to the use of acetate solutions
it was discovered that the best results were obtained with one
or two percent solutions of the copper salt. Very dilute solu-
tions, caused a troublesome evolution of hydrogen at the
cathode; Saturated Solutions gave rise to dark brown, non-
adherent deposits.
(e) Duration of the Electrolysis
Under the standard conditions previously determined
and described, five minutes was sufficient time for the prepara-
tion of a satisfactory deposit. Prolonged electrolysis caused
the copper to blacken and to become heavily coated with gela-
tine concentrating at the cathode.
(f) Effect due to Acidity of Electrolyte
When acetic acid was added to the electrolyte, it was
found that the films became lighter in color and gave poorer
blues on development. With strongly acid solutions the
development became very slight indeed. The electrolyte
should be neutral or contain but a slight excess of free acetic
acid.
(g) The Nature of the Cathode
The nature of the metal used as cathode is of little im-
portance in this process as long as it does not in itself cause
the decomposition of the copper solution. Thus, with nickel,
brass and platinum good deposits of gelatine-copper were
obtained and all these gave good blue colorations upon de-
velopment.
- Quantitative Experiments
The electrolysis of copper acetate solutions containing
the usual quantity of gelatine was next carried out with a
3O8 T. Roland Briggs
carefully measured current and with carefully weighed elec-
trodes. A copper coulometer was placed in series with the
acetate cell and, using Faraday's law as the basis of computa-
tion, the cathode efficiency was determined. The solution
employed contained I.5 percent of crystallized cupric acetate
and O.6 percent of gelatine. The experiments were performed
at room temperatures, the developments at 30°–35° C.
(a) WITH A CATHODE OF SHEET-COPPER


Coulometer Test cathode gain Efficiency Weight change on de-
ga1n Gram Percent velopment
Gram Gram
O. O.I.98 O. O23 I II 7 + O. OOO8
O. OI 25 O. O.I.6O I 28 -
O. O2O3 O. d283 I39 —O. OO43
(b) WITH A CATHODE OF SHEET-PLATINUM

| g
Coulometer Test cathode gain Efficiency Weight change on de-
ga1n Gram Percent velopment
Gram Gram
|
O. O.25 I O. O34O I3 I —O. OO47
O. O3 I3 O. O376 I 20 | —O. OOO7
*=º O. O.I.4O * — —O. OOO5
The numbers in the first column are the weights in grams
of the copper deposited upon the coulometer cathodes while
those of the second column show the corresponding gain by
the test-cathodes in the acetate solution. The cathode effi-
ciency is shown in the third column and the changes in weight
occasioned by careful development of the test cathodes consti-
tute the fourth column.
It is evident from the data so obtained that the deposit
from the gelatine-acetate solution is distinctly, abnormal.
The high efficiency calculated for the cathode deposition is,
of course, meaningless as the deposit was certainly contami-
nated with gelatine, oxide, acetic acid or copper acetate.
Electrochemical Production of Colloidal Copper 309
After electrolysis the gelatine was present upon the deposit
in a visible layer and was removed by careful rinsing in cold
water before the electrode was weighed.
The weight changes upon development are small and show
nothing. The high loss in weight in two experiments (over
four milligrams) may have been due to incomplete removal
of gelatine from the electrodes before they were weighed to
determine the cathode efficiency. *3
These experiments are vitiated by the errors met with
in handling such small quantities of a very instable substance,
possessing enormous surface and an undoubted tendency
towards oxidation and the adsorption of cupric oxide and
gelatine.
The Development of the Blue Color
We have seen that the copper deposited by the electric
current from certain copper Solutions containing gelatine pos-
sesses the striking property of developing a remarkable blue
coloration in aqueous solutions of these same copper salts.
This development is equally good in both acetate and pro-
pionate solutions while the formate gives a decidedly less
satisfactory result, the blue color being duller, thinner and less
evenly distributed over the surface. The presence of gelatine
in the developing solution is of no effect and plays no dis-
coverable part in the rather mysterious process of develop-
IIle11t.
