A CONTRIBUTION TO THE CHEMISTRY OF THE TELLURATES A BY EDGAR BURTON HUTCHINS, Jr. A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF WISCONSIN, 1905 Reprinted from the Bulletin of the University of Wisconsin Science Series,“Vol. 3, No. 3 MADISON, WISCONSIN July, 1905 L>tVV\ v/ A CONTRIBUTION TO THE CHEMISTRY OF THE TELLURATES. introduction. 5 The first systematic study of tellurium and its compounds was made by Berzelius in the early part of the last century. He showed that tellurium forms two oxygen acids, H2Te03 and H2Te04. Berzelius prepared telluric acid in the crystalline form and showed that it has the composition H2Te04.2H20. He prepared a large number of the salts of both tellurous and telluric acids and found that nearly all of the metals yield insoluble com- pounds with these acids. Consequently the tellurates were largely obtained in the form of precipitates. Berzelius made quite a detailed study of some of these compounds. In the case of others, however, the tellurate was prepared and but little more is recorded concerning it. The work of Berzelius is of great importance in that it has not only furnished us with a large num- ber of facts concerning these compounds but also paved the way for a fuller investigation of their properties by later chemists. Oppenheim, Becker, Staudenmaier, Retgers, Mylius, and Gut- bier have each made important contributions to our knowledge "of telluric acid and its salts. All of the alakli metals, sodium, potassium, rubidium, and caesium yield soluble crystalline tellurates and these have re- ceived the attention of investigators to a greater extent than have the tellurates of the heavy metals. Indeed many of the tellurates that were obtained as precipitates by Berzelius appear not to have been studied at all by later investigators. An examination of the literature on the tellurates shows that detailed knowledge of most of them is wanting. Many of the properties that have been observed appear to be those of mixtures. This is doubtless 3 42 BULLETIN OF THE UNIVERSITY OF WISCONSIN. due to the fact that telluric acid possesses weak acid properties and has a great tendency to form basic salts, particularly with the heavy metals, and, in general, to form complex salts. It is frequently found that the line of demarcation between two of the tellurates as they separate from solution is so narrow that one salt is contaminated by another and sometimes by^a third or a fourth. The tellurates of 'the alkali metals are the only ones that have hitherto been described as crystalline. The following work was undertaken with a view of preparing a number of the tellurates and studying their properties. The work has been confined for the most part to the salts of potassi- um, silver, and mercury. Careful attention has been devoted to their preparation in the crystalline form. The ease with which the tellurates of silver and mercury are decomposed by water has made this problem particularly difficult. For the most part the salts appear at first as amorphous precipitates of varying com- position and quickly alter in the mother liquor, their subsequent composition depending on the conditions in the solution. IIUTCPIINS CHEMISTEY OF THE TELLUEATES. 4 ^ TELLURIC ACID. Berzelius 1 prepared telluric acid by oxidizing tellurium diox- ide in alkaline solution with chlorine. From this solution he precipitated barium tellurate by means of barium chloride and subsequently treated the barium salt formed, with sulphuric acid. Sulphuric acid liberates free telluric acid from barium tellurate, insoluble barium sulphate being formed at the same time. Ber- zelius also oxidized tellurium dioxide by fusing it with potassium nitrate. From the potassium tellurate thus formed, he obtained barium tellurate from which, in turn, he liberated the free acid by means of sulphuric acid. Oppenheim^ obtained the acid by fusing tellurium dioxide with potassium hydrate and potassium chlorate. He used the method of Berzelius for preparing the free acid from the alkali tellurate. Becker® dissolved tellurium in nitric acid and oxidized the solu- tion by means of lead peroxide. The lead tellurate obtained was decomposed by sulphuric acid. Gutbier'* * and'Resenscheck have shown that telluric acid can be prepared by oxidizing tellurium dioxide by means of hydrogen peroxide in a solution which is strongly alkaline with sodium or potassium, hydroxide. The alkali tellurate which is obtained in this manner is treated with nitric acid and the telluric acid ob- tained in The free condition. The telluric acid used in this work was obtained by purifying and oxidizing electrolytic tellurium prepared by the Baltimore Copper Co. This tellurium^ contains silver, copper, and selenium as its principal impurities. The acid was prepared by the method described by Staudenmaier.® By this method free telluric acid is obtained by oxidation of tellurous acid by means of chromic acid. In many respects this method is superior to those in which sul- phuric acid is used to set the telluric acid free. The yield is greater, the operaton is move direct, and the sulphuric and selenic ^Pogg. Ann., Vol. 32, p. 23. 2Jour. f. pr. Chem., Vol. 71, p. 266. 3 Ann. CEera., Vol. 180, p. 256. *Zeit. f. anorg. Chem., Vol. 40, p. 260. ’Zeit. f. anorg. Chem., Vol. 10, p. 189. 44 BULLETIN OF THE UNIVERSITY OF WISCONSIN. acids are eliminated in one of the first steps of the process which consists in purifying tellurium dioxide. These acids are apt to contaminate telluric acid prepared by the other methods, unless special precautions are taken. The method as carried out in this work is as follows : Tellurium is dissolved in aqua regia and the resulting solution freed from nitric acid by evaporating with excess of hydrochloric acid. After the hydrochloric acid solution has been diluted and filtered the tellurium is precipitated by means of sodium acid sul- phite and repeatedly washed with hot water. The moist tellurium is oxidized with nitric acid and the resulting basic nitrate purified by recrystallization. The purified basic nitrate is covered with several times its bulk of dilute nitric acid. A little more chromic acid is added to the solution than is required by the equation 3Te02+2Cr03-|-3H20 =3HoTe04ACr203. When the mixture is boiled for some time the tellurium dioxide is completely oxidized to telluric acid according to the above equation. The solution is then con- centrated over the water bath until a crystalline crust begins to form and allowed to cool slowly. The telluric acid separates as a compact crystalline layer which is easily removed from the solu- tion and rinsed to free it from the mother liquor. If the nitric acid solution to which the chromic acid has been added is quite concentrated, telluric acid will separate from the hot solution in the form of fine crystals. The solution is cooled, decanted, and again concentrated over the water bath until crystals form. This operation is repeated as long as telluric acid separates from the mother liquor. The telluric acid, which is green from the presence of chromium nitrate, is dissolved in hot water and recrystallized. If a white residue remains when the acid is dissolved, it is an indication that the amount of chromic acid was insufficient to oxidize all of the tellurium dioxide. A residue may remain even when the hot nitric acid solution is clear. This is due to the fact that tellurium dioxide is much more soluble in hot nitric acid than in cold, and so may separate with the telluric acid when the solution is cooled. In order to remove the chromium nitrate from the telluric acid and obtain a pure white product the acid must be recrystallized 6 HUTCHINS CHEMISTUY OF THE TELLURATES. 45 from ten to twenty-five times, depending upon the amount of the chromium salt present. It has been found possible to prepare fifteen hundred grams of the recrystallized acid in six weeks. The yield is about So%. Telluric acid remains in the mother liquor along with the chro- mium nitrate, from which it cannot be readily separated as the solution becomes syrupy upon evaporation. Telluric acid is readily soluble in either hot or cold water. It is much more soluble in hot water than in cold. Its solubility is decreased by the addition of nitric acid. This fact may be made use of in the separation of telluric acid from chromium nitrate. When concentrated nitric acid is added to a concentrated solution of these compounds telluric acid is precipitated; the chromium nitrate being soluble in nitric acid, remains in solution. If a solu- tion of telluric acid is concentrated rapidly it does not crystallize but appears as a syrupy or gummy mass. Telluric acid is dimorphous® crystallizing in the hexagonal- rhombohedral and isometric systems. It usually crystallizes from its solutions in the rhombohedral system but regular octahedra are sometimes formed. The crystals are colorless and transparent. They have the composition represented by the formula H2Te04. 2U^O. Staudemaier^ describes a second hydrate of telluric acid having the composition H2Te04.6H20 which he obtained by cooling an aqueous solution of telluric acid to zero. Telluric acid is a weak acid showing but a feeble reaction to litmus. Gutbier® found that telluric acid has about the same molecular conductivity as hydrocyanic acid or hydrogen sulphide. He determined the molecular weight of telluric acid by the freez- ing point method. From the results which he obtained he con- cludes that telluric acid should be represented by the formula Te(OH)6 and not H2Te04-|-2H20. When heated above iio° telluric acid loses water. According to Gutbier® constant weight is obtained at 145°. He shows that the loss in weight at that temperature is not as great as that re- ®Gutbier — Studien iiber das Tellur, p. 18. H. c. 81. c. / ® Studien iiber das Tellur., p. 16. 