of gelatin - American Chemical Society

T H E ELECTRO-DEPOSITION OF COPPER I N T H E P R E S E S C E

OF GELATIN’,’ B Y ROBERT TAFT A N D H.4ROLD E. MESSMORE

A number of investigations upon the deposition of copper in the presence of gelatin have been made; a bibliography of many of these investigations, together with others bearing upon the topic of addition agents in general, will be found in the excellent paper of Frolich3 published in 1924. Since that time the important papers of Fuseya and his students4 and of Marie and Buffatj and Marie and ClaudeP have been published. While the beneficial effect of gelatin (or glue) in copper plating baths has long been known and utilized, the explanation of the process by which gelatin produces its effects has never been satisfactorily given. Frolich has advanced the view that in plating baths of copper sulfate possessing a lower p H than 4.7 (isoelectric point of gelatin) any added gelatin will be present in the bath as particles having a positive charge. Upon the passage of an electric current these gelatin particles tend to move toward the cathode and unless removed, will tend to accumulate around this electrode. Removal of gelatin does occur, however, as it has been shown by Marie and Buffat, and others, that the cathode deposit from baths containing gelatin and copper sulfate is greater in mass than that obtained under otherwise similar conditions from baths free of gelatin. Moreover, Marie and Buffat have shown that in a t least one instance, the major portion of this difference in mass between two such cathode deposits is actually gelatin. There seems to be sufficient evidence for Frolich’s explanation of the advance of gelatin toward the cathode to consider it the true explanation of this process, but, this, of course, is a phenomenon entirely distinct from the actual electrode processes. It is in connection then, with the cathode processes that the uncertainty exists, i.e. there is no accepted explanation of the mechanism whereby gelatin finds its way into the deposit. This problem, for addition agents in general, has been reviewed by Blum’ who suggests the following possibilities to account for those cases in which the Presented a t the Indianapolis meeting of the American Chemical Society,’April I , 1931, Constructed, in part, from a thesis presented by Harold E. Messmore to the Faculty of the Graduate School of the University of Kansas in partial fulfillment of the requirements for the degree of Doctor of Philosophy, May, 1930. Trans. Am. Elec. SOC.,46, 67 (1924). Fuseya and Murata: Trans. Am. Elec. Soc., 50, 2 3 j (1926) (An extensive bibliography is also given in this paper;) Fuseya and Kagano: 52, 249 (1927). These papers do not deal specifically with the deposition of copper in the presence of gelatin hut are listed here as bearing upon the general topic of the deposition of copper in the presence of addition agents. J. Chim. phgs., 24, 470 (1927). Compt. rend., 187, 170 (1926). Colloid Symposium Monograph, 5 , 300 (1926)

2 j86

ROBERT TAFT ASD HAROLD E. MESSMORE

addition agent has actually been found in the cathode deposit?. ‘ f i r ) Codischarge of colloid particles antl metal ions, ( 2 j diwhsrge of coniplrx ions containing the metal an(! the colloid, (3) adsorption of colloid upon the face of the metal deposit, or (4) mechanical inclusion in the deposit.” Uluiii further states in regard t o these various possibilities that “it is difficult froin the ineager data available t o suggest the relative probabilities of thesc processes. . . .” I h w v e r , other workers h a v expressed ~ preferencr for one or thc other of the particular views outlined above. Thus Hancroftl t:icitly wmiii’s that adsorption accounts for the inclusion of the addition agent in the rlepoGit; Fuseya and l f u r a t a po.stulatc the existence of comples cations forinetl from the metal ion and addition agent and produce evidence to show that snch complex ions :ire doubtless formed. They further assiinie, without producing esperiniental evidence however, that the complex ions are tiischnrgeti :it thc. cathode and become thercby a portion of the deposit. 3lnrir antl Butlfxt have shown that the m a s of the cathode deposit incrc:iws to a liinitirig inasiniriin value \Yith increasing content of addition :igent in tlii: c:ihe of tlic particular cell, C‘u/ (‘uPOr, (kl:itin,’Cii. In fnct, if the ni is plotted against the gelatin content of the solution, the resulting curve resembles in form those typical of adsorption phenomena. lye have confirrnetl this observation of lIarie and Ruffat under ~ o i r i ( ~ hniore a t vxicd conditions than have these investigzitors.? It would sppcar that this evidence would support the third vien as t o the inc~ch:misniof the c:cthotle process as outlined above by Bluni. In order t o obtain further evidenci. for one rir the other of Hlum’s postulate* we have examined the variation of the m of the cathode deposit froin cells of the type Cu!CuSO4, Ge1;itin ‘(,‘ii as functions of gelatin content, copper sulfate content, current clrmsity, tinit’, and temperature.? I n addition we have determined the form of tlic cnthotle cleposit a.7 n function of the gelatin content of such cells, arid have 1iken.ise studied the influencci of gelatin upon the magnitude of the cathode polarizntioii. lye \vi11 coiisid(sr below our experimental results in the order (A) the influence of gelatin upon the forin of the deposit; (B) the effect of gi’latin upon the weight of the cathode deposit; (C) the influence of gelatin 11pon the polnrization at tlic cathode during d p positon of copper. 11-e will then discus these results in the light of the four possibilities suggestd h y 13luni. A . The Influence of Gelatin upon the Form of the Cathode Deposit After numerous preliminary trials, some of which have already heen reported in our previous paper, the following procedure was adopted to study the effect of varying the conccritration of gelatin upon the form of the deposit. -~

Trans. Am. Electrochem. Soc., 23, 266 (191,3). Taft and Messmore: Trnns. I i m . .-1c,n(I. Science. 32, .jz (1929). We recognize, of course, that :my evidence for one of the views which may he ohtaineii from the studv of n pnrticuhr tvlie of eiectrolvtic cpll nerd not, of neceesitv, lead t o the proper esplanntion of the functio;i of d/:iiiditio;i :igiPnts. Considering the est;.emelv v:iried chkaracter of the addition agents uard i n suc.11 cells i t is unlikely thnt the same expianation \vi11 hold lor nll CRSCS of :iddition agent nction. .\nv conclusions which u-e may r e x h can hold only for the cells which \ye here consider until more general evidence is avhilahle.