The effect of temperature upon the rate of development
was then studied. The cathode films were prepared under
constant conditions and were then developed in Saturated
copper acetate solution at various temperatures. The de-
velopment was continued until the same shade of blue was
visible upon each copper electrode and the time required for
this to occur was determined with a stop-watch. Electrolyte
A contained 3 grams of copper acetate, I gram of gelatine
and 325 cubic centimeters of distilled water. Electrolyte
B contained 4 grams of acetate, I gram of gelatine and 325
cubic centimeters of distilled water. The data follow :
3 IO T. Roland Briggs
COPPER FROM ELECTROLYTE A
Temp. of developer Time required
17.5°C 42O Sec.
2O. O°C I8O Sec.
24. O’C I I O Sec.
28. O* C 65 Sec.
31 o°C 50 Sec.
34. O’C 4O Sec.
COPPER FROM ELECTROLYTE B
Temp. of developer Time required
23°C 3OO Sec.
289 C I4O Sec.
33°C 90 Sec.
38°C 6O Sec.
45°C 4O Sec.
These data show that the rate of development increases
very rapidly with a rise in the temperature of the developing
solution. They also make prominent the fact that very small
differences in the concentration of the electrolyte apparently
have a marked effect upon the reactivity of the gelatine-copper.
Indeed, films prepared under seemingly identical conditions
ofttimes have displayed extraordinary differences in rate
and nature of development in the same acetate developer.
The experimental conditions are not easy to control.
The concentration of the developer has an uncertain effect,
good development being obtained in both weak and strong
solutions of acetate. With weak solutions the development.
is rapid but gives rise to thin and uneven colors. A five
percent solution has usually been employed in this work.
The nature of the salt employed as a developer was next
studied. A very great number of salts were used in aqueous
solution but in no instance was a blue development noticed
except in the following cases. Normal copper sulphate solu-
tion was carefully neutralized with ammonium hydroxide
and used as a developer for gelatine-copper deposited from
Electrochemical Production of Colloidal Copper 31 I
an electrolyte made up of 325 cubic centimeters of 1.5 percent
copper acetate and I gram of gelatine. A blue-black colora-
tion ensued and the film on immersion in warm dilute hydrazine
hydrate gave distinct evidences of the presence of copper
oxide. With an eighth normal solution of copper sulphate
the development was better and a dull but distinctly blue
color was obtained from the warm solution. A control ex-
periment with 5 percent copper acetate gave a magnificent
blue. A mixed developer of 50 cc of N/5 K.SO, and 50 cc
of 5 percent copper acetate gave a much duller and darker
color. --
These experiments show that development of gelatine-
copper does occur to a limited extent in copper sulphate solu-
tions. -
In fiftieth normal sulphate developer the blue color was
fairly bright and evenly distributed, although the develop-
ment required the high temperature of 45°-50° C. In none
of the experiments with sulphate developers, however, were
the gorgeous colors of the acetate developers obtained.
The failure of the sulphate solution to give the best re-
sults must be ascribed to the presence of the SO, ions and per-
haps a trace of free H.SO,. This is borne out by the fact
that the mixed potassium sulphate-copper acetate gave re-
sults little better than those obtained with copper sulphate,
while potassium acetate had little effect upon copper acetate
developer in retarding the coloration. Previous treatment of
the film of gelatine-copper with very dilute H,SO, also pre-
vented the formation of the blue patina.
Fifth normal copper nitrate at both room temperatures
and at 40° C gave a black surface coloration to the copper
film. In this case it is certain that the nitrate acted as an
oxidizing agent, and that nothing more than brown or black
cupric oxide resulted. Copper chloride gave no blue develop-
IIle11t. *
Sodium and potassium acetate solutions of various con-
centrations gave not the smallest trace of blue surface colors
nor did solutions of acetic acid alone. Hence we cannot
3I 2 T. Roland Briggs
ascribe the color development to the acetate anions of the
copper acetate—a copper Salt is essential.
But is the blue color the result of an oxidation process?
Various oxidizing solutions, such as potassium dichromate,
permanganate, perchlorate, chlorate and persulphate, were
employed as developers without the formation of a trace of
blue color. A dirty brown discoloration of the film was the
usual result, especially if the solutions were warmed to 50° C.
A series of hydrogen peroxide solutions with a wide concen-
tration range also afforded no indications of development.
We are thus assured that the blue coloration is not simply
and solely an oxidation process.
Reducing solutions gave negative results. The film
of gelatine-copper was unaffected, by dilute, warm, hydrazine
hydrate solution and underwent accelerated blue develop-
ment in copper acetate after such treatment.