7 i 46 BULLETIN OF THE UNIVEESITY OF WISCONSIN. quired by the change of H2Te04.2H20 to H2Te04. Experiments carried out during the progress of this work have given similar results to those obtained by Gutbier. Mylius^® states that H2Te04.2H20 is changed to (HgTeO^) ^ at about 140°. He calls the polymerized acid allotelluric acid and shows that its solutions have a much greater conductivity than the acid that has not been dehydrated. The conductivity of solu- tions of allotelluric acid, according to Mylius, diminishes upon standing. This may indicate that the acid gradually assumes the composition HgTeOg when in solution. While telluric acid may be considered a weak acid, a hot aque- ous solution of it will attack many metals such as silver, mercury, lead, bismuth, copper, zinc, arsenic, antimony^ tin, aluminum, cad- mium, and nickel. A number of these metals are attacked by a cold solution of the acid. SALTS OF TELLURIC ACID. The types of salts that telluric acid forms may be represented as follows: (2M20.3Te03), (M'20.2Te03), (M'20.Te03), (3M'20.2Te03), and (3M'20'.Te03). Crystalline salts of all of these types are now known. With the possible exception of mer- curic orthotellurateii (Hg3TeOg) no anhydrous crystalline salt has been described. It is claimed by Nandi and von Lang^^ that K2Te04 exists as a crystalline salt in the anhydrous condition but its existence has been disputed by Retgers, Rammelsberg, Stau- ^ denmaier, and Gutbier. Berzelius considered telluric acid a dibasic acid and accord- ingly called salts of the type M'2Te04 normal tellurates and salts of the other types basic or acid tellurates according as they con- tain more or less metallic oxide than the normal salt. This nomenclature has been universally adopted in describing the tel- lurates and is adhered to in the following description. However, lOBer., Vol. 34, II, p. 2208. “New salt. “Wiener Akad.; Ber., Vol. 43, p. 117. “Gutbier, Studien iiber das Tellur., p. 36. 8 9 HUTCHINS CHEMISTRY OF THE TELLUEATES. ' 47 salts of the type M'gTeOe are termed ortho-tellurates from their evident relation to orthotelluric acid HgTeOg. We have both amorphous and crystalline salts of the type M'gTeOg such as lAg^TeOgd^ Hg^TeOgd^ CugTeOgd® and Zn^ TeOgd® These salts appear to be derived from the hexabasic acid HgTeOg which is identical in composition with ordinary tel luric acid. The crystalline salts of the type MsO.TeOg contain two or more molecules of water^ for example, (Na20.Te03.2H20),^'^ (^2^- Te0,.2U,0') ( Cs2O.TeO3.3H2O ) ( Rb2O.TeO3.3H2O ) (Ag2O.TeO3.2H2O) (Hg0.Te03.2H20).^® These compounds may be considered acid salts of orthotelluric acid of the types — (Na2H,TeOg), (K.H.TeOg), (Cs2H,Te06.H20'), (Ag2H,Te OJ, (HgH.TeOJ. Likewise the silver salt (3Ag2O.2TeO3.3H2O') which accord- ing to the nomenclature in vogue is a basic salt, may be consid- ered an acid salt of the type Ag3H3Te06. This view of the tellurates finds some confirmation in the ex- periments of Gutbier and Mylius on the molecular weight and electrical conductivity of telluric acid. It is also supported by the behavior of the hydrous tellurates when heated. Gutbier-^ has shown that the alkali tellurates decompose with the libera- tion of oxygen before the water which they contain is entirely driven off. The silver salts mentioned do not give up all of their water until a temperature of 200° is reached. Oxygen is evolved from these salts at a temperature only a few degrees above this point. Some of the crystalline tellurates contain more water than is required to make them derivatives of orthotelluric acid ; such salts are K2Te04.5H20^^ and HHgTe04.3H20.^^ These com- Amorphous, Berzelius. , ‘^Crystalline, new salt. Amorphous, new salts. ‘^Berzelius. ‘SRetgers, Zeit. f. phys. Chem., Vol. 10, p. .536. ‘9 Norris and Kingman, Am. Chem. Jr., Vol. 26 , p. 320. 29 New salts. 2iStudien iiber das Tellur., p. 34. 22 Berzelius. 22 New sait. ; i 9 48 BULLETIN OF THE UNIVEKSITY OF WISCONSIN. pounds may be considered acid salts containing water of crystal- lization, thus, — K2H4Te06.3H20 and HgH5Te06.H20. Such crystalline salts as CsHTe04.^H20 and RbHTeO^.^ H20“^ would have to be considered salts of a higher acid, as H6Te209. Corresponding to the type M'2Te04 a large number of anhy- drous tellurates have been prepared but they are all amorphous. These salts may also be considered metatellurates. Their rela- tion to orthotelluric acid is similar to that which the metaborates bear to orthoboric acid. It is problematical how much water these salts would contain were it possible to prepare them in the crystalline condition. In addition to the types of tellurates enumerated above a num- ber of tellurates corresponding to M2Te207 have been prepared. They have been called the pyrotellurates. Berzelius also de- scribed the salts Na2O.4TeO3.5H2O and K2O.4TeO3.4H2O. METHODS OF ANALYSIS. Many of the metals may be precipitated from solutions of the tellurate under conditions that will allow all of the tellurium to remain in the filtrate. Thus silver was determined in some cases by precipitation as chloride from a solution of the tellurate in dilute nitric acid. In the analysis of the tellurates of those metals where chlorides are not volatile below a red heat the tellurium has been volatil- ized as Te02.2HCl and the non-volatile chloride weighed. This method of determining the metal is preferred to the method of precipitation because it is less time-consuming; there are fewer operations required for the analysis and less opportunity for ex- perimental error. In cases where it is applicable, that is, where the chloride of the metal is not readily volatile, it gives at once a clean separation of the metal from telluric acid. Moreover the tellurium is obtained in a suitable solution for precipitation with sulphur dioxide. 24 Norris and Kingman, 1. c. 10 i HUTCHINS CHEMISTRY OF THE TELLURATES. 49 Professor Lenher^^ has shown that when dry hydrochloric acid gas is passed over a tellurite heated to a temperature somewhat below redness the tellurium is completely volatilized as Te02. 2HCI. If the chloride of the metal is not volatile it remains in the boat. He has employed this method in the analysis of a number of the tellurites. The method has been found to work equally well for the analysis of salts of telluric acid. Following is a description of the method as it has been carried out. A sample of the tellurate tO' be analyzed is placed in a porce- lian boat which has been weighed. The boat is loosely covered with a thin glass plate and introduced into a hard glass tube. Hydrochloric acid gas is generated by allowing concentrated hydrochloric acid to drop slowly into a flask containing concen- trated sulphuric acid. In this manner a steady stream of the gas is provided. The gas after having been dried by sulphuric acid is passed through the tube, which is heated to' a dull red heat. The hydrochloric acid decomposes the tellurate with the forma- tion of the chloride of the metal, chlorine, and Te02.2HCl accord- ing to the following equation : M2Te04 + 6HC1 = 2M.CI + 2CI -[- Te02.2HCl 2H2O. The tellurium compound sublimes upon the cold part of the tube in the form of crystalline flakes leaving only the chloride of the metal in the boat. The boat is removed from: the tube, cooled in a desiccator and weighed. The escaping hydrochloric acid gas is collected in a receiver containing cold water. The tellurium may be washed from the tube into the receiver with hydrochloric acid and precipitated with sulphurous acid. Tellurium was repeatedly determined by this method. The method which Norris and Kingman^® used in the analysis of the tellurates was largely employed for the estimation of tel- lurium. This method depends upon the fact that hot aqueous hydrochloric acid completely reduces telluric acid to tellurium tetrachloride with the liberation of free chlorine, according to the following equation : H2Te04 -f 6HC1 = CI2 + TeCl^ -f 4H2O. This reaction goes on only slowly, if at all, in the cold. The analysis is carried out as follows : Unpublished notes. c. 11 i 50 BULLETIN OF THE UNIVERSITY OF WISCONSIN. The sample to be analyzed is placed in a small round-bottomed flask together with a small piece of magnesite. About fifty cubic centimeters of concentrated hydrochloric acid is added. The solution is heated until about one-third of it has distilled over, the distillate being collected in a solution of potassium iodide kept cool by a freezing mixture. The liberated iodine is titrated with a solution of sodium thiosulphate. To test the accuracy of this method a weighed amount of re- crystallized telluric, acid was decomposed by hydrochloric acid. The chlorine liberated by the reaction was absorbed by a solution of potassium iodide and the liberated iodine titrated with a solu- tion of sodium thiosulphate. The solution of sodium thiosul- phate was standardized against pure iodine. 0.4465 grams of telluric acid liberated chlorine equivalent to 0.2482 grams of tel- lurium. H2Te04.2H20 requires 55.56% Te; found 55.59%.^^ If it is desired, the metal may be determined in the solution after the tellurate has been reduced and the chlorine distilled. This was done in some cases where the amount of material at hand was small. Tellurium can be determined in telluric acid or its salts much more quickly by this method than by any method requiring pre- cipitation of elementary tellurium. The entire determination requires less than one hour. Moreover the method has proven more reliable as carried out in this work than precipitation of elementary tellurium by sulphur dioxide. In separating mercury from telluric acid use has been made of the fact that tellurium sulphide is soluble in a solution of ammonium sulphide. The tellurate of mercury is dissolved in dilute hydrochloric acid. Mercurous compounds are oxidized by chlorine or bromine water. If the acid solution is saturated with hydrogen sulphide in the cold only a trace of telluric acid is re- duced.