ELECTRODE POTENTIAL OF MANGANESE 
 
 By 
 
 DAVID SCHLESINGER 
 
 THESIS 
 
 FOR THE 
 
 DEGREE OF BACHELOR OF SCIENCE 
 
 IN 
 
 CHEMICAL ENGINEERING 
 
 COLLEGE OF LIBERAL ARTS AND SCIENCES 
 UNIVERSITY OF ILLINOIS 
 
 1921 
 

 * ’ ‘ •» 
 
28 SEP 2] 
 
 UNIVERSITY OF ILLINOIS 
 
 \^ 2 .\ 
 
 ^c3sf\« w 
 
 192I 
 
 THIS IS TO CERTIFY THAT THE THESIS PREPARED UNDER MY SUPERVISION BY 
 
 ^yJ.d__Siil2ljeaing.eii 
 
 ENTlTLED___Ele_ctrode_JPia.t-eiiti.ai._oX_i/langanase- 
 
 IS APPROVED BY ME AS FULFILLING THIS PART OF THE REQUIREMENTS FOR THE 
 DEGREE OF __B§_CiL©lAr_J3l’__Si3lejlC_^ 
 
 
 Instructor in Charge 
 
 Approved : 
 
 HEAD OF DEPARTMENT OF 
 
 4T3694 
 
Digitized by the Internet Archive 
 in 2016 
 
 https://archive.org/details/electrodepotentiOOschl 
 

 Table of Contents. 
 
 page 
 
 I. 
 
 Introduction • 
 
 1 
 
 II. 
 
 Theoretical . 
 
 2 
 
 III. 
 
 Historical. 
 
 6 
 
 IV. 
 
 Experimental . 
 
 7 
 
 
 Apparatus . 
 
 
 
 Preparation of Materials. 
 
 
 
 Results . 
 
 
 V. 
 
 Acknowledgement . 
 
 20. 
 
1 
 
 THE ELECTRODE POTENTIAL OF MANGANESE. 
 
 INTRODUCTION. 
 
 The electrode potential of many of the elements has never 
 been determined because of the great difficulties which arise 
 in making electrolytic measurements between the metal and ions 
 of the metal in solution. Many other obstacles are encountered 
 duetto electrical phenomena such as over^joltage and passivity. 
 The reactivity of others, as the alkali metals, necessitates 
 indirect means of getting results. This research was carried 
 out for the purpose of determining the potential difference 
 between metallic manganese and a solutiori of a manganese salt 
 normal with respect to manganese ions. The potential of this 
 element has never been satisfactorily obtained. 
 
 In the laboratory work in electrochemistry, electromotive 
 force measurements were made on some of the moer common metals 
 as zinc, copper and silver whose potentials are accurately 
 known. The results obtained in these experiments by the students 
 v;ere so poor that it seemed advisable to attempt to find the 
 cause, at the same time gaining experience in methods used by 
 the various investigators. 
 
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 theoretical. 
 
 The elctromotlve force produced by a voltaic cell is the sum 
 of the electromotive forces produced at the junction between the 
 electrodes and the solutions and of those produced' at the jun«?tions 
 between the different solutions that may be present in the cell. 
 
 The electromotive forces produced at the electrodes are caned 
 the electrode potentials. 
 
 The methods used in determining electrodes potentials are by 
 measuring the electromotive force of an ordinary cell. It is there- 
 fore customary to adopt as the value of the electrode potential of 
 the half cell, the electromotive force of the v<hole cell vrhich 
 consists of the half cell under consideration, combined with the 
 standard half cell whose electrode potential is thereby arbitrarily 
 assumed sometimes as zero. There are two important standard half 
 cells, one being the tenth normal calomel electrode, and the other 
 the molai hydrogen electrode. The hydrogen electrode is assumed to 
 be zero and the tenth normal calomel electrode has a value of 0.?36 
 volts as compared to the hydrogen electrode. The tenth normal 
 calomel electrode consists of the system O.ln KCl - HggClg, Hg. 
 
 The electrode potential of any half cell is therefore equal 
 to the electromotive force of the whole cell formed by combining it 
 with the standard half, cell, assuming the value of the standard as 
 zero. Thus the electrode potential of the half cell Zn,0«0ln ZnCl 2 
 referred to the moiai hydrogen electrode, is equal to the electro- 
 motive force of the whole cell Zn,0.0<ln ZnClg 1.0m H, H 2 ( 1 atm.). 
 