Contact with dilute solutions of mineral acids decreased
the activity of the gelatine-copper toward copper acetate
developer. About one-half of the surface of a film of gelatine-
copper was immersed for a short time in tenth normal hydro-
chloric acid, the treatment resulting in a slight cloudy dis-
coloration of the clear brownish layer. The whole film was
then immersed in the usual copper acetate developer. A
bright even blue color developed upon the surface that had
not been treated with the hydrochloric acid; wherever the acid
had been in contact with the deposit the developments were
poor and uneven.
In stronger hydrochloric acid solutions the deposit took
on a yellow-brown color which on standing, even under a
collodion lacquer, became iridescent in appearance. Stronger
nitric acid solutions turned the deposit into a non-adherent
dark brown form while solutions of arsenic trioxide gave
rise to an almost black coloration.
All of these observations make it evident that the gelatine-
copper is very sensitive indeed to the action of almost any
substance and solution but that the blue color-reaction can
Electrochemical Production of Colloidal Copper 313
be brought about only in Solutions of copper acetate, propionate
or formate or to a less degree in dilute copper sulphate.
Reverse Development in Hydrazine Hydrate
An electrode, covered with a layer of the deep blue
copper, was immersed in a cold and dilute solution of hydrazine
hydrate (5 to Io drops of N.H.OH in IOO ce of water). In a
short time bubbles of nitrogen began to rise from the blue
surface and the color commenced to fade away, giving place
first to a purplish red and then to a red. A golden brown
color then overspread the surface until finally this disappeared
and there was left the pale brown of the undeveloped gelatine-
copper film. Indeed, this series of color changes was about
the reverse of that exhibited during the usual process of de-
velopment and will be termed “reverse development.”
After the reverse development was completed, the
electrode was rinsed in distilled water and once more immersed
in copper acetate developer. The usual series of color changes
occurred, the final product being a fairly good blue in color,
even though a bit thin and uneven.
Further experiments in this direction led to the conclusion
that in the powerfully reducing hydrazine solution the usual
development could be reversed. The gelatine-copper so treated
retained to a greater or less extent its property of turning
blue in copper acetate. The obvious conclusion to be drawn
from these experiments is that the process of reverse develop-
ment is a reduction. This point is of prime importance in
this investigation as it leads to an explanation of these phenom-
ena which will be advanced later.
Effect of Oxidation
It was observed early in my experiments that a deposit
of gelatine-copper, if dried and allowed to stand for an hour
or longer exposed to the oxygen of the air, deepened slightly
in color and completely lost its power of developing the blue
color in copper acetate solutions. The colloidal, finely divided
structure of the film being recognized, this striking decrease
in activity was attributed to the “ageing'' or coagulation
3I4. T. Roland Briggs
that colloid gels undergo upon standing and dehydration.
In the case under discussion, however, while the “ageing ”
of the gel must be a factor, it is quite probable that the loss
of developing power is due to the oxidation of the copper de-
posit. This conclusion resulted from the following experi-
IIle11tS :
A deposit of gelatine-copper was prepared in the usual
manner and allowed to remain untouched for about fourteen
hours. It was then treated with the acetate developer with-
out the slightest color change and without any visible surface
or internal change. On rinsing in water and immersing the
electrode in dilute, cold hydrazine hydrate solution a very
decided evolution of gas, presumably of nitrogen, took place
from the surface of the deposit. The reduction completed,
the clean electrode was dipped into copper acetate with an
almost immediate development of a bright blue color. The
power of development had been restored to the inactive gela-
time-copper by the reducing action of the hydrazine. On
standing in the air the film of copper became transformed to
cupric oxide and lost its power of development.
A second electrode was coated with the copper film and
treated with warm dilute hydrogen peroxide solution. The
film turned a dirty brown and would not develop blue in
copper acetate. The electrode was then dipped in hydrazine
solution until no further reduction occurred, i. e., until the
gas evolution ceased. On placing the film Once more in the
acetate developer, the copper deposit was given a fairly good,
though uneven, blue color, showing, as before, that the power
of development had been restored by the reducer.