^® The solution of telluric acid is then decanted from the 2 ^ All calculations have been made on the basis of Te = 127.6. 2®Brauner, Jr. Chem. Soc., Vol. 67, p. 545. 12 i HUTCHINS CHEMISTKY OF THE TELEUEATES. 51 precipitated HgS. The precipitate is washed by decantation with hydrogen sulphide water to free it from telluric acid, and then digested with ammonium sulphide to remove the trace of tellu- rium precipitated with the mercuric sulphide. Instead of decanting the solution of telluric acid from the pre- cipitated mercuric sulphide it may be made strongly ammoniacal and saturated with hydrogen sulphide. Telluric acid is reduced at once by hydrogen sulphide in alkaline solution. The precipi- tate that is formed dissolves immediately in ammonium sulphide forming a yellow solution. All of the tellurium will now be found in the solution. The mercuric sulphide is washed with a solution of ammonium sulphide and hydrogen sulphide v/ater, dried at iio° and weighed. Water was determined in most cases by heating the tellurate in a current of dry air and collecting the water in sulphuric acid. 13 52 BULLETIN OF THE UNIVERSITY OF WISCONSIN. I THE TELLURATES OF SILVER. NORMAL SILVER TULLURATU. Berzelius describes normal silver tellurate (Ag2Te04) as a dark yellow precipitate. He states that he obtained this com- pound in the form of bulky flakes by adding a concentrated solu- tion of silver nitrate to a solution of normal potassium tellurate. If the precipitate is treated with boiling water it is converted into a brown basic silver tellurate. Gutbier^^ states that it is hardly possible to prepare normal silver tellurate in pure condition by the method described by Ber- zelius. According to Gutbier the precipitate obtained from con- centrated solutions of silver nitrate and potassium tellurate is always contaminated with ai larger or smaller amount of basic silver tellurate. It has not been found possible to prepare normal silver tel- lurate (Ag2Te04) in pure condition by precipitation as described by Berzelius. When solutions of silver nitrate and normal potas- sium tellurate are brought together, the precipitate contains basic silver tellurate. The formation of a basic salt may be obviated by using an acid tellurate of potassium, for example, KHTe04, place of the normal tellurate. But under these con- ditions the precipitate may contain an excess of telluric acid. Moreover when the precipitate is washed it is decomposed form- ing a basic tellurate of silver. It has been found possible to prepare well crystallized normal silver tellurate. Crystals of this salt are not rapidly attacked by cold water and may be obtained free from basic salts. Silver tellurate has been obtained by the following methods : the action of telluric acid upon silver oxide; the double decom- position of a soluble silver salt with a soluble tellurate or telluric acid; the action of telluric acid upon metallic silver. 29pogg. Ann. d. Phys. u. Chem., Vol. 32, p. 577. 30Zeit. f. Anorg. Chem., Vol. 31, p. 340. 14 i HUTCHINS CHEMISTRY OF THE TELLURATES. 53 Action of Telluric Acid on Oxide of Silver, When silver oxide is treated with a slight excess of a solution of telluric acid, normal silver tellurate is formed according to the equation : Ag20 + H3Te04.2H20 = Ag2Te04.2H20 + H2O. The silver tellurate obtained by this method has quite different physical properties from the yellow precipitate described by Ber- zelius and Gutbier. It is a heavy granular powder nearly white in color. It has the composition Ag2Te04.2H20. It is much more stable in the presence of water than the yellow precipitate hitherto described. When the powder is heated to 100° it loses a part of its water and assumes a brown color ; but it does not give up all of its water until a much higher temperature is reached. It is soluble in ammonium hydrate, sodium thiosul- phate, and the acids. The white powder reacts with silver oxide suspended in hot water to form a brown basic tellurate of silver. If a solution of Ag2Te04.2H20 in acetic or nitric acid is evap- orated to dryness in vacuo telluric acid and silver acetate or nitrate are formed. When the salt is strongly compressed be- tween steel rolls it detonates and assumes a brown color. The same property is possessed by the tellurates of mercury. This work will be continued later. For analysis the salt was dried over sulphuric acid and dis- solved in dilute nitric acid. The silver was precipitated from the solution by dilute hydrochloric acid. The filtrate was evap- orated with an excess of hydrochloric acid to free it from nitric acid. The tellurium was precipitated in the hydrochloric acid solution by sulphurous acid. Analysis gave the following results : 0.9597 grams gave 0.6172 grams AgCl and 0.2755 grams Te. 0.9051 grams gave 0.0724 grams H2O. Ag Te H2O Calculated for Ag2Te04.2H20 — 48.68% 28.77% 8.11% Found 48.40% 28.06% 7-99% 15 i 54 BULLETIN OF THE UNIVERSITY OF WISCONSIN. Crystalline AgoTeO ^.2H^0. It has been found possible to obtain crystals of normal silver tellurate by slowly evaporating a solution made by bringing to- gether dilute solutions of telluric acid and silver acetate contain- ing free acetic acid. The dilute acetic acid holds the tellurate of silver in solution and allows it to crystallize as the solution is evaporated. The acetic acid solution must be quite dilute, as a strong solution of the acid prevents the formation of silver tellu- rate. When solutions of silver acetate and telluric acid of moderate concentration are brought together a lemon yellow precipitate is formed. The precipitate closely approximates Ag2Te04 in com- position. When a solution of silver acetate containing 2.0 grams of silver acetate and a few drops of free acetic acid per liter, is mixed with an equal volume of a solution of telluric acid con- taining 1.4 grams of the acid per liter, nO' precipitate is formed. When such a solution is allowed to evaporate in the dark at room temperature yellow crystals of Ag2Te04.2H20 are formed. The free telluric acid in the solution prevents the decomposition of the normal salt by the water. Only small amounts of the salt have been prepared in this manner. Larger quantities of the salt are more conveniently prepared in the following manner : upon adding an excess of a solution of silver nitrate to a con- centrated solution of potassium tellurate containing a little free acetic acid a, yellow or brown precipitate is formed. Analysis showed that this precipitate approximates normal silver tellurate in composition. Its exact composition depends largely on the concentration of the solution. If was observed that when such a solution is allowed to remain in contact with the precipitate for a few hours a heavy yellow salt separates at the bottom of the beaker. The salt is composed of distinct crystals of a straw yel- low color. As the crystals have a much higher specific gravity than the amorphous precipitate, they can easily be separated from it by elutriation. If the precipitate is allowed to remain in the mother liquor for several days, red crystals of a basic silver tel- 16 ( HUTCHINS— CHEMISTKY OF THE TEELURATES. 55 9 lurate will usually be found among the yellow crystals of the normal salt. Analysis of the yellow crystals gave the following results : 0.7638 grams gave 0.0605 grams H2O. 0.4847 grams gave 0.31 17 grams AgCl. 0.4980 grams when heated with hydrochloric acid gave suf- ficient chlorine gas to liberate 0.2828 grams of iodine from potas- sium iodide. H2O Ag Te Calculated for>g2Te04.2H20 8.11% 48.68% 28.77% Found 7.92% 48.41% 28.57% In this analysis the silver was precipitated with hydrochloric acid from a dilute nitric acid solution of the salt. Silver acetate or sulphate may be used instead of silver nitrate for the preparation of the salt by double decomposition with potassium tellurate and subsequent change of the amorphous pre- cipitate to the crystalline salt. Likewise the free acetic acid may be replaced by nitric acid, or the acid may be omitted altogether. The salt crystallizes much better, however, when the solution contains free acid. Ag2Te04.2H20 crystallizes in the orthorhombic system. The crystals resemble those of sulphur both in the forms that occur and in the axial ratios. They are insoluble in hot or cold water but soluble in ammonia, potassium cyanide, sodium thiosulphate, and nitric, acetic and sulphuric acids. Concentrated nitric, acetic, or sulphuric acids completely decompose silver tellurate, giving free telluric acid and the corresponding silver salt. When crystals of the normal salt are allowed to remain for some time in contact with a cold solution of a silver salt, red' crys- tals of the basic tellurate 3Ag2O.2TeO3.3H2O are formed. The change is much more rapid in a ten per cent solution of silver nitrate at a temperature of 50° than in the cold. It takes place very slowly even in cold water. Crystals of Ag2Te04.2H20 darken slowly when exposed to the light. When the powdered crystals are compressed between steel rolls they detonate. When heated above 100° the crystals lose water and take on a brilliant purplish black lustre. They maintain this lustre until a temperature of about 200° is reached. IT 4 56 BULLETIN OF THE UNIVERSITY OF WISCONSIN. As the temperature is raised still higher oxygen is evolved, the mass assumes a gray brown color and fuses. If the mass is al- lowed to cool as soon as it fuses, it becomes black and crystalline, but if it is heated to bright redness it is completely changed to silver tellurite, which is white when cold. The crystalline normal salt may be prepared entirely free from basic silver tellurate by allowing the precipitate obtained by mix- ing solutions of silver nitrate and acid potassium tellurate to re- main in the mother liquor until it assumes the crystalline form. Ag2Te04.2H20 Orthorhombic. Axes — a :b :c = 0.722 :i 12.107.