Flu. I Form 01 dapoait ns function 01 variety o i gelntin (XI). Up m i row thrrty m i n i m a eiectioiysis; towor row ninety minutes. L e f t to riJt Eetiitinr are

Difcu, Knon, irsh free, Coignet. Electrolyris n t gelatin content of 1 1 ~and 5 ~1 M

gC, I uS0,.

nmpldmz with

Fig. I shows the typicd result oi such n series of electrolysis in still baths with grlstin content ranging From 0.02; per ernt to 3 . 0 per cent. l'h carried out in a constant temperaturt' batli :mtonr;itic:rlly k q t t at 30' current density of i ampere per square deeinrc%erior thirty minutes. The concentration of copper S U l f R t C was the same in dl sixtcrn cells contnining gelatin, namely i i m h . Tlie coulometer deposits were obt:tind imni the familiar bath (Oettel'sj contnining sulfuric acid uliil ttleohol in nddition to tho copper sulfait,. r _

l h e irnpor!:ur! fenlures to be ub~scrwdin eooncctim with the cicposits in tliosc solutions rontaining up to 0 . 1j per cent gr1:itin arc tiis niiiiii:rous small circular deposits of coppw. Thest! stand tibow blie I< 1 ui the bm? metal and are rlarkcr colorrd than the rcrnniniiig cappcr. A continuous d e p s i t oi copprr is ohsrrv~nhlc,I r, betwmn these raised :ire&-. Fwilrer it will be noted that the ituiiilx oints" of deposit dirrrinisbcs wit,h increasing eonwntrat,ion of gdatin. 7:p to this conecntration of &itin ( 0 . 1 5 per c m t j there is no definite nlignrncnt of these raisrd areas but :is tiic conoentrdivn bcyond this vnluc., definitr striations appenr. 'These

ELELTHO-DEPOSITION OF COPPEH IN P n E s m c E OF e experiments upon the fundamental questions involved will be considered in connrction with the final discusion of our results.

B. The Effect on Gelatin upon the Weight of the Cathode Deposit The procedure for determining the weight of the cathode deposit was somewhat sirnilnr to that described under Part A. A number of cells were .set up in series; the cells consisted of 1 5 0 cc. pyres beakers, the cathode was usually of sheet platinum 2 . 5 X 2 . j em. and the anode of heavy sheet copper carefully cleaned and plated electrolytically before use. The electrolyte was the stock solution of copper sulfate described above hut the gelatin used was an ash-free product, obtained from the Eastnian Kodak Co.* The same procedure for getting the gelatin into solution mas used as we have described above. We have in the majority of cases, determined the pH of the solutions used in the experiments reported in sections 13 and C. S o t that m y great significance is to be placed upon the meaning of the pH of these solutions but chiefly because the pH measurements furnkh :in easily determinable “constant” or property of the systems which we have used; the work of previous investigators has frequently been criticized for failure to state specifically the properties of the solutions employed. Furthermore as the direction of migration of gelatin particle, or ion, is also determined by the pH value of the solution, it is essential if uniformity in the direction of this movement is to be secured in every c n e , that the pH he lower than 4.7. I t will be observed by the inspection of the dntn to follon that the pH in every instance was considerably lower than thi. v:iluc. Our pH valurb were determined by means of the quinhydrone electrode and a saturated calomel electrode at zs0C. and were computed from the relation, 0.6992-3.2j&E, pH = where 0.6992 is the potential of the normal quino.oj91

hydronc electrode, 0.2458 that, of the saturated calomel electrode, and O . O j 9 1 the value of the thermodynamic function In RT/SF, all at z j”C. E, is the measured potential of the cell comprised of the quinhydrone and the saturated calomel electrodes. As O’Sullivan’ has shown that there is a slow drift in the potential of such cells with time, the measurements were made within a few minutes after the quinhydrone had been added to the solutions kept at zj°C. Our results concerning the weight of the cathode deposit are given in terms of “excess weight.” By excess weight we mean the difference in weight beThis material contained approximately 1 2 ‘ ;moisture upon drying to constant weight at 100°C. All of our concentrations of geiatin are given, however, i n terms of the undried product and are nccordingly I Z ~ ;too high. Trans. Faraday Soc., 23, 52 (1927:.

ELECTRO-DEPOSITION OF COPPER ISPRESENCE OF GELATIN

2 591

tween that of the cathode deposit in a particular cell and that produced in a copper coulometer bath kept in ice and stirred with natural gas and in series with the cell in question. The copper coulometer bath consisted of a solution of copper sulfate, sulfuric acid and alcohol in the proportions recommended by Oettel.‘ Our reasons for choosing the copper coulometer as the reference standard rather than a cell containing copper sulfate alone, are these: (a) it more nearly indicates the actual quantity of electricity flowing through the circuit; (b) it gives one standard rather than the several which would be obtained by using a cell containing only copper sulfate. By the use of copper sulfate alone as electrolyte the mass of the deposit obtained depends upon the concentration and temperature employed as is already well known; (c) as the result of every extensive experience with deposits from baths of copper sulfate alone we have found that duplicate determinations differ to some extent among themselves. This we ascribed to oxidation of the copper cathodes in the drying process, as despite all precautions the surfaces of the electrodes obtained from such baths darkened in the course of drying.* Such darkening did not occur in the case of deposits obtained from acid baths or from those containing gelatin. In the case of the acid baths the deposit is evidently more compact, thus not offering such a large surface to the atmosphere as deposits from “neutral” baths; in the case of the third type of cell, the gelatin evidently protects the copper by its presence in the deposit. In order to confirm these suppositions we have determined the magnitude of the deposits in baths containing copper sulfate alone as compared to the

TABLE I Coulometers kept at o°C. Copper sulfate cells a t 3ooC. Current density 2 amp/dm2. Deposit, grams

Coulometers

.4851 1.48j2 I . 4856 I

I.