 As there is an electromotive force at the junction of the liquids, 
 this value has to be subtracted from the value of the whole cell, 
 unless Some liquid is placed there which will eliminate this 
 

 
 
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 potential. Saturated potassium chloride forms an excellent bridge 
 and practically eliminates the liquid junction. 
 
 Just as the electromotive force of a cell is determined 
 solely by the change in state that takes place in it, so any 
 electrode potential i^ determined by the change in state that 
 takes place at the electrode. With the aid of this principle 
 and the general expression for the free energy change attending 
 the transfer of a substance from a solution of one concentration 
 to one of another and for the free energy change involved in 
 the production of work in voltaic cells, the change of the 
 electrode potential with the concentration of the ions can be 
 calculated provided the concentrations are so small that the 
 dissolved substances conform to the perfect gas laws. 
 
 The free energy change attending the transfer of a substance 
 from a solution of ■'one concentration to that of another concen- 
 tration is, 
 
 - Af = NRT In 2 . > 
 
 Cz 
 
 at the absolute temperature T, of that quantity of substance 
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 concentration is Cg. The free energy change involved in the 
 production of work in a voltaic cell is, 
 
 - AF = ENP 
 
 where E is the electromotive of the cell, N the equi vaients 
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 place at each electrode. Equating the free energy equations we get 
 
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 c^ 
 
 then 
 
 E = 
 
 _ NRT 
 
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 through the solution. Then 
 
 E - in 
 
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 where E is the specific electrode potential of the electrode in 
 question, the specific electrode potential being defined as the 
 electrode potential when the concentrations of the ions »re one 
 moiai and the pressure is one atmosphere. 
 
 ^ RT 
 
 E -E = 
 
 n7^ 
 
 In 
 
 c^. 
 
 = E - log-^ at 18° C. 
 
 n c -7 
 
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 the metallic ions in solution. In order to determine the concen 
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 ionization of the salt in the solution at the concentration in 
 question. The degree of ionization may easily be determined by 
 conductivity measurements according to the standard method of 
 
 Kohlrausch, where the degree of ionization is equal to 
 where A is the conductivity at the concentration in question 
 and Aothe conductivity at infinite dilution assuming the salt 
 completely ionized at infinite dilution. 
 

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 By measuring the potential difference between manganese 
 and solutions of a manganese salt v/ithrespect to different ion 
 concentrations, the electrode potential may be calculated by 
 means of Nernst's equation given above. Due to the reactivity 
 of the metal in acqueous solutions direct measurements cannot be 
 made. Lewis and Keyes (Jour. Am. Chem. Soc. 3^0, 19^^) 
 
 determined the potential of lithium by first measuring the poten- 
 tial between lithium and lithium amalgams and then the potential 
 of lithium amalgam with respect to the normal calomel electrode 
 obtaining good results. Carrara and Agostini Elektrochem- 
 
 1 1 , 385 » 1905 ) have measured the electromotive force of metals 
 in acqueous and aicohS)lic solutions expressing the relation 
 between the two as 
 
 EP — ITIP ~ 1 os ^w 
 
 •^W -"-^a nP 
 
 in which EP, and EP^ ^-re th®- electrolytic potentials in water 
 
 V7 ci 
 
 and alcohol, respectively, and P^ and P^^ the solution pressures 
 
 of the metals iin each solution. According to the authors the 
 
 value of the fraction is the same for all metals. These 
 
 KT 
 
 constitute the more important indirect ways of measuring the 
 potentials of metals . 
 
6 
 
 HISTORICAL. 
 
 B. Neumann (Zelt. phislk. Chem. J_4, ^93- 230, 1894) measured 
 the electrode potential of manganese amalgams in different 
 manganese solutions. The system he measured consisted of 
 
 (MnSO, ) 
 
 Mn^Hg j MnCl^ indiff .Salt , 
 3 CL^ 
 
 1.0n KCl, Hg 2 Cl 2 , Hg. 
 
 The results he obtained for the electrode potential of manganese 
 was -1.0 • This determination has not been considered as satis- 
 factory. Since then no attempts to determine the potential of 
 the metal have been made . 
 
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7 
 
 EXPERIMENTAL. 
 