^^ pp'". III : iTi = iii° 26'. pp'. III :Tii— 77° 25'. cp, 001 : III = 105° 40'. cn, 001 :oii = 115° 29'. cs, 001 : 113 = 103° 02'. 3^Tlie author is indebted to Professor Hobbs for his kindly advice in that part of this work which deals with the crystallography of the tellurates. 18 IIUTCHINS CI-IEMISTRY OF THE TELEURATES. 57 The crystals that were used for measurement were obtained by double decomposition of solutions of silver nitrate and potas- sium tellurate containing a little free acetic or nitric acid. The amorphous precipitate that formed on mixing the solutions slowly dissolved in the free acid, while at the same time crystals of Ag2Te04.2H20 were formed upon the walls of the containing vessel. In order to determine whether silver sulphate and silver tel- lurate are isomorphous, crystals of AgoTe04.2H20 were intro- duced into a saturated solution of silver sulphate and the solution allowed to evaporate. Silver sulphate crystallized out of the solution entirely independent of the silver tellurate. Silver sul- phate (Ag2S04) and silver tellurate ( Ag2Te04.2H20) are there- fore not isomorphous. It may be stated that the tellurate of sil- ver was decomposed by the solution to some extent, a few red crystals of a basic tellurate being formied. The sulphate and selenate of silver are isomorphous, their axial ratios being as follows : Silver sulphate a :b :c = 0.5712 :i : 1.238 Silver selenate a :b :c = 0.5945 : 1 : 1.256 However, these salts cannot properly be compared with the tellurate of silver since they are anhydrous, while silver tellurate contains two molecules of water. In addition to the tellurate of silver described by Berzelius Op- penheim^^^ describes a crystalline precipitate, which he obtained in small quantity by adding a saturated solution of telluric acid to a concentrated solution of silver nitrate. He states that it dif- fers from any tellurate of silver obained by Berzelius in that it is crystalline and colorless. When exposed to the air it assumed a yellow color, according to Oppenheim. He states that he did not make a quantitative analysis of the substance but concluded that it was a double compound of silver tellurate and nitrate, since he found that it contained nitric and telluric acids together with silver. 32Jr. pr. Chem., Vol. 71, p. 266. 19 58 BULLETIN OF THE UNIVERSITY OF WISCONSIN. Gutbier^^ attempted to prepare the crystalline compound de- scribed by Oppenheim. When a concentrated solution of telluric acid is added to a concentrated solution of silver nitrate a red brown or yellow precipitate is formed. This precipitate is the only product that Gutbier obtained from such solutions. When a solution of silver nitrate is added to a dilute solution of telluric acid no precipitate is formed. But if the solutions are sufficiently concentrated and an excess of telluric acid added, a yellow flocculent precipitate is formed in small quantity. If the solution and precipitate are allowed to remain together in a dark place at room temperature for a few hours the amorphous precipitate changesi to straw yellow crystals of Ag2Te04.2H20. This is the crystalline compound that Oppenheim obtained. Un- der the conditions of the experiment the crystals might easily be contaminated by silver nitrate. ACID TELLURATKS OF SILVER. Berzelius states that concentrated solutions of KHTe04 and K2'I'e4043 react with concentrated solutions of silver nitrate to form the corresponding tellurate of silver. If the solution con- taining the precipitate is evaporated to^ dryness, a white earthy residue remains. Berzelius states that the acid tellurates are transformed to a brown basic tellurate of silver in dilute solutions. Attempts were made to prepare the acid tellurate of silver de- scribed by Berzelius. It was not found possible to obtain pre- cipitates having either the composition AgHTe04 or Ag2Te40j3. The precipitates obtained in the manner described by Berzelius contain more telluric acid than is required by normal silver tel- lurate (Ag2Te04), but upon remaining in the mother liquor for some hours they are largely transformed to crystalline Ag2Te04. 2H2O. It would hardly be expected that a solution of KHTe04 would yield AgHTe04 with silver nitrate, when it is known that telluric acid and silver nitrate give the crystalline normal tellurate. When concentrated solutions of KHTe04 and silver nitrate are brought together a bright yellow precipitate is formed. The 331. C. 20 I HUTCHINS CHEMISTRY OF THE TELLURATES. 5'J lemon yellow color of the amorphous precipitate is permanent, there being no tendency to form a brown basic tellurate in the medium present. Analysis of the precipitate obtained in this manner showed that it contained 46.6% silver and 32.9% tel- lurium. The ratio between the silver and telluric acid in the pre- cipitate lies between those required by the formulae Ag2Te04 and AgFlTeO^. Some of the precipitate was allowed to remain in the mother liquor for forty-eight hours. At the end of that time a large part of it had been converted into crystalline normal sil- ver tellurate. When a concentrated solution of silver nitrate is added to a solution of acid potassium tellurate made by dissolving potas- sium hydrate and telluric acid together in the proportions re^ quired by the formula K2'I'^40i3 ^ bright yellow precipitate is formed. When left in contact with the mother liquor for some time the precipitate is largely changed to crystalline Ag2Te40. 2H2O. Analysis of the crystalline salt obtained in this manner gave the following results : 0.4006 grams gave 0.0329 grams H2O and 0.2571 grams AgCl. 0.5384 grams with HCl liberated chlorine equivalent to 0.1555 grams Te. Found H2O, 8.21%; Te, 28.88%; Ag, 48.31%. The silver tellurate prepared in this m.anner is not well crys- tallized. This is due to the fact that the precipitate is only slightly soluble in the mother liquor. The crystals darken only very slowly in direct sunlight. When solutions of HKTe04 and silver nitrate are brought together in equimolecular proportions and the whole evaporated to dryness a white residue rem.ains. Analysis of the residue ob- tained in this manner and washed with a little hot water shows that it contains telluric acid in excess of that required by normal silver tellurate. The residue also contains potassium. It is not possible to determine how much of the telluric acid is combined with the potassium, or how much of it is in the form of the free acid. Consequently no conclusion can be reached as to the exact composition of the silver tellurate in the residue. It is not feas- 21 60 BULLETIN OF THE UNIVEESITY OF WISCONSIN. ible to wash it thoroughly because of the decomposition that sil- ver teilurate undergoes in the presence of water. When crystals of normal or basic silver teilurate are boiled with a concentrated solution of telluric acid, a bright yellow amorphous precipitate is formed which is similar to the one obtained from solutions of acid potassium teilurate and silver nitrate. BASIC SILVUR TULLURATES. Berzelius describes two basic tellurates of silver, 3Ag20. 2Te03 both of which were obtained as brown precipitates. He states that boiling water decomposes the nor- mal teilurate of silver into 3Ag20.2Te03 and telluric acid. When a dilute solution of silver nitrate is added to a dilute solution of potassium teilurate, the dark yellow precipitate which is first formed quickly assumes a dark browir color. Berzelius found this dark brown precipitate to be 3Ag20.Te03. He also prepared this compound by adding an ammonical solution of sil- ver nitrate to a solution of normal silver teilurate in ammonia and evaporating the solution. The basic teilurate separates from the solutions as a brown black precipitate. Gutbier®® states that the precipitate formed by the action of silver nitrate on potassium teilurate may be completely changed to 3Ag20.Te03 by long continued washing with cold water. He states that when the precipitate is washed with hot water the final product is 3Ag20.2Te03. According to Gutbier this salt may be prepared by bringing together hot dilute solutions of silver nitrate and potassium teilurate. 3Ag.0.2Tc0^,3H^0. In addition to the brown precipitates of 3Ag20.2Te03 and 3Ag20.Te03 already described, a crystalline teilurate of silver having the composition 3Ag2O.2TeO3.3H2O was prepared dur- ing the progress of this work. When the precipitate obtained by ( c. 351. C. 22 HUTCHINS — CHEMISTUY OF THE TELLUEATES. 61 the double deconiiposition of moderately dilute solutions of sil- ver nitrate and potassium tellurate assumes the crystalline con- dition, both yellow crystals of Ag2Te04.2H20 and red crystals of 3Ag2O.2TeO3.3H2O are formed. The relative amounts of these salts obtained depends largely upon the temperature of the solutions and their concentration. When the solutions are cold and concentrated but little of the basic salt is formed. On the other hand, if the solutions are very dilute and warm, it is pos- sible to obtain the crystalline basic tellurate free from the nor- mal salt. Analysis of the red crystals gave the following results : 0.2812 grams gave 0.0653 grams Te 0.3763 grams gave 0.2917 grams AgCl 1.6094 grams gave 0.0797 grams H2O Te Ag H2O Calculated for 3Ag2O.2TeO3.3H2O 23.18% 58.83% 4.90% Found 23.22% 58.35% 4.95% 3Ag2O.2TeO3.3H2O crystallizes in ruby red, transparent crys- tals belonging to the monoclinic system. The crystals remain unchanged in cold water. They are gradually decomposed by boiling water with the formation of a brown residue, probably d^g'^O-TeOg. When boiled with an excess of a concentrated solution of telluric acid a bright yellow amorphous precipitate re- sults which probably contains silver bi-tellurate (AgHTeO^). 