2.

3.

Av. Copper sulfate cells Conc. (molarity)

Gain over average coulometer deposit, milligrams.

PH

4.

1.25

j.

1.00

6. 7. 8.

0.50

3.11 3.26 3.37 3.55

0.25

3.77

0.75

1.48j3

1.486; 1.4874 I . 4878 I . 4891 I . 4878

1.2 2.1 2.5

3.8 1.5

Chem. Ztg., 17, 543 (18g3).) * The lack of reproducibility is also doubtless due, in part, to adsorption of cuprous and cupric hydroxides as the data, shortly to he presented, indicate.

ROBERT TAFT ASD HAROLD E . YESSNORE

2592

coulometer deposits, obtained as above described, by using a series of cells of various concentration of copper sulfate, electrolysis being continued for a long period of time (9 to I O hours) in order to minimize the relative effects of oxidation upon drying. Results were obtained as shown in Table I. The excess weights in the case of copper sulfate cells 4, 5 , 6, are apparently due to surface oxidation occurring in drying as an analysis for copper (by a method to be described) in these deposits gave results as shown in Table 11.

TABLE 11 Cell

Deposits, grams

Copper upon analysis, grams

4.

I . 4865

I . 4848

-0.

5.

,4874 I . 4878 I . 4891 1.48j8

1.4849 1.4854 1.4881 1.4845

-0.4 +o. I

6. 7.

8.

I

Difference, milligrams j

+2.8

-0.9

The “Difference” tabulated in Table I1 is t,hat between the mass of t’he copper found upon analysis and the average ralue of the coulometer deposit. The magnitude of these differences for cells 4, 5 , and 6, are of the same order as the experimental error and hence we have concluded that the actual mass of copper deposited in the cells is, wit,hin our experimental error, t,he same. In cell 7 the increased copper content of the cathode is likely due to adsorbed cuprous oxide as Richards, Collins, and Heimrodl have shown that deposits from more dilute copper sulfate solutions are heavy for this reason. In the case of cell 8 the low copper content is likely due to the fact that at or near this concentration of copper sulfate, hydrogen liberation sets in. This fact combined with the greater hydrolysis of cuprous sulfate in a dilute solution and the possibility of cupric hydroxide formation and adsorption as a result of hydrogen liberation could account for a deposit greater in mass than the coulometer deposit but of smaller copper content. The maximum weight due to oxide or hydroxide in these eases is 4 milligrams; the maximum excess weights in the presence of gelatin for a similar quantity of electricity would be in the neighborhood of ninety-five milligrams (cf Table VIII). That is, t h e excess weights obtained in the presence of gelatin can scarcely be due to oxides or hydroxides of copper. T o further eonfirm this fact we have in a large number of instances analyzed the deposits for copper and have found that the major portion of this excess weight must be due to gelatin itself. Marie and Buffat have directly determined this fact for several cases by analysis for nitrogen upon large deposits (4 to j grams) of copper obtained in the presence of gelatin. We have determined the variation of this excess weight as functions of (a) the gelatin conteni the bath, of (b) the current density, of (c) the time and of (d) the temper: i e . R o c . -4m.Acad. -Arts Sci., 35, 123 (1899).

ELECTRO-DEPOSITION O F COPPER IS PRESENCE O F GELATIN

2593

Excess Weight as a Function of Current Density and Concentration of Gelatin Four sets of electrolyses were carried out in 0.5 M copper sulfate solutions a t five concentrations of gelatin and a t three current densities. The first series consisted of ten ce!ls containing five concentrations of gelatin in duplicate and two coulometers. I n this set the platinum cathodes were approsimately 2 . 5 x 2 . 5 cm. in cross-section, a current density of 0.5 amperes per square decimeter (.03 15 amperes) being employed for ten hours. The results obtained are given in Table I11 and graphically in Fig. 4 (lower curve). In this table there are tabulated the deposit obtained, the excess weight as

f L R 'CCN7 G h L R T l N

FIG.4

TABLE 111 Series

I .-0. j

amperes per square decimeter (0.03 I j amperes) ; weights in milligrams. Temperature 30OC.

Conc. gelatin

0.5%

2% I

Deposit Excess weight Copper Excess copper

2

389.9 389.7 11.4 11.2 380.3 380.0 1.8 1.5

I


2

386.j 8.0 379.5

386.4 7.9 379.6

1.0

1 . 1

2

387.8 388.4 9.3 9.9 379.5 379.3 1.0 0.8

387.2 389.6 8.7 11.1 379.5 380.1 1.0 1.6

0.03%

Coulometers

0.12yo I

0.25%

I

2

I

2

3 8 2 . 9 383.6 4.4 5.1 379.9 3 7 9 . 8 1.4 1.3

I

2

378.3

378.7

Av.