The Structure of the Films
It now remained to investigate the structure of the blue
film formed by development. Owing to the very small quanti-
ties of substance deposited at the cathode and the impurities
undoubtedly present, an analysis of the deposit was not made
and would have shown little if it had been made. A micro-
scopic examination of the blue surface by reflected light
Electrochemical Production of Colloidal Copper 315
revealed little save that the color was fairly continuous and
homogeneous.
A sheet of platinum was coated with an exceedingly thin
film of vaseline and the usual layer of gelatine-copper de-
posited. A fairly good blue was obtained on development.
After being dried, the film of colored copper was easily re-
moved from the platinum in little flakes, blue on their ex-
ternal surfaces but possessing a bright copper reflection
from the surfaces that had been in contact with the platinum.
On being pulverized, a dark indigo powder was obtained.
The coloration thus exists chiefly upon the surface of the films
and not throughout their whole mass. N
Summary of Experimental Results
The electrolysis of solutions of copper propionate, acetate
and formate in the presence of gelatine gives rise to a peculiar
form of copper at the cathode.
This gelatine-copper, on immersion in certain copper
solutions, undergoes a striking “development” with the
formation of a series of colors, blue being the most charac-
teristic.
Evidences of development are given by dilute copper
sulphate solutions, moderate development by copper formate
and good development by solutions of copper acetate and pro-
pionate.
The blue coloration is due neither to a reduction nor to
an oxidation of the gelatine-copper film. By Oxidation the
deposits lose their power of giving the color-reaction; by
reduction in dilute hydrazine this power is restored. Oxi-
dizing and reducing agents destroy the blue color of the de-
veloped films and the same result is obtained by dilute acids,
alkalies and salt solutions as well as with heat, ageing and
other means. In dilute hydrazine hydrate a blue film undergoes
a reversal of the development.
The blue seems to be largely a surface coloration.
Under special circumstances, a red, brown or gold color
can be imparted to copper, brass or platinum by making these
3 I6 T. Roland Briggs
metals cathode in copper acetate Solutions containing gelatine.
PART III—CONCLUSION
Gelatine-copper
In the first part of this paper it has been shown that the
“allotropic” copper of Schützenberger was the normal metal
in the modified physical form of a colloidal gel. The formation
of such a gel was ascribed to the action of the gelatinous,
hydrous copper oxide suspended in the acetate electrolyte.
The second part of the paper dealt with the production
of a similar deposit of copper upon the cathode during the
electrolysis of copper acetate containing gelatine. To return
again to the paper of Müller and Bahntje, it was shown that
the presence of certain colloid bodies in copper solutions
undergoing electrolysis may have a marked influence upon
the copper deposited. The formation of the copper deposit
is a cathodic process and, as a result, it is essential that the
colloid substances should be present in appreciable quantities
in the region of the cathode if they are to exert any influence
upon the structure of the copper film. The direction of
migration (cataphoresis) of the colloid particles must hence
be from the anode to the cathode; if the reverse is the case,
the colloid has little influence upon the cathode deposit.
In neutral copper sulphate solutions starch did not concen-
trate upon the cathode and did not affect the copper being
formed there; but in more strongly acid solutions the starch
moved to the cathode and modified the structure of the metal
film. Gelatine, however, in solutions containing a trace of
free acid, moved to the cathode and there exerted its peculiar
influence.
Copper formed by electrolysis from sulphate solutions
containing gelatine has a burnished appearance, the result of
the very finely crystalline nature of the deposit. This is due,
say Müller and Bahntje, to the so-called “protective” action
of the hydrophile colloid upon the metal particles, inhibiting
the growth of the copper nuclei into larger aggregates.
The rôle of gelatine in copper acetate solutions submitted
Electrochemical Production of Colloidal Copper 317
to electrolysis is similar to that played by gelatine in copper
sulphate. In this case the copper comes out in an intensified
colloidal form and does not give the least indication of crystal-
line structure. There is thus formed a compact, irreversible
gel of finely divided copper, containing gelatine and other ad-
sorbed impurities.
The Development
To what is the blue coloration of gelatine-copper in copper
acetate due? The answer to this question was not easy to
find and was obtained mainly by a process of elimination.
That the blue color is not the color shown by thin films
is proven by the uniformity of the blue color upon uneven
surfaces. Thus a coin or an embossed copper dish can be
colored a very even blue by the application of the process.
No iridescent effects are noticed. Examination of the blue
surface under a microscope by light reflected upward from
the surface, reveals the homogeneity of the colored film.