'fhe crystals are soluble in the same reagents that dissolve nor- mal silver tellurate. In the sunlight the crystals become opaque and assume a copper color. When heated above 110° they lose water and become black, but all of the water is not given up until a temperature of about 200° is reached. Just as it is possible to obtain crystals of hydrous normal sil- ver tellurate by evaporation of solutions of silver tellurate under proper conditions, so it is possible by varying the conditions to obtain crystals of 3Ag2O.2TeO3.3H2O directly from clear solu- tions. If a dilute solution of potassium tellurate is added to a large excess of silver nitrate containing a little free nitric acid, the solution filtered and allowed to evaporate at room tempera- ture, red crystals of the basic salt are formed. The formation of crystals when the solution is evaporated is made possible by the 62 BULLETIN OF THE UNIVERSITY OF WISCONSIN. fact that the dilute nitric acid is saturated with silver tellurate dissolved from the amorphous precipitate. If the solution con- tains much nitric acid the silver tellurate is entirely decomposed with the formation of silver nitrate and telluric acid so that no crystals of the tellurate of silver appear when the solution is evap- orated. Consequently a large volume of solution is necessary in order to obtain a small amount of the crystalline salt. Crystals of 3Ag2O.2TeO3.3H2O have been repeatedly obtained in this manner, but only in small quantities. Larger quantities of the salt have been prepared by double de- composition of solutions of silver nitrate and potassium tellurate and subsequent change of the amorphous precipitate thus obtained into the crystalline salt. If the solution contains no free acid the crystals form only in contact with the amorphous precipitate, since silver tellurate is insoluble in water. If, however, the solution contains a little free nitric or acetic acid, red crystals of 3Ag20. 2Te03.3H20 frequently form on the walls of the containing ves- sel. By working with dilute solutions at temperatures from 15° to 50° and using a large excess of silver nitrate, it has been found possible to obtain the red crystalline salt entirely free from yellow crystals of the normal salt. In most cases, however, more or less of the normal salt was formed. Both of the salts appear in distinct crystals. Their colors are so different that it is easy to ascertain when one salt contaminates the other and to remove it mechanically if it is present in small amount. HUTCHIN-S CHEMISTEY OF THE TELLURATES. 63 3Ag2O.2TeO3.3H2O. Monoclinic. Axes — a ;b :c = o.5290 : i :o.27o6. /3 = 77° 12'. mm'", no :iTo=I25° 24'. 11 , III : III = 154° 28'. nn'", 210 ;2 To=: 150° 00'. rb, 13T :oio=i27° 43'. vb, 12T : 010 =118° 00'. , ub, iiT :oio= 104° 33'. zz'", 230 1230= 104° 30'. 25 64 BULLETIN OF THE UNIVEESITY OF WISCONSIN. SAg^O.TeO^. Silver orthotellurate (3Ag20.Te03) has not been obtained in the crystalline form. It is known only in the form of the brown amorphous precipitate described by Berzelius. The salt, some- what contaminated by 3Ag20.Te03, was obtained by adding a hot dilute solution of potassium tellurate to a solution of silver nitrate containing free nitric acid. Analysis of the precipitate, which had been thoroughly washed with hot water and dried over sulphuric acid, gave 72.9% silver. 3Ag20.Te03 requires 74.3% silver. The filtrate was allowed to evaporate at room temperature. Crystals of 3Ag2O.2TeO3.3H2O formed in the solution. 1. The precipitate obtained by double decomposition of a silver salt with telluric acid or a soluble tellurate, may vary in compo- sition from a mixture containing more telluric acid than Ag2Te04 to 3Ag20.Te03, depending upon the conditions in the medium'. 2. It has not been found possible to obtain the acid tellurates of silver, AgHTe04 and Ag20.4Te03 described by Berzelius. 3. Silver forms two crystalline tellurates — Ag2Te04.2H20, which appears as yellow crystals belonging to the orthorhombic system, and 3Ag2O.2TeO3.3H2O, whose crystals are red and be- long to the monoclinic system. 4. The crystalline compound that Oppenheim described as a double salt of silver tellurate and silver nitrate, is Ag2Te04.2H20. 5. No sulphates or selenates of silver have been prepared that are analogous to the crystalline tellurates of silver. 26 HUTCHINS CHEMISTEY OF THE TELEUEATES. G5 THE TELLURATES OF POTASSIUM. NORMAL POTASSIUM TULLURATE. Berzelius^® states that normal potassium tellurate can be pre- pared by treating potassium carbonate with telluric acid. These compounds were dissolved together in equimolecular quantities and the solution evaporated to dryness. The residue was dis- solved in water and the solution evaporated in vacuo over sul- phuric acid until a crystalline crust formed. According to Berzelius this crust consisted of normal potassium tellurate. If the solution is evaporated at a moderate temperature a gummy mass is deposited. Gutbier^’^ states that he was not able to obtain normal potas- sium tellurate by this method. Experiments were undertaken to ascertain whether normal potassium tellurate could be obtained by this method. It was found that solutions made by dissolving equivalent quantities of potassium carbonate and telluric acid in water always yield a gummy mass when they are evaporated in vacuo. Potassium tellurate crystallizes from its solutions if they are allowed to evaporate very slowly. This separation of the crystal- line salt is aided by the addition of a few crystals of the salt. When solutions, made according' to the directions of Berzelius, are evaporated in this manner no crystals are formed. The gummy mass even separates around the crystals of K2Te04.5H20, introduced into the solution. This pasty mass evolves carbon dioxide when treated with hydrochloric acid. The same result is obtained if the solution is cooled to zero instead of evaporated. That all of the carbon dioxide appearing in the gummy mass had not been absorbed from the air is shown by the following ex- periment : 1.42 grams of anhydrous potassium carbonate and -•37 grams of telluric acid were dissolved in a little water. The solution was at once evaporated to dryness over a free flame. scpogg, Ann., Vol. 32, p. 579. 2^Zeit. f. anorg. Chem., Vol. 31, p. 340. 27 6G BULLETIN OF THE UNIVERSITY OF WISCONSIN. The residue copiously evolved carbon dioxide when treated with hydrochloric acid. It is evident that telluric acid cannot replace all of the carbonic acid in an equimolecular quantity of potas- sium carbonate. This fact is in harmony with the fact, observed by Berzelius, that carbon dioxide reacts on normal potassium tellurate to form acid potassium tellurate, KHTeO^, and potas- sium carbonate. No carbon dioxide is evolved when saturated solutions of acid potassium tellurate (KHTeO^) and potassium carbonate are brought together at 95° in the proportion of two molecules of the acid tellurate to one of the carbonate. KHTeO^ is more sol- luble in a solution of potassium carbonate than in water. K^TcO,.5H^O. Berzelius^® prepared crystalline K2Te04.5H20 from concen- trated solutions of potassium hydrate and telluric acid in the fol- lowing manner : A solution of potassium hydrate was added to a solution of telluric acid until a permanent precipitate formed, indicating the presence of an excess of the alkali. The gummy precipitate was dissolved by warming the solution. Crystals of K2Te04.5H20 separated from the solution upon cooling it to zero. The salt is decomposed by carbon dioxide with the for- mation of KHTe04 potassium carbonate. Gutbier^^ obtained K2Te04.5H20 in the form of long slender needles by employing the method of preparation used by Berze- lius. Gutbier states that it is very difficult to obtain the salt free from adhering alkali since it cannot be recrystallized without add- ing free alkali to the solution of the salt. Retgers^® describes a second crystalline tellurate of potassium (K2Te04.2H20) which he prepared by dissolving telluric acid in a concentrated boiling solution of potassium hydroxide and al- lowing the solution to cool slowly. He states that crystals of K2'f"e04.2H20 are isomorphous with those of K2OSO4.2H2O. sn. c. 30 1. C. ^oZeit. f. phys. Chem., Vol. 10, p. 536. 28 HUTCHIIs^S CHEMISTHY OF THE TEELUEATES. 67 Gutbier^^ also prepared by dissolving telluric acid in hot concentrated potassium hydroxide and cooling the so- lution. He found the salt to be isomorphous with K2OSO4.2H2O. Nandi and von Lang^^ describe crystalline anhydrous potas- sium tellurate (K2Te04) which they found to be isomorphous with potassium sulphate. Gutbier, Rammelsberg, Staudenmaier, and Retgers have been unable to prepare the anhydrous crystal- line tellurate.'