 Apparatus • 
 
 Electrode potential measurements were made with a student 
 potentiometer. This type being of a very simple design, the 
 accuracy of it had to be detemined. The apparatus was standard- 
 ized by means of the decade-box type potentiometer. The results 
 obtained showed that equally accurate measurements could be made 
 with the student potentiometer as with the more accurate decade- 
 box type. The diagram of "the connections are given in Fig. 1. in 
 which P is the potentiometer, R the resistance box to regulate 
 the curpent so that only O.OOl ampere flows through the circuit, 
 
 B the storage battery and G- the galvanometer. A double pole 
 double throw switch was used so that at any time reference to 
 the standard cadmium cell could be made. 
 
 The reference ceil used was the standard cadmium cell. The 
 cell consists of an H shaped vessel as shown in Fig. 2. It consists 
 of, (0 mercury, (2) paste of cadmium sulphate and mercurous 
 sulphate, (3) saturated solution and large crystals of cadmium 
 sulphate, (4) cadmium amalgam, (5) small crystals of cadmium 
 sulphate, (6)paraffin, (7) corh and (8) sealing wax. The latter 
 three keep the vessel air tight. The potential of this cell is 
 1,0186 volts at 20 
 
 Conductivity measurements were made by the standard method 
 of Kohlrausch. In Fig. 3 the diagram of connections is given. 
 
 S is the Slide v;ire bridge, R is the known resistance, T the 
 telephone receiver. The induction coil gives the necessary 
 alternating current for the measurements. The coil is actuated 
 by the storage battery B. 
 

8 
 
 Figure 3 . 
 
9 
 
 The conductivity vessel used is of the type shov/n in Fig* 4* 
 The two plates P are circular platinum plates which have been 
 coated with platinum black* This is done electrolyticaiiy from a 
 platinizing solution, by passing a current through the solution 
 for ten minutes, its direction being reversed every half minute. 
 The electrodes are then thoroughly washed with dilute sulphuric 
 acid and passing a current through the solution for 15 minutes 
 reversing the current every minute. This removes the last traces 
 of piatanizing liquia and also any occluaea gases. 
 
 The sLanaard electrode vessel was of the type shown in Fig. 5* 
 At the bottom was a sma-ll amount of mercury (1) and (2) was 
 calomel paste in one tenth normal potassium chloride and the 
 liquid above was one tenth potassium chloride saturated with 
 calomel. The funnel shaped reversoir above the tv7o way stop-cock 
 contained some of the tenth normal pots-ssium chloride to flush 
 the side arm of any liquid which may have diffusdd up. The 
 potential of this electrode is C.536C volts. For the non-acqueous 
 solutions the same type cell wasused except that the calomel 
 paste was mixed With lithium' chloride and the solution above the 
 paste was absolute alcohol saturated with lithium chloride. The 
 potential of this cell was C.11S5 volts as compared to the tenth 
 normal calomel electrode. 
 
 For determining the potential of metals the electrode vessel 
 as shown in Fig. 6 wasused. The metal for which the electrode 
 potential wasto be measured waspiaced in the bottom and contact 
 was made by a seaied-in-giass platinum wire. The two half cells 
 are connected by immersing the side arms in a saturated solution 
 of potassium chloride. The saturated solution eliminates the 
 

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10 
 
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3 
 
 1 1 
 
 Fig. 7. 
 
tn 
 
12 
 
 liquid junction potential. 
 
 For standardizing the silver-silver chloride electrode the 
 dipping hydrogen electrode was used. Hydrogen is passed into the 
 electrode at A and passes down the tube into the solution at the 
 bottom of the vessel, saturating the solution around the platini 
 izfed platinum electrode P with hydrogen ions. The excess gas 
 escapes at B. T/^Taen measurements are taken the flow of hydrogen 
 is by-passed into the air and the vessel closed at B, the hydrogen 
 in the vessel therefore being at atmospheric pressure. 
 
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13 
 
 Preparation of Materials. 
 
 Finely divided zinc v/as prepared according to the method 
 used by Horsch( Jour. Am. Ghem. Soc . £1,1787, l9l9)* The metal 
 was prepared by a rapid electrolysis of a concentrated solution 
 of zinc chloride. A rod of the pure metal served as the anode 
 and a platinum wire of about number 18 B&S gauge served as the 
 cathode. A current of about 0.3 ampere was used. Varying the 
 current density had no effect except that at very high densities 
 the metal became so spongy aa to pack together and prevent proper 
 washing. Y/hen a sufficiently large amount had been obtained, the 
 metal was removed and thoroughly washed. The zinc was kept under 
 zinc chloride solution of the concentration in which the electro- 
 motive force measurements v/ere to be made. After standing for some 
 time a small amount of the zinc reacted with the solution to 
 form zinc hydroxide. This hydrolysis did not affect the utility 
 of the zinc. After washing the metal with dilute hydrochloric 
 acid and then with distilled watery it was again possible to 
 use the zinc. 
 