378.5

ROBERT TAFT ASD HAROLD E. MESSMORE

2594

previously defined, the copper present in the cathode deposit found as a result of analysis, and the excess copper. This last quantity is the difference between the copper found upon analysis of the cathode deposit and the average coulometer deposit. The “copper” tabulated above in this and subsequent determinations was found as follows: the platinum cathode bearing the deposit was placed in a 400 cc. beaker and 40 to 50 cc. of dilute nitric acid was added slowly enough to prevent any loss of copper by volatilization. The platinum cathode was washed and removed. Enough sulfuric acid was then added to convert the copper present to the sulfate and the solution slowly evaporated to dryness. When dry the beaker was held over the open flame of a Meeker burner to remove the excess sulfuric acid and to destroy any remaining gelatin. When cooled the copper sulfate was taken up in a little water and I cc. of nitric acid was added. Ammonium hydroxide was then added until neutral and a slight excess added. The ammoniacal solution was then neutralized with a small amount of nitric and sulfuric acids and diluted to 150 to 2 0 0 cc. The solution was then electrolyzed between platinum electrodes. The cathodes were light cylinders (3-4 grams) of platinum gauze and were rotated a t about 300 revolutions per minute. Electrolysis was continued until a few drops of the solution gave no brown or black deposit with water saturated with hydrogen sulfide. This method of analysis x a s developed after many trials when other procedures had failed to give satisfactory results. The following results obtained by analysis of coulometer deposits give some idea of the accuracy of the procedure. Coulometer deposit, gms.

0.3740 0.3878

Copper upon analysis, gms.

0.3740 0.3879 0.3797 0.3788

An examination of the data of Table I11 and of Fig. 4, shows that the excess weight increases to a maximum with increasing concentration of gelatin. The greatest variation in the duplicate determinations occurs in the gelatin cell. Judging from the remaining data 8.7 is more case of the 0.~57~ nearly correct than I 1.1 and it is that value that is used in plotting the results in Fig. 4 (lower curve). Large variations of this type were frequently encountered and are difficult to explain. A possible explanation will be given later. It should be noted that the excess copper is approximately the same in all cases. In the second set of electrolyses exactly similar conditions were used as described above save that platinum cathodes 1.77 cm. on an edge were used, this giving one half the area and therefore twice the current density. The data for this set of experiments are given in Table IV.

ELECTRO-DEPOSITION O F COPPER IN PRESENCE O F GELATIN

2595

TABLE IV Series z.-One

ampere per square decimeter (0.03 15 amperes); weights in milligrams. Temperature 30°C.

Conc. of Gelatin

Deposits Excess weight Copper Excess copper

0.5%

2%

I

2

3 9 2 . 8 394.2 13.2 14.6 3 7 9 . 9 379.6 0.3 0.0

1

391.9 12.3 380.0 0.4

0.12% I

Deposits Excess weight Copper Excess copper

0.25% 2

392.2 12.6 379.8 0.2

389.4 389.5 9.8 9.9 3 7 9 . 9 379.6 0.3 0.0

1

383.8 4.2 380.3 0.7

2

3 9 1 . 9 392.2 12.3 12.6 3 7 9 . 8 379.6 0.2

0.0

Coulometers

0.03% 2

1

2

383.9 4.3 379.8 0.8

1

2

379.5

379.7

379.8

379.6

Av.

379.6

/7

/6

The values of the excess weight a t the higher current density are somewhat greater than a t the lower. This is brought out more clearly in Fig. 4. It will be noticed again that the excess copper is practically the same in each case, although somewhat lower t h a n in the first series. The third and fourth series were similar to the first and second, respectively, save that a current of 0.0625 amperes was employed for approximately five hours. This would give a current density of I ampere per square decimeter in the third series and of 2 amperes per square decimeter in the fourth. The data are given in Tables V and VI, and graphically in Fig. 5.

2596

ROBERT TAFT AND HAROLD E. NESSMORE

TABLE T' Series 3 .--One

ampere per square decimeter, (0.0625 amperes) ; weights in milligrams. Temperature 30°C.

Conc. of Gelatin

2 5 I


-

402.0

14.I 388.9

-

1.0

2

I

401.2 401.0 13.3 1 3 . 1 389.2 389,4 1.3

1.j

2

398.8 398.9 I O .9 I I .o 389.7 389.5 1.6 1.8

I

2

392.9 393.0 j.0

358.9

I

2

401.3 400.2 13.4 1 2 . 3 389.j 389.2 1.6 1.3 Coulometer3

0.03'';

0.12';

I


o.2jrc

0.jr;

2

I

Av.

2

387.8 388.0 387.9

j.1

389.0

1.0

387.9 387.4

1.1

TABLE TI Series 4.-Two Conc. of Gelatin

amperes per square decimeter (0.062j amperes) weights in milligrams. Temperature 3ooC. 2% 0.5i'; 0.2,jC; I

Deposit Excessweight Copper Excess copper

2

406.8 406.4 18.2 17.8 390.2 389.9 1.6 1.3

I

1j.4


400.2 I I.6

400.2

I I .h 1.2

I

I

2

304.6 4 0 4 . 4 16.0 15.8 389.9 389.8 1.3

1.2

Coulometers

0.037 2

389.7 3 8 9 . 8 1.1

404.3 15.7

389.5 3 8 9 . 9 0.9 1.3

0.1zc;

I

2

404.0

2

I

Av.

2

393.3 393.4

388.7 388.6 388.6

4.8 388.; 389.4 0.1 0.8

388.3 388.3

4.7

Our reaion for running vrics t w o and thrre a t the u n i e current density but a t different currentk, n n s t o determine if the rate of deposition had an3 material effect upon the magnitude of the excess neight The data of Tables IV and V are not exactly comparable as the coulonieter deposits are not the same but if the avcrage excess neights of Table Is' are multiplied by the 388 o ratio of the coulometer depoqits, __ aq hiLq been done in Table VII, the 379 6 results can be compared The fact that the excess weight5 are slightly greater for the larger current would indicate that the process of taking up gelatin by the electrode must be a n instantaneous one, for if time Rere involved the excess meight should be

ELECTRO-DEPOSITION O F COPPER IN PRESEXCE O F GELATIN

2597

TABLE VI1 Conc. of Gelatin

Excess weights a t I ampere per square decimeter. 0.031j amperes, average 0.062 j amperes, average values values

2 .‘;c 0.55

I4.I*

12.8

13.2

0.2jy;

12.8

12.9

0 .12yc

10.0

11.0

4.3

5.0

0.037~

* Based

14.2

on a single value (See Table V).