^® K2'^'eO4.5H20 may be obtained in the form of crystals by slow evaporation of solutions of the salt at room, temperatures. When- ever solutions of potassium tellurate are allowed to evaporate in vacuo over sulphuric acid they solidify to a gummy mass which does not become crystalline until it is dry. In order to bring about slower evaporation oi the solution the loosely covered dish containing the solution may be placed in a desiccator over sul- phuric acid. The desiccator should be filled with air freed from carbon dioxide to prevent the formation of potassium carbonate and acid potassium tellurate. Potassium tellurate has been re- peatedly crystallized from its solutions in this manner. The solu- tion may be concentrated rapidly until its specific gravity is about 1.2 and then crystallized by slow evaporation. The fact that potassium tellurate is very soluble renders it somewhat dif- ficult to obtain a nicely crystallized product. Frequently the crystals form radiating clusters but in other cases the crystals are distinct. Solutions of potassium tellurate that have become supersaturated can frequently be made to crystallize by adding a few crystals of 1x2^004. 5H2O and preventing further evaporation by closely covering the solution. Even under these conditions crystallization proceeds slowly. Crystals of K2Te04.5H20 may be obtained by slow evapora- tion of the solution resulting from the double decomposition of equivalent quantities of normal silver tellurate and potassium bromide solution. This may be readily accomplished in the fol- lowing manner. The potassium bromide and silver tellurate are ^^Wiener Akad. Ber., Vol. 43, p. 117. *®Gutbier, Studien uber das Tellur., p. 36. 29 68 BULLETIN OF THE UNIVEESITY OF WISCONSIN. thoroughly ground together, treated with a little warm water and again triturated. After standing for some time in an atmosphere free from carbon dioxide the solution is filtered and crystallized in the manner described above. Analysis of crystals obtained in this manner gave the follow- ing results : 0.9742 grams gave 0.2420 grams H2O. 0.9742 grams gave 0.4010 grams KCl. 0.2839 grams with hydrochloric acid liberated chlorine equiv- alent to o.iooi grams Te. Calculated for K2Te04.5H20 Found H20 25.01 24.84 K 21.76 21.60 Te 3545 35-26 Crystals of K2Te04.5H20 are soft and crumble under a slight pressure. Solutions of the salt are alkaline to litmus and attack glass. Solutions of potassium tellurate containing two molecules of free potassium hydroxide to one of the tellurate yield crystals of K2"re04.5H20 when evaporated slowly. The yield is small on account of the free alkali in the solution. Solutions of this kind were evaporated in order toi ascertain whether potassium tellurate would crystallize with two or five molecules of water under these conditions. Crystals that had been washed with water and dried with filter paper gave the following results upon analysis : 1.1936 grams gave 0.3000 grams H2O. 1.1936 grams gave 0.4916 grams KCl. 0.6443 grams with hydrochloric acid liberated chlorine equiv- alent to 0.2306 grams Te. Calculated for K2Te04.5H20 Found H2O 25.01 25-15 K 21.75 21.63 Te 3545 35-79 30 HUTCHINS CHEMISTHY OF THE TELLURATES. 69 K,Te0,.2H^0. This salt was obtained in the form of distinct crystals by dis- solving telluric acid in a hot concentrated solution of potassium hydrate and allowing the solution to cool. The yield was very small. Analysis of the salt after having been washed with water at zero and dried for a day over calcium chloride gave the follow- ing result: 1.4806 grams gave 0.1767 grams of water. Calculated for K2Te04.2H20 ii 47 Found 1 1 .93 When the solution of potassium hydrate, from which the crys- tals of K2Te04.2H20^ had been removed, was diluted a copious amount of K2Te04.5H20 separated as small crystals. This fact indicates that the formation of the dihydrate in the concentrated solution is due to the dehydrating effect of the strong alkali. It is also evident that K2Te04.5H20 is less soluble in a dilute solu- tion of potassium hydrate than K2Te04.2H20 is in a concentrated solution. ACID Te:uUURATES OF POTASSIUM. Berzelius states that potassium bitellurate (KHTe04) may be obtained from equivalent quantities of telluric acid and potassium carbonate, but that it is produced with greater certainty by dis- solving potassium carbonate and telluric acid in a small amount of hot water in such proportions that there shall be two molecules of telluric acid present to one of potassium carbonate. When such! a solution is cooled the bitellurate of potassium separates upon the walls of the containing vessel in granular tufts. The formula K2O.2TeO3.4H2O is assigned to the crystalline salt. Berzelius states that potassium quadrotellurate (K20.4Te03) can be obtained by the same method as the bitellurate. In the case of the quadrotellurate, however, telluric .acid and potassium carbonate were dissolved in the proportion of four molecules of the former to one of the latter. The formula K2O.4TeO3.4H2O 31 70 BULLETIN OF THE UNIVEESITY OF WISCONSIN. is assigned to the crystalline salt. Berzelius states that these salts may also be obtained by treating a solution of normal potas- sium tellurate with the requisite amount of an acid (nitric or sul- phuric) to withdraw potassium. Berzelius also describes a yellow insoluble quadrotellurate of potassium obtained by heating either K20.2Te03 or K20.4Te03. He states that this compound may also be obtained by heating tel- luric acid with potassium nitrate at a temparature below a red heat. The compound is insoluble in cold nitric, hydrochloric, and sulphuric acids, and potassium hydrate. KHTeO,. A tellurate of potassium containing more telluric acid than KHTe04 cannot be obtained by the action of carbon dioxide on normal potassium tellurate. When carbon dioxide is passed into a concentrated solution of potassium tellurate a white precipitate of potassium bitellurate (KHTe04) is immediately formed. The precipitate may settle as a gummy mass, but it becomes granular when cooled to zero. The composition of this precipitate is not changed by prolonged contact with carbon dioxide. KHTe04, obtained in this manner, was repeatedly covered with distilled water, cooled to zero, and treated with carbon dioxide. Analysis of the residue, after having been dried with filter paper, gave the following results : 0.5428 grams with hydrochloric acid liberated chlorine equiva- lent to 0.2415 grams Te. 0.9906 grams gave 0.251 1 grams KCl. 1.5444 grams gave 0.3492 grams H2O. Found 44.49% Te; 13.30% K; and 22.62% HoO. After deducting the water, some of which was evidently held mechanically, the values obtained for potassium and tellurium correspond closely with those required by the formula K20.2Te03. In another experiment 18% of water was obtained from a sample dried with filter paper, while the same sample gave 13.2% of water after having been dried over phosphorus pentoxide for sev- eral days. The calculated value for water in KHTe04.2H20 is 32 HUTCHINS OHEMISTllY OF THE TELLURATES. 71 i6.8%, and for KHTeO,.i>4HO (or K2O.2TeO3.4H2O) 13.9%. The latter formula is the one assigned to the bitellurate by Berze- lius. Potassium bitellurate may be obtained in the form of a trans- parent crystalline crust by slow evaporation of its solutions. It is only sparingly soluble in water at 0° ; but is readily soluble in boiling water. It is alkaline to litmus. When a solution of potassium bicarbonate is added tO' a con- centrated solution of normal potassium tellurate a white precipi- tate is formed. The precipitate appears flocculent when it is first formed, but it settles as a gumm.y mass. The precipitate is so soluble in the menstruum present that a large amount of potassium tellurate is required to obtain a small amount of the solid ma- terial. The precipitate is probably potassium bitellurate. Its solubility in the menstruum may be due to potassium carbonate formed by the reaction — KHCO3 + K^TeO^ = KHTeO^ + K2CO3. This reaction is in harmony with the fact, mentioned above, that potassium bitellurate does not react with potassium carbon- ate to give carbon dioxide. The ready formation of potassium bitellurate may be due largely to the insolubility of the compound, but from the facts that have been mentioned it seems probable that telluric acid is a weaker acid than carbonic. K,0.3Te0,.5H,0. When solutions made by dissolving potassium carbonate and telluric acid together in water in the proportion of one molecule of the carbonate to four of the acid are evaporated, a white gran- ular precipitate of K2O.3TeO3.5H2O is deposited. Analysis of the salt, after having been dried for eleven days over phosphorus pentoxide, gave the following results : 1.6971 grams gave 0.2075 grams H2O. 1.6971 grams gave 0.3495 grams KCl. 1-3340 grams gave (with HCl) chlorine equivalent to 0.7207 grams Te. 72 BULLETIN OF THE UNIVERSITY OF WISCONSIN. Calculated for K2O.3TeO3.5H2O Found H20 12.65 12.22 K II.OI 11.06 Te 53-83 54.02 The salt reacts alkaline to litmus. Like the bitellurate it is much more soluble in hot water than in cold. This acid salt may be obtained by dissolving potassium carbon- ate and telluric acid together in the ratio of one molecule of potas- sium carbonate tO' three of telluric acid. MERCUROUS TELLURATES. Berzelius describes normal mercurous tellurate (Hg2Te04) as a yellow-brown precipitate obtained by adding powdered mer- curous nitrate to a solution of potassium tellurate. When a solu- tion of mercurous nitrate is used instead of the powdered salt, the yellow-brown precipitate gradually changes to a pale yellow color. Berzelius states that this change in color is probably due to the formation of the bitellurate of mercury (HgHTe04) brought about by the free acid in the solution of the mercury salt. Oppenheim^^ states that when a concentrated solution of mercurous nitrate is treated with telluric acid a white cheesy pre- cipitate forms which becomes yellow in the air. He considers this precipitate a double salt of mercurous nitrate and tellurate analogous to the double compounds of silver and lead that he describes. The precipitates obtained by double decomposition of solutions of mercurous nitrate and a soluble tellurate or telluric acid may vary in composition from HHgTe04 to 3Hg0.2Te03. However, any precipitate of mercurous tellurate may be changed to crystals of HHgTe04.3H20 by treatment with a cold concentrated solution of telluric acid. This acid tellurate is not only the only crystalline mercurous tellurate that has been' pre- pared, but it is the only mercurous tellurate of definite composi- tion that has been obtained during the progress of this work. «Pogg. Ann., Vol. 32, p. 577. Jr. f. pr. Chem., Vol. 71, p. 266. 