 The zinc chloride stock solution v/as prepared by dissolving 
 pure zinc chloride in distilled water. Considerable hydrolysis 
 occured. The zinc hydroxide was filtered off and the concentration 
 of the clear solution then determined by the Volhard chloride 
 method. The more dilute solutions were prepared from this stock 
 Solution. 
 
 The Silver-Silver chloride electrode was prepared by first 
 plating a sealed- in-glass platinum gauze with silver from a 
 silver cyanide solution and then the silver chloride formed as 
 
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14 
 
 a dark deposit by using the silver as the anode in a dilute 
 Sodium chloride solution to which a small amount of hydrochloric 
 acid had been added. This electrode was standardized against 
 hydrogen in one hundredth normal hydrochloric acid and found to 
 have a value of C.45b3 volts, this reading being the average of 
 a number of determinations . 
 
 The anhydrous manganese chloride was prepared by tv^o different 
 methods. The first sample was prepared by passing hydrochloric 
 acid gas over heated hydrous manganese chloride. The hydrochloric 
 acid gas was prepared by treating pure sodium chloride with pure 
 concentrated sulphuric acid and then thoroughly dried by passing 
 the gas through concentrated sulphuric acid and calcium chloride. 
 This did not eliminate any impurities which may have been in the 
 original salt so the anhydrous salt was prepared by direct union 
 of the elements. The manganese was finel^r divided by crushing. 
 Chlorine gas was prepared by oxidizing concentrated hydrochloric 
 acid with potassium permanganate and washing tv/ice with water 
 to remove any hydrochloric acid gas and then drying by passing 
 through concentrated sulphuric acid calcium chloride. The man- 
 ganese was placed at one end of a long tube and strongly heated. 
 
 All air was first removed by passing chlorine through the tube 
 for Some time. A strong current of chlorine was kept flowing 
 over the heated manganese. The manganese chloride sublimed at the 
 other end of the tube and then collected. Manganese chloride of 
 veryhigh purity was obtained by this method. 
 
 Absolute alcohol was prepared by redistilling 95 % ethyl 
 alcohol over pure cs-lcium oxide. The conductivity of the absolute 
 
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15 
 
 alcohol was 2.151 x lo" 
 
 Manganese chloride stock solution in absolute alcohol was 
 prepared by weighing out an accurate amount of the anhydrous 
 salt and dissolving it in the alcohol- The concentration of this 
 solution was accurately determined by the silver chloride precip- 
 itation method. The more dilute solutions were prepared from 
 this stock Solution. 
 
 The metal manganese was prepared by electrolyzing a concen- 
 trated Solution of manganese chloride in either water or absolute 
 betv/een platinum electrodes. A hard coating of the metal formed 
 on the cathode v/hich was then thoroughly washed and dried. Due 
 to the great reactivity of the pure metal it had to he prepared 
 immediately preceding electrode measurements. 
 

 
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16 
 
 Results . 
 
 In measuring the electrode potential of zinc against the 
 Silver-Silver chloride electrode, thepure finely divided metai 
 was used. The observed voltages of the cell Zn, ZnClg, AgCl, Ag 
 in the concentration noted are given belov/. The values given 
 are the results of determinations made on different samples 
 prepared under different conditions. The concentration of the 
 zinc chloride solution was one hundredth normal. 
 
 Sample 1 
 
 2 
 
 3 
 
 4 
 
 5 
 
 6 
 
 1 . 1628 
 
 1.1580 
 
 1 . I600 
 
 1 . I605 
 
 1 . 1649 
 
 1.1563 
 
 t . 1629 
 
 1.1550 
 
 1 . 1610 
 
 1 . 1607 
 
 1 .1647 
 
 1 . 1572 
 
 1 . 1620 
 
 1 . 158c 
 
 1 . 16 lo 
 
 1 . I605 
 
 1 . 1652 
 
 1 .1580 
 
 The results are 
 
 typical 
 
 of the 
 
 electrode 
 
 potential 
 
 of thii 
 
 cell inasmuch as they agree even closer than some data that were 
 previously obtained. While only the values for one concentration 
 are given here, this gives an indication of the lack of constancy 
 between different' ceils of the same concentration • The results 
 did not warrant the adoption of this method for a, class room 
 experiment in preference to the amalgamated zinc method. 
 