90 80 70

? a 60

i $

5

$@

2

3 so Y 2 20 10

0011 0.06

I I I I I I I I I 1 I I

I I I I I I I I I I I I I I

050

2.0 P C R CEnr

GcLArm

FIG.6

greater for the slower rate of deposition. The slight increase a t the greater current, if r e d , is evidently due to the increased amount of gelatin brought in contact with the electrode by the greater current. To obtain further information upon the magnitude of the excess weight we have made determinations of this quantity a t various concentrations of copper sulfate but a t a constant current density of two amperes per square decimeter, i.e. at the current density where the effect is greatest. In the data of Tables I11 to T’I we were not satisfied with the determinations of excess copper. The differences between duplicate determinations were, in several instances, of the same order of magnitudes as the individual quantities themselves. I n order to increase the relative precision in the determination of this quantity, the deposits in the series of runs about to be described were made some three times larger. The results obtained are given in Table VI1 and the excess weight as a function of gelatin content is shown graphically in Fig. 6.

ROBERT TAFT AND HAROLD E. MESSMORE

2598

TABLE VI11 Current density, 2 amperes per square decimeter. * Temperature 3ooC. Excess weights as functions of copper sulfate and gelatin concentration. Per Cent Gelatin

1.25

M . C U S O ~pH ,

Deposit (grams) Excess weight (mgm) Copper (upon analysis) Excess copper (mgm)

Current Density, z amp./dm* 2.0 0.5

3.18

-

-

3 ' I4 I . 2958 49.5 I .2524

6.1

0.06

3.11 I . 2632 16.9 I . 2503 4.0

1.00&.I.C U S O ~pH , Deposit (grams) Excess weight (mgm) Copper (upon analysis) Excess copper (mgm)

3.28 I ,3181 71.8 I . 2 j46 8.3

3.27 I . 2950 48.7

0.75 M . C U S O ~pH , Deposit (grams) Excess weight (mgm) Copper (upon analysis) Excess copper (mgm)

3.44 1.3091 62.8

3.41 I . 2891 42.9 1.2506 4.3

3.38 I . 2671 20.8

0.50 M. C U S O ~pH , Deposit (grams) Excess weight (mgm) Copper (upon analysis) Excess copper (mgm)

3.60 I . 3 106 64.3 1.2537 7.9

3.58 I , 2922 45.9

3.56 I . 2694 23. I I , 2483

M. C U S O ~pH , Deposit (grams) Excess weight (mgm) Copper (upon analysis) Excess copper (mgm)

3.84 1.3871 80.8

3.82 1.3126 66.3 1,2534 7.'

0.25

Coulometer deposits (grams) Copper (upon analysis)

1.2542

7.9

1.2544

8. I

I . 2509

4.6

I ,2 5 0 1

3.8

1,2459

I . 2464

1.24jj

I , 2464

3.25 I . 2657 19.4 1.2495 3.2

1.2501

3.8

2.0

3.80 1.2749 28.6 I.2jOj

4.5

I . 2464 = Av. I . 2456

I.

2462

* During the last half hour of electrolvsis (total time approximately I O hours) the soluI. gelatinized. This increased the resistance tion containing 2 % gelatin and 1.25 >"CUSOI somewhat and as a result the current density fell. It did not fall below 1.5 amperes per square decimeter, however. It is for this reason t h a t no excess weight is recorded for this cell.

ELECTRO-DEPOSITION O F COPPER IN PRESENCE O F GELATIN

2 599

The two important features of these observations show that first, for all concentrations of copper sulfate employed a limiting v d u e of excess weight is reached, the limiting value being greatest in the most dilute solutions and second, that the excess copper increases with gelatin content, although a t a decreasing rate with increasing gelatin content. The form of the curves in Fig. 6 are somewhat more rounded than in Figs. 4 and 5. This variation is likely not a real one but is due to the fact that the curves of Fig. 6 are based upon three experimental points, whereas those of the previous figures are based upon five. A discussion of the bearing of this data upon our problem will again be postponed until all of our data have been presented.

Excess Weight and Time From our data we have calculated the dependence of the mass of the cathode deposit upon time. If the entire mass of the deposit arises from electrolytic causes then the mass of the deposit a t a constant current should be proportional to time. Comparisons of deposits to coulometer deposits at a given current and current density show that the two are roughly proportional over small differences of coulometer deposits. For greater differences of coulometer deposits, the mass of the deposit depends upon several factors including concentration of gelatin, concentration of copper sulfate, temperature, etc. The data of the following table is a specific example.

TABLE IX I

&ICuSO4 at Series

Cone. of Gelatin o.% (Coulometer deposit)

Deposit

z.%

0.5% 0.06%

z

amp/dm2, 3oOC. Series

I

Excess Cu.

0.3684 gms. 0.3846 0 . 7 mgm 0.3847 1.7 0.3740 1.9

Deposit, Calculated I .3010

1.3013 1.2651

Deposit

2

Excess Cu.

1.2462 gms. 1.3181 8 . 3 mgm I , 2950 4.6 I ,2657 3.2 Calculated Excess Cu. z .36

mgm

5.75 6.43

Vnder each coulometer deposit is given the mass of the deposit obtained from cells in series with it but a t various concentrations of gelatin. I n column six are given the masses of the deposits obtained by multiplying the data under the smaller coulometer deposit by the ratio of the coulometer deposits, 1.2462 - These values should be identical with those of the fourth column, 0.3684'

if proportionality to coulometer deposits exists.

Since the current and


2600

current density are the same in both c a w , the mass of the coulometer deposits are proportional to time. I t is evident that the deposits are not strictly proportional to time. On the other hand in a 0.5 11 copper sulfate solution containing 0.jUc gelatin a t z 5' when electrolyzed at a current density of two amperes per square decimeter, the mass of the deposits were found to be practically proportional to coulometer deposits for deporits whose masses lay between 0.j grams and I . 7 grams; below the lower value considerable discrepancies from the proportionality occurred. In general, exact proportionality is not the rule.