Since the blue color is formed chiefly upon the surface
of the copper deposit, it seems reasonable to exclude the
possibility of a definite chemical reaction occurring between
the gelatine-copper and the developer. If a chemical reaction
were actually taking place we should be able to obtain equally
good development in either the acetate salts of other metals
Or in other copper salts, such as sulphate, chloride, nitrate,
etc. If a chemical reaction occurred, the whole cathode
deposit would doubtless be converted into a blue layer with
a distinct increase in weight, the reaction being necessarily
between the copper in the cathode film and the copper as ion
or the copper acetate itself. The change in weight of the
film on development is small and uncertain, and at present
gives little indication of the actual changes that may occur.
It will be recalled that Wiedemann noted, in the case of
the so-called allotropic copper, that it possessed the power
of absorbing copper oxide from the solution of acetate from
which it had been prepared. This observation, discussed in
the first part of this paper, leads directly to a most plausible
explanation of the development phenomenon. The hypothesis
318 T. Roland Briggs
is this: the color changes that take place on the surface of
the gelatine-copper film during immersion in the developer
are due to a surface adsorption by the colloidal copper of
hydrous copper oxide or hydroxide from the solution. As
already explained, the copper hydroxide is formed by the
hydrolysis of the copper salt and is present in suspension to a
greater or less degree depending upon the nature of the copper
salt employed as developer. The weaker the acid combined
with the copper the greater the degree of hydrolysis; thus
the sulphate solutions are much less hydrolyzed than the form-
ate and the latter is still less hydrolyzed than the acetate
and propionate. The development may be considered as
dependent upon the concentration of oxide or hydroxide
in the developer; we should thus expect little development
in sulphate solutions, better development in formate solu-
tions, and the best results in either copper acetate or copper
propionate. This prediction is confirmed by the experimental
facts. *
These blue adsorption compounds of colloidal copper
and copper Oxide are analogous to the familiar adsorption
compound of Stannic oxide and gold known as the “Purple
of Cassius.” Silver in suspension forms a similar compound.
and of late years the idea has gained credence that the different
“ultramarines” are similar adsorption compounds containing
sulphur Or are solid suspensions of sulphur in sodium aluminate,
borate and other bodies."
The reversal of development by hydrazine is due to the
reduction of the copper hydroxide adsorbed by the colloidal
copper, the latter being restored to nearly its original condi-
tion. The redevelopment that can then be brought about
in copper acetate is a repetition of the regular process of de-
velopment.
There seems to be a certain definite ratio of hydroxide
to copper in the surface film in order that the color reflected
shall be blue. As the concentration of the adsorbed hydroxide
* Cf. Hoffmann: Zeit. Kolloidchemie, Io, 275 (1912).
Electrochemical Production of Colloidal Copper 319
increases, the color changes from brown to gold, to red and
finally to blue. Long treatment with the developer seems
to dissolve the deposit and either a pale blue-gray or a pale
olive-green is the final shade obtained. On standing either
exposed to the air or protected by a lacquer, the blue film
probably loses water and then assumes the pale Olive-green
color of a thin film of copper oxide."
Much work remains to be done in this most interesting
field both in establishment of the above hypothesis as to the
color effects and in finding a method of applying these colors
to practical needs. At present the blue colors are instable
to an extreme, and no lacquer has yet been found sufficient
for their protection. It is hoped that further work will
throw light on the subject.
The general results of this paper are the following:
Schützenberger’s “allotropic” copper is a solid gel of
normal copper in a finely divided form, analogous with similar
forms of lead, silver, and gold.
The electrolytic production of a new form of colloidal
copper by the electrolysis of certain copper solutions contain-
ing gelatine has been described in detail.
The remarkable color action shown by these films of
gelatine-copper has been studied and termed a development.
The development seems to be an adsorption by the copper
film of hydrous copper oxide (hydroxide) formed by the hydrol-
ysis of the developing solution.
Electrolytic methods have been devised for coloring ob-
jects of copper and other metals a variety of hues, the more
prominent being blue, red and gold.
This work was carried out under the direction of Pro-
fessor Bancroft, and I wish to thank him for his interest and
kindly criticism. -
Cornell University
* Cf. Turner: Proc. Roy. Soc., 81A, 301 (1908).
MICHIGAN
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