34 HUTCHINS CITEMlSTilY OF THE TELLURATES. I i o Mercurous tellurate is a striking example of a salt of a weak acid and a weak base. Slight variations of the conditions under which a salt is placed may bring about a change in its composi- tion, causing it to become either more acid or more basic. This, together with the fact that the salt is readily decomposed giving free mercury and mercuric tellurate, renders the preparation of the tellurates of monovalent mercury somewhat difficult. HHgTe0,.3H,0. This crystalline salt may be obtained either by double decom- position of solutions of mercurous nitrate and telluric acid or by the action of telluric acid upon mercurous oxide. When mercurous oxide is treated with a cold concentrated solu- tion of telluric acid it is slowly attacked and colorless crystals of HHgTe04.3H20 are formed. As the resulting crystals are mixed with the unattacked oxide, this is not a suitable method for the preparation of the salt in pure condition. Large amounts of the salt may be conveniently prepared by treating a solution of mercurous nitrate with telluric acid and subsequent change of the precipitate to the crystalline salt. When a solution of telluric acid is added to a solution of mercur- ous nitrate a bright yellow precipitate is formed. If a large ex- cess of telluric acid is added, the yellow precipitate becomes nearly white and slowly changes to colorless crystals of HHgTe04.3H20. The presence of considerable free nitric acid in the solution aids greatly in the formation of crystals of the salt. Under these conditions crystals frequently form; on the walls of the containing vessel. Analysis of the colorless crystals gave the following results : 1.9265 grams gave 0.2717 grams H2O. 0.8459 grams gave 0.4358 grams HgS. 1.2416 grams with hydrochloric acid liberated chlorine equiva- lent to 0.3501 grams Te. Calculated for HHgTeO^.sH^O Found Hg 44.78 44.40 Te 28.57 28.21 H2O I4.II 14.11 35 74 BULLETIN OF THE UNIA'ERSITY OF WISCONSIN. For the estimation of the mercury, all of which was present in the mercurous condition, the salt was treated with dilute hy- drochloric acid and the precipitated mercurous chloride oxidized with bromine water. The mercury was precipitated with hydro- gen sulphide. Tellurium was estimated by decomposing the salt with hydro- chloric acid, absorbing the liberated chlorine in a solution of potassium iodide and titrating the free iodine with sodium thio- sulphate. The chlorine absorbed by the solution of potassium iodide represents only a portion of the chlorine liberated by the oxidation of the hydrochloric acid. When the tellurate is treated with hydrochloric acid in the cold, mercurous choride is precipi- tated. Upon heating the solution the chlorine formed by the oxidation of the hydrochloric acid converts the mercurous chlor- ide into mercuric chloride. The amount of chlorine thus reduced was calculated and added to the amount absorbed by the iodide solution in order to determine the amount of tellurium in the salt. The reactions involved may be represented as follows : 2HHgTe04 + 2HCI = Hg^Cl^ 2U^TqO, -f 12HCI =: 2TeCl4 + 8H2O+2CI2. Hg2Cl2 + Cl2 = 2HgCl2. One-half of the chlorine liberated during the reaction is re- duced by the mercurous chloride in the solution. HgHTe04.3H20 crystallizes in the triclinic system. It is in- soluble in water but soluble in dilute acetic or nitric acid. Am- monium hydroxide reacts with the salt tO' form a black precipitate. Sunlight blackens the salt, but it is stable in the air if protected from the light. Crystals of the salt have been preserved in closed tubes for a year without losing their transparency or changing in any respect. With boiling water the crystals quickly decompose, telluric acid and a yellow amorphous tellurate being formed. An excess of cold concentrated mercurous nitrate solu- tion brings about the same result. If the yellow basic precipitate is treated with an excess of telluric acid it may be changed to- crystals of HHgTe04.3H20. When strongly compressed be- tween steel rolls the salt detonates and assumes a brown color. The amorphous mercurous tellurates possess the same property. It has been observed that when a glass rod is drawn over the wet 36 i HUTCHINS CHEMISTEY OF THE TELLUEATES. 75 precipitate obtained by bringing together solutions of mercurous nitrate and potassium tellurate, it changes from a yellow to a brown color. The same change may be brought about by press- ing the precipitate between layers of filter paper. 37 7G bulletin of the UNIVEKSITY OF WISCONSIN. HHgTeO^.sH^O. Triclinic. Axes — a :b :c= 1.044 • i • i-055 106° 03 y= 1103'^ ' 59'- ca, 001 : 100 = : 109° 3S'. ab, 100 :oio = 108° 00'. cb, 001 :oio = :I05° 30'. cs, 001 :oi I = :I4I° 45'- cx, 001 :Tii = : 127° V * 0 sx, on :Tii = :I4I° 30'. c'r, ooT : TiT= 131° 32'. as, 100 :oii = 114° 05'- bx. 010 :Tii = 1130° 15'- c'h, ooT : :oiT= 126° 08'. hs, oiT :oii = : 92° 23'. ub, 223 : :oio= 133° 25'- uc', 223 ; ooT= 103° 30'. rb, Tii : 010= 106° 00'. xa', Tii : Too= 104° 25'- 1 HUTCHINS OHEMISTEY OF THE TELLURATES. 7T Other Mercurous Tcllurates. All attempts to prepare normal mercurous tellurate in the crys- talline form have been unsuccessful. Indeed, it has not been found possible to obtain an amorphous tellurate of mercury hav- ing exactly the composition Hg2Te04. The salt is likely to be contaminated by either a basic or an acid tellurate of mercury, depending upon the conditions in the solution. The preparation of the salt is still further complicated by the fact that the precipi- tate obtained by double decomposition of mercurous nitrate and potassium tellurate contains nitric acid which it is difficult to remove by washing. Moreover, long continued treatment with water decomposes the precipitate. When a large excess of a cold concentrated solution of potas- sium tellurate is added to a concentrated solution of mercurous nitrate the yellow precipitate, which is first formed, quickly changes to a brown. The precipitate is slimy and difficult to wash. It approximates Hg2Te04 in composition. Impure 3Hg20.2Te03 may be obtained as a yellow amorphous precipitate by treating a hot dilute solution of potassium tellurate with an excess of mercurous nitrate solution and washing the pre- cipitate with hot water. It is hardly possible to obtain the pre- cipitate free from nitric acid. A mercurous tellurate analogous or silver orthotellurate (AggTeOg) has not been prepared. When the bulky yellow pre- cipitate, obtained by treating a hot dilute solution of potassium tellurate with an excess of mercurous nitrate solution is washed with hot water and boiled for some time in water it is decom- posed giving free mercury. The precipitate may be boiled for several hours without changing in appearance, but at length it settles as a heavy brown granular powder containing free mer- cury. This change may be hastened by rubbing some of the pre- cipitate against the side of the beaker with a glass rod. When the brown powder is heated with hydrochloric acid ele- mentary tellurium is precipitated. 39 78 BULLETIN OF THE UNIVERSITY OF WISCONSIN. MERCURIC TELLURATES. Berzelius d'escribes mercuric, tellurate (HgTe04) as a white flocculent precipitate. Divalent mercury forms two salts with telluric acid, mer- curic orthotellurate (HggTeOe) normal mercuric tellurate (HgTe04), both of which have been prepared in crystalline form. HgTe04 may be obtained as a white flocculent precipitate as de- scribed by Berzelius. Under favorable conditions it crystallizes with two molecules of water, giving HgTe04.2H20. The in- termediate compound 3Hg0.2Te03 has not been isolated, al- though there are indications that it may exist. All precipitates of mercuric tellurate, .whether crystalline or amorphous, show a strong tendency in the presence of water to change to mercuric orthotellurate. This property of mercuric tellurate renders it dif- ficult to prepare HgTe04 in pure condition. M erciiric Orthotellurate — Hg^TeO^. When an excess of mercuric nitrate is added to a hot dilute solution of potassium tellurate, a heavy yellow granular precipi- tate of mercuric orthotellurate is formed. In order to obtain this salt in crystalline form, a somewhat different method of pro- cedure must be employed. If a cold concentrated solution of mercuric nitrate is added to a concentrated solution of potassium tellurate made acid with nitric acid, a white flocculent precipitate of HgTe04 is formed. At room temperatures a portion of the precipitate usually as- sumes a yellow color immediately, probably due to the formation of a basic salt. If the acid solution containing this white or yellow flocculent precipitate is allowed to stand undisturbed for several days the precipitate is largely converted into transparent amber colored crystals of HggTeOe. Frequently the walls of the containing vessel are covered with crystals that have separ- ated from the acid solution. In many cases the amber colored crystals form in contact with the white amorphous precipitate, but 40 4 HUTCHINS CHEMISTRY OF THE TELLURATES/ '79 in all cases the crystals are entirely distinct from the precipitate. HggTeOe crystallizes in the isometric system, the predominat- ing form being the dodecahedron. Combinations of the cube and