Excess Weight and Temperature As is well known, the extent of adsorption at, a solid-liquid interface diminishes niarkedly with rising temperature, but is with less influence upon the magnitude of many electrode potentials. As the data obtained up to this point, resembles in some measure adsorption phenomena, wc have carried out several series of experiments at' temperatures ranging approximately from 2ooC to 40°C to determine the effect of temperature upon the magnitude of the excess deposit. Deposits of approximately a gram were made a t the temperatures ZIT, 25OC, 3ooC, and 39'C. The solutions contained 0 . 5 ~ ~ gelatin and were made 0.5 91 with respect t o copper sulfate and electrolyzed a t a current density of 2 amperes per square decimeter. In order to show the various series of trials graphically t'he deposits were all computed (by simple propationality) to the iiiasr of deposit, equivalent to one gram of copper in the coulometer deposit. As the deposits werc all very close to one gram in mass, no great error was introduced as the results of the previous sectinn (on excess weight as a function of time) has already shown. The results are shown in Fig. i . The point plotted at 21' was the average of 8 detr-rininations, those at 2 joand 30' of three trials each, and that at 39' was the mean of six values. The individual values are given in Table 1,where the excess weights are given in milligrams. Each of these deposits was analyzed for copper and the weight of copper produced in the redeposit above that of the coulometer deposit is recorded below as "escess copper.''

TABLE S 21°C

2j"C

Exc. Cu

Exc. K t .

62 3 61.9

7.3

__

jI.2

6.3

31.1

51.2

29.2

2.6 0.9

62.3 60. 61.9 59.5 58.7

7.3 8.j 8.8 9.3 8.1 7.2

j z .j

6.9 6.9

28.1

2.3

2S.I

1.; 2.7 3.3

57.2

Exc. Cu.

g0'C

Exc. Wt.

43.8 41 .; 41.9

Exc. Cu. 3.0

3.7 3.9

Exc.

39°C

Exc. IYt.

Kt.

30.6 30.7

Exc. Cu

ELECTRO-DEPOSITION OF COPPER IN PRESENCE OF GELATIN

2601

Gray' has pointed out that the deposit obtained from a solution of pure copper sulfate is also a function of temperature, diminishing slightly in mass as the temperature rose. Gray's work was based upon solutions containing nearly double the concentration of copper sulfate used in our above trials. This diminution in mass (which is relatively small) has been ascribed to the

increasing solvent action of the copper sulfate solution as the temperature rises.2 In our more dilute solutions the mass of the deposit increases slightly with increasing temperature in baths containing no gelatin but obtained otherwise under conditions exactly similar to those stated in Table X. For example in a bath 0.j M with respect to copper sulfate but containing no gelatin the excess deposit (for a coulometer deposit of I gram) was 2.5 milligrams at Z I O C ; a t 3ooC the excess deposit was 6.1 milligrams. The differences between our results and those of Gray's may be attributed to the decreased solvent Phil. Mag., (5) 25, 183 (1888). Richards, Collins, and Heimrod: Proc. Am. Acsd. Arts Sci., 35, 123 (1899).

2602


action in the more dilute solution and the increased hydrolysis of the dilute solutions; this increased hydrolysis results in the formation of a greater s u p ply of cupric (and cuprous) hydroxide and its subsequent adsorption by the electrode. The data upon excess copper confirms this view as it will be noted there is always “excess” copper present and that the excess copper diminishes a t a different rate than does the excess weight with rising temperature; evidently the increased solubility of copper in the copper sulfate solution as the temperature rises accounts for the diminution of this excess copper. It is evident from our data, then, that the gelatin content of the deposit (which forms the larger part of the excess weight) diminishes very rapidly with increasing temperature.

C. The Influence of Gelatin upon the Polarization at the Cathode during Deposition of Copper Many workers in this field have recognized the usefulness of determining the magnitude of polarization during electrolysis as an aid in determining the mechanism of the electrode process. Blum in particular has emphasized this mode of attack. Obviously, of the various methods of measuring polarization, one must be chosen that will give the average value of polarization over the entire cathode surface. Our results in Section A can only be interpreted by assuming that different areas of the cathode are a t different potentials, as the processes taking place vary from locality to locality over the cathode surface. The method of Haring‘ while doubtless not as precise as other methods, is admirably adapted for our present needs, and was the method which we have employed in the present investigation. The method, in brief, consists in the employment of a cell, divided by means of two copper gauze partitions into three equal compartments, and containing a t the two ends of the cell, the cathode and anode, respectively, of the same dimensions as the gauzes. The potential drop across each compartment is then measured while the current is flowing through the cell; that across the middle compartment is assumed to be due wholly to the I R drop. That across the other two compartments is due to the I R drop plus the cathode (or anode) polarization. As the compartments are made of equal dimensions the IR drop in each will be the same and consequently the cathode (or anode) polarization will be the differences in potential across the cathode (or anode) compartment and the middle compartment. As will be seen this method gives the average polarization over the entire surface of the electrode and not that due to any particular area. The cell which we employed was constructed of wood and was approximately 20 centimeters in length and 2.6 cm. in cross section. The interior of the cell was thoroughly coated with a black asphalt paint which was found to be insoluble in the copper sulfate solutions used. Two copper sheet electrodes, 2 . j centimeters in cross section were then cemented into the cell with the same asphalt, the distance between these electrodes being exactly I j 1

Trans. Am. Electrochem. Soc., 49, 417 (1926).