 In taking up the work of determining the electrode potential 
 of manganese in absolute alcohol it first became necessary to 
 find the degree of ionization of manganese chloride. The degree 
 of ioniza.tion is determined by making conductivity measurements 
 of several solutions of various concentrations. In this way A 
 may be detemined by extrapolation. The data given represents 
 the average of a number of determinations . 
 
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 • Zj- : o . X ‘*-i) io .1 lo 
 

 18 
 
 
 
 c 
 
 
 A 
 
 (X 
 
 0 . 1 
 
 0.695 
 
 4.80 
 
 0.053 
 
 0.01 
 
 0 . :^- 4 o 
 
 1 1 . 6 c 
 
 0.129 
 
 o.ool 
 
 0.173 
 
 30.20 
 
 0.335 
 
 By extrapolating A-^owas found to be 90*0 
 
 It v/as very evident from the start that potential measure- 
 ments of manganese in acqueous could not be made. Short circuiting 
 several manganese electrodes in a manganese chloride solution 
 showed the reactivity of the metal, the latter dissolving readily 
 in the acqueous solution. 
 
 The next attempt in measuring the electrode potential was 
 by means of solutions of manganese chloride in absolute alcohol. 
 Manganese electrolyzed from an acqueous solution was made up 
 and measurements attempted. This metal was very reactive with 
 alcoholic solution* so v/as dropped. Manganese prepared by 
 electrolyzing it from a dry solution of manganese chloride in 
 absolute alcohol v?as next tried. Not a.s good a deposit was 
 obtained but the reactivity of the metal v/asso small that the 
 fact was neglected and electromotive force measurements made. 
 
 The data obtained quickly showed that even this slight reaction 
 had a very decided effect. The electrode potential quickly dropped 
 from the time the metal and solution were brought in contact. 
 
 The readings were taken as soon as possible after the cell had 
 been set up in order to obtain the maximum value of the electrode 
 potential. The values for the system, 
 
 L'In, cn IvInCl 2 aic.,sat.KCl ale. LiClsat,Kg 2 Cl 2 alc. Kg 
 
 are given belov;. 
 
' T T . i ' . ■ 
 
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 ■ ’ ; ‘;.".'-ivi X' . a* 
 
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 Co.: r.- y,0‘ri ^ . -.J:'" fc* ^ 
 
 ;>.u iync;.\ rl-.Tf' v , h -• 
 
 ■•'* ■ - ^ ^ -3 ■*. ... .; „ J'-'X*^ 
 
 0'3 ,.SB Jv ij V .'vX .:.'jjl..:j ifiJtiivntiMi 
 
 xb . ..t ;x.:, ::X*i!iaq> 
 
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 oii7 
 
 
 
 
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19 
 
 0.1 n LlnCl^ 
 
 0.9815 
 
 C.9794 
 
 0 . 0 I n I^nClg 
 
 0.9520 
 
 0.9380 
 
 0.9653 
 
 0.9280 
 
 0.9142 
 
 0.9504 
 
 0.9363 
 
 0.8974 
 
 0.8758 
 
 0 . 86 16 
 
 0.7770 
 
 0.8423 
 
 It will be noted that the maximum value for the tenth normal 
 solution is 0.9815 volts and for th& one hundredth normal 0.9520 
 volts. These maximum values are reproducible and v/ere obtained 
 with each nev7 attempt* The difference in potential between the 
 
 two solutions 0.0295 volts is the theoretically correct difference 
 for a tenfold change in concentration of a bivalent ion and 
 evidently shows that electrode potential measurements are possible 
 in non-acqueous solutions for manganese. It is very evident 
 though that absolute alcohol cannot be used due to its reactivity 
 with the pure electrolytic metai . 
 
 It seems that the reactivity is due to the OH radical as there 
 is quite an evolution of gas similar to that obtained in acqueous 
 Solutions. Other organic solvents as acetone, ethyl amine, propyl 
 amine have been used in electrode potential measurements and show 
 promise of giving more conste.nt results. The necessity of finding 
 a more inert solvent is very evident. Due to a lack of time it was 
 possible to try out any organic solvents. 
 
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20 
 
 ACKNOVJLEDGMENT . 
 
 The writer wishes to take this opportunity to thank 
 Dr. Gr. Dietrichson under whose direction this work v^as carried 
 on for his valuable suggestions and criticisms and for the 
 personal interest he has taken in this work.