dodecahedron are frequent, while many crystals show a broad octahedral face on which the crystal has grown. Single crystals have been obtained which are over an eighth of an inch in diam- eter. The crystals contain a little nitric acid and about o.6% of water. This is considerably less than one-half of a molecule of water to one of HggTeOs. The crystals are insoluble in water and unchanged by boiling with water. They are soluble in nitric acid but more readily soluble in hydrochloric acid. Hot potassium hydrate precipitates mercuric oxide. The powdered salt does not change in compo- sition when kept in contact with a cold saturated solution of telluric acid for '48 hours. When heated the salt assumes a red color, but it regains its original color upon cooling. The crystals are not altered by heating to 140°. Analysis of the salt, after having been dried over phosphorus pentoxide for several days, gave the following results : 4.6696 grams gave 0.0279 grams H2O. 1.7853 grams gave 1.4940 grams HgS. 1.0894 grams with hydrochloric acid liberated chlorine equiva- lent to 0.1727 grams Te. Found 72.13% Hg 15-85% Te 0.59% H2O The value obtained for tellurium is about 0.5% too high. This discrepancy is due to the presence of a little nitric acid in the crystals. The corresponding sulphate of mercury, Hg^SOg, exists as tur- peth mineral. This compound may be prepared as a heavy yel- low powder or as yellow crystals by treating the normal sulphate with water. In color the sulphate is very similar to the tellurate, but it does not crystallize in the isometric system. 'Amorphous Hg^TeO^, When concentrated solutions of mercuric nitrate and potassium tellurate or telluric acid are brought together at zero a white 41 { 80 BULLETIN OF THE UNIVERSITY OF WISCONSIN. flocculent precipitate of mercuric tellurate is formed. The pre- cipitate is quickly decomposed by water at room temperature into telluric acid and basic mercuric tellurate. This decomposition goes on even when the precipitate is washed with water at zero; consequently it is impossible to obtain a pure white product in suitable condition for analysis. The precipitate remains white indefinitely in a saturated solution of telluric acid at room tem- perature. Analysis of the light yellow powder obtained by washing the white precipitate with water at o° showed that it approximates HgTe04 in composition. Crystalline HgTeO^^.^H^O. If the white precipitate of mercuric tellurate formed by mixing concentrated solutions of mercuric nitrate and potassium tellurate containing free nitric acid, is allowed to remain undisturbed in the mother liquor for several days at room temperature, nearly all of it is converted into crystalline HggTeOe. In some cases colorless crystals of HgTe04.2H20 were obtained mixed with the amber colored crystals of mercuric orthotellurate. Analysis of the colorless crystals gave the following results : 0.2413 grams gave 0.0202 grams H2O ; 0.2413 grams gave 0.0887 grams Te02 ; Calculated for HgTe04.2H20' Found Te 29.84 29.39 H,0 8.41 8.37 HgTe04.2H20 appears as transparent crystals belonging to the orthorhombic system. It is slowly decomposed by cold water. Boiling water quickly decomposes it into telluric acid and basic mercuric tellurate. Copper Ortho tellur a,fe, Cu^TeO^. When a solution of copper nitrate is treated with a solution of potassium tellurate a green flocculent precipitate of copper tel- lurate is formed. Berzelius describes this compound as normal copper tellurate (CuTe04). If the precipitate is boiled for some 42 HUTCHIInS CHEMISTEY OF THE TELLURATES. 81 hours with water it is converted into copper orthotellurate, which settles as a heavy green powder. It is insoluble in water but soluble in ammonia, potassium cyanide, and acetic, hydrochloric, and nitric acids. Analysis of the green powder after having been dried over phosphorus pentoxide for a week gave the following result : 0.6675 grams heated with hydrochloric acid liberated chlorine equivalent to 0.2069 grams tellurium. CugTeOe requires 30.79% tellurium; found 31.00%. Numerous attempts were made to prepare copper tellurate in crystalline form, but none were successful. Copper hydrate was treated with an excess of telluric acid ; the resulting tellurate of copper was allowed to remain in the solution for several weeks. Likewise the precipitate obtained by bringing together solutions of potassium tellurate and copper nitrate was allowed to remain in mother liquors of varying composition for weeks. In no' case were any crystals of copper tellurate formed. Zinc Orthotellurate, Zn^TeO^. The white flocculent precipitate obtained by bringing together solutions of zinc nitrate and potassium tellurate may be changed into a heavy granular precipitate of ZugTeOo by long continued treatment with hot water. Analysis of the powder prepared in this manner and dried over phosphorus pentoxide for a week gave the following result; 0.6331 grams with hydrochloric acid liberated chlorine equivalent to 0.1940 grams tellurium. Calculated for ZusTeOg, 30.40% tellurium-; found 30.64%. ZngTeOg is insoluble in water but soluble in acetic, nitric, hy- drochloric, and sulphuric acids. Gutbier^® prepared zinc tellurate in the form of a white insol- uble precipitate by bringing together solutions of potassium tellu- rate and zinc chloride. He does not give the composition of the tellurate of zinc that he obtained in this manner. ^ Zeit. f. anorg. Chem., Vol. 31, p, 349. 43 82 bulletin of the university or Wisconsin. Attempts were made to prepare zinc tellurate in crystalline form by allowing the amorphous salt to remain in contact with mother liquors of various compositions. The results were all negative. GOLD TELLURATU. No tellurate of gold has been prepared. From the experiments that have been made in an attempt to prepare this salt it seems probable that water immediately decomposes the salt if it is formed in aqueous solutions. When a solution of gold chloride is added to normal silver tel- lurate suspended in water, silver chloride and gold oxide are pre- cipitated. When solutions of normal potassium tellurate and gold chloride are brought together and the resulting solution allowed to evap- orate over sulphuric acid, yellow crystals of potassium chlor- aurate are formed. No tellurate of gold separates from the solu- tion. The fact that gold chloride takes up potassium from potassium tellurate to form potassium chlor-aurate is a good illustration of the weak affinity that telluric acid exerts towards bases. separation of gold from tellurium. While working with gold and telluric acid it became necessary to separate gold from tellurium. It was found that nitrous acid would not reduce either telluric or tellurous acids. Nitrous acid completely reduces gold from its solutions. Gold may be sepa- rated from tellurium in dilute acid solution by the addition of potassium' nitrate. After filtering out the precipitated gold, tellu- rium may be determined in the filtrate by precipitation with sul- phur dioxide. Fischer, Pogg. Ann., Vol. 17, p. 480. HUTCHINS CHEMISTEY OF THE TELLURATES. 83 SUMMARY. 1. -Telluric acid forms ortho-tellurates of the type M'gTeOg, with a number of the metals. AggTeOg, HggTeOg, Zn.TeOg, and CugTeOg have been prepared. The normal tellurates (e. g. Ag2Te04) of these metals are changed tO' the ortho compounds by treatment with water. 2. The crystalline normal tellurates that have been prepared, viz. : Ag2Te04.2H20, Rb^TeO^-sH^O, HgTe04.2H20, Cs2Te04. 3H2O, Na2Te04.2H20, and K2Te04.2H may be considered acid salts of orthotelluric acid of the types M'2H4TeO'g and M'2H4Te0e.H20. 3. Silver forms two crystalline tellurates — Ag2O.TeO3.2H2O and 3Ag2O.2TeO3.3H2O. The former salt is yellow and crystal- lizes^ in the orthorhombic system ; the latter is red and crystallizes in the monoclinic system. Both of these salts may be considered acid salts of orthotelluric acid. No selenate or sulphate of silver is known that is isomorphous with either of these salts. 4. HgHTe04.3H20 crystallizes in the triclinic system. It is the only crystalline mercurous tellurate that has been prepared. 5. Two crystalline salts of divalent mercury have been pre- pared, viz. : Hg3Te06 and HgTe04.2H20. The former appears in the form of amber colored crystals belonging to the isometric system ; the latter is white and crystallizes in the orthorhombic system. 6. The crystalline compound described by Oppenheim as a double salt of silver nitrate and tellurate is normal silver tellurate {Ag,Tt0,.2H,0). 7. It has not been found possible to prepare the acid tellurates of silver described by Berzelius. 8. Potassium tellurate may be prepared in crystalline form by slow evapouation of its solution, provided care is exercised to pre- vent supersaturation of the solution. Potassium forms a second tellurate — K^TeO^. 5H2O. 45 84r BULLETIN OF THE UNIVERSITY OF WISCONSIN. 9. Telluric acid does not completely replace the carbonic acid in an equivalent quantity of potassium carbonate. Crystalline normal potassium tellurate cannot be obtained from potassium carbonate and telluric acid as described by Berzelius. 10. Although telluric acid is a weak acid, hot concentrated solutions of it attack mercury, silver, lead, tin, arsenic, antimony, bismuth, nickel, zinc, aluminum, and cadmium. 11. There are no well authenticated cases of isomorphism be- tween sulphates and tellurates or between selenates and tellurates. This work was undertaken at the suggestion of Professor Lenher and carried out under his guidance. I wish to take this opportunity of expressing my thanks for the inspiration that I have received from him in the laboratory as well as in the class- room. 1 III III III III III III III j III Tl 3 0112 072M463 J j