2603

centimeters. Two 14 mesh copper gauze partitions were also cemented into the cell, dividing it into three compartments, each 5 centimeters in length. Each electrode and gauze was cut with a cmsiderable lead, to which was soldered the leads to the potential measuring device. Before each trial the electrodes and gauzes were given a heavy coat of electrolytic copper from an acid plating bath. Potentials were measured by the use of a Leeds and Northrup student potentiometer having a range of o to 2 . 2 volts, the current through the potentiometer being adjusted to its proper value by a certified standard cell. I n some instances the drop of potential across the cathode compartment exceeded the range of the potentiometer and in these instances a potential divider, constructed of precision resistances, was employed and exactly onehalf (or in some cases, exactly one-third) of the potential drop was measured. By means of suitable switches the potentials across the compartments, after considerable practice, could be very rapidly determined. The anode polarization was also measured as it required but little additional effort to obtain it. We have not considered it in our discussion, however, as we were interested only in the cathode process, but have included it for the sake of any who might be interested in such data. Our actual measurements of polarization were made as follows: The cell, having been thoroughly cleaned was placed in a large glass cell, which was in turn immersed in a water thermostat whose temperature was automatically kept constant. The temperature within the glass cell was z jo i 0.1’for all the measurements recorded. T h e control of temperature was necessary as it was soon found on attempting polarization experiments a t room temperature that the value obtained fluctuated with comparatively small differences of external temperature. The electrolyte was added to the cell until it just reached the top of the electrodes and gauzes, thereby giving a cross sectional area of electrolyte very nearly equal to 6.25 square centimeters. As soon as the temperature of the electrolyte had reached z jothe current was allowed to flow through the cell. Regulation of this “working” current was obtained by means of a sensitive rheostat included in this circuit. Preliminary trials had given us a knowledge of the proper setting of the rheostat for a desired current, the magnitude of which was measured by means of a precision ammeter placed in series with the cell. Approximately one minute after the circuit was closed the ammeter became steady a t the desired current and polarization readings were taken in the order cathode, middle, and anode compartments. These readings required about one minute to complete; two or three minutes were then allowed to elapse and a second set of polarization values taken. These, in general, differed from the first set by one or more millivolts. After several minutes a third set of data was recorded; the final polarization values at this current density being obtained by the averaging of the three sets of data. As soon as the third set of readings had been obtained, the working current was adjusted to a higher value and three sets of data again obtained as outlined above. This procedure was carried out over the entire range of our

2604


experiments the current densities of which were varied between 0.16 and 3.20 amperes per square decimeter. The majority of the polarization measurements were made upon solutions containing one half of one gram mole per liter of copper sulfate. These were compared with similar solutions containing in addition one-hnlf of one per cent of ash-free gelatin. Duplicate determinations were made in each instance. The data for 0. j M copper sulfate a t 2 j o is presented in Table XI and for a similar solution containing o.jYc gelatin in Table XII. The polarization values are expressed in millivolts, results for the first trial being recorded under I, and for a second solution under 11.

TABLE XI Current Density

Cathode polarization

Anode polarization I i1

Brnpsldm2

I

I1

0.16 0.32 0.48 0.64 0.80 0.96 1.12 1.28 1.44 1.60

31. 41.

39.

9.

50.

21.

55. 71. 8;.

61.

30. 39. 46. 49.

2

.oo

2.40

2.80 3.20

100.

117.

I33 ' 149. 164. 208. 256. 296. 345.

72.

84. 98. 113. 129. 145. 162, 201.

245' 295. 339.

2.

IO. 20.

51.

33 ' 42. 46. 49 '

j4.

54.

57. 59. 69. 77. 84. 105.

57. 62. 72.

83 ' 99. 112,

TABLE XI11 Current Density Amps/dm*

0.16 0.32 0.48 0.64 0.80 0.96 1.12 1.28 1.44 1.60 2 .oo 2.40

2.80 3.20

Cathode polarization

I '59. 156. 176. 199. 214. 355, 386. 412. 434. 453. 506. 555. 612. 651.

I1 164. 140. 168. 188. 207.

365. 409. 441. 470.

490. 542.

597 ' 639' 686.

Anode polarization I 11 IO.

I.

24. 33. 41. 46. 49. 53.

5. 8.

58. 63 ' 66. 13'

- ?

86. 104.

129.

13 ' 19. 25. 33 ' 38.

44. 49. 61. 76. 95. 128.


2605

It will be noticed that polarization in the copper sulfate solution is very nearly a linear function of the current density whereas in the bath containing gelatin, the values are initially much higher and furthermore show a considerable rise a t a current density of 0.8 amperes per square decimeter. This point will be considered again in the discussion which follows. The variation of the polarization values with current density can more clearly be seen in graphical form which we have presented in Fig. 8. The experimental points

in these diagrams are the average values of both trials. As will be seen froni Tables XI and XII, the polarization values are more reproducible in the copper sulfate solution alone, than when in the presence of gelatin. I n addition to determining the polarization values with copper cathodes, measurements were made substituting a platinum cathode for that of copper. Our reason for this step was to furnish polarization data more directly comparable to that obtained in the determination of our excess weights as described in Section B. It will be recalled that in those instances the cathode was always of platinum. The polarization values with the two types of

2606


cathodes would not be expected to be greatly different save for a possible initial difference, for after electrolysis had proceeded for some time we would, in either case, be dealing with a copper cathode. On the other hand, as the base metal influences to some extent the form of the deposited metal, there might possibly be considerable difference in the two cases. In order to make certain of the magnitude of these differences, measurements with a platinum cathode were carried out. The data are presented graphically in

POL#..IZ#~ON IN VOLTJ FIG.

9

Fig. 9 each point again representing the average of six measurements (two solutions). When these are compared with the curves of Fig. 8 it will be noticed that the general form of the functions is the same whether a platinum cathode be used or a copper one. The values for the cathode polarization a t each current density are, however, somewhat greater for the copper cathode than for the platinum cathode. We attributed this difference to the fact that the grain size of the deposits was somewhat smaller when depositing upon platinum than upon copper. The effective current density in the case of deposi-

ELECTRO-DEPOBITION O F COPPER IN PREBENCE O F GELATIN

2607

tion upon platinum would therefore be somewhat smaller than for the same value of applied current in the case of copper and consequently a lower value of polarization would result.

Discussion of Results Of the various views which have been advanced to explain the mechanism of the cathode processes under study we believe that our data is more satisfactorily explained by regarding the inclusion of gelatin in the copper deposit as a purely adsorption or surface phenomenon. That it is a case of simple occlusion within the interstices of the deposit is cleerly not tenable, for in this case the excess weight should increase continuously with increasing gelatin content, and further should become greater (rather than smaller) as the temperature rose.' Such is not the case as an examination of Figs. 4-7 will show. For the same reason it is not probable that the gelatin finds its way into the cathode deposit as a result of electrolytic discharge of gelatin ions or particles, as under these conditions the amount of excess deposit should likewise increase continuously with increasing gelatin content in the bath. This case would be somewhat analogous to the deposition of alloys, say of copper and zinc from a solution containing both copper and zinc salts. In such cases it has been found that for a fixed amount of one metal in solution the proportion of the second metal in the deposit becomes greater as the amount of the second metal m solution becomes greater.2 Not in direct proportion, to be sure, but nevertheless no limiting value is indicated in the data available in the literature for such cases. The view that the gelatin is found in the deposits as a result of the discharge of complex ions containing gelatin and copper is only a t all tenable on the assumption that other processes than the discharge of simple cupric ion and complex ion are taking place. For if only these ions were discharged the mass of the copper in the deposit, should in every instance, be considerably less than the mass of copper produced in the coulometer deposit. Our data, however, show that there is always more copper in these deposits than in those obtained from the coulometers. That other processes than the discharge of simple cupric ion may take place a t the cathode we freely admit. Any hydrolysis of cupric or cuprous sulfate to their respective hydroxides, would produce small amounts of these insoluble substances in the vicinity of the electrodes. These hydroxides may be adsorbed by the metallic copper of the electrode. We believe t h a t this is the explanation of the excess copper found in those baths containing no gelatin, and in part, is the explanation for the excess copper found in those baths containing gelatin. Another possible process which would tend to increase the copper content of the deposit over that of the coulometer deposit Increasing tern erature tends to increase the size of the crystals of electrodeposited copper, and hence &r the same mass of copper deposited the volume of the intersticea should become greater. a See for example the data of Ferguson and Sturdevant: Trans. Am. Electrochem. SOC., 38, 176 (1920).

2608

ROBERT TAFT A S D HAROLD E. MESSMORE

mould be discharge of cuprous ion. Cuprous ions are undoubtedly present and might owe their origin to at least three processes: ( I ) to the solution of the cathode, according to the equation Cu++ Cu = zCuL; ( 2 ) to the solution of the anode, according to the equation Cu --E = Cu+; (3) the reduction of cupric ion by the gelatin itself. Any cuprous ions formed as a result of ( I ) above, however, would, if discharged, mutually cancel their effects if the efficiency of the cathode process is computed upon the basis of discharge of cupric ion. The possibility of cuprous ions being formed as a result of (3) above has been tested experimentally by treating 0.5 gram of ash free gelatin dispersed in 7 0 cc. of water by 3 j cc. of standard Fehling’s solution in the cold. S o red cuprous oxide was produced even after standing 48 hrs. X similar solution gave a very slight residue of the red oxide after boiling and standing hot’ for several hours. It should be noted that conditions are presumably far more favorable for reduction in the highly alkaline Fehling’s solution than in the slightly acid solutions which actually existed in our electrolytic cells. It should also be stated that the amount of cuprous oxide formed by boiling the solution depended upon how long the solution was allowed to remain hot. If chilled immediately after boiling the mass of precipitate was a very small fraction of that obtained when the solution was allowed to remain hot for several hours. It would appear that the reducing ability is the result of chemical action of the hot alkaline solution upon the gelatin, forming other substances which possess reducing properties. As a result of the second possibility mentioned above, cuprous ions are formed in neutral solution.’ Such inns could only reach the cathode as a result of migration and convcct,ion. If it he assumed that initially the concentration of cuprous ions is s~ small as to preclude the possibility of discharge of cuprous ions, such ions would tend to accumulate about the cathode. When the concentration of cuprous ions had become such that the potential Cu/Cu+ had been raised to the operating electrode potential, discharge of cuprous ion would commence, In this case the proportion of excess copper, as we have defined it, would tend to show marked increases with the passage of time. I n the majority of cases which we have examined, the reverse seems to be true (See Table IX) i.e. the proportion of excess copper diminishes as the electrolysis is continued. Further, Richards, Collins, and Heimrod have shown that at a current density of I amp/dm2 at o°C the cathode deposits from a solution of “neutral” copper sulfate (approximately 0.4 &I)when compared to deposits of silver obtained from the silver coulometer, lead to atomic weights of copper ranging from 63.53 t o 63.55 depending upon the size of the cathode used, but computed upon the basis of divalent copper. The present accepted value for this atomic weight is 63.57, so that discrepancies arising from the deposition of cuprous ion could not amount, to more than four parts in sixty-three hundred. Although many of our results were obtained under conditions of current density, temperature, and concentration different than that of these

+

Kiliani: Berg. Hutt. Z., 44, 294, (188j).

ELECTRO-DEPOSITION OF COPPER IN PRESENCE OF GELATIX

2609

investigators, the small difference obtained when comparing our deposits to those obtained from the copper coulometer, indicates to our minds that the discharge of cuprous ion plays an extremely small part in the electrode reaction. The maximum variation which we have observed (from some hundreds of quantitative experiments) between the mass of the coulometer deposit and the total copper in a given deposit amounted to nine parts in a thousand, and this in a case where the copper sulfate was 0.2 j M. This large value may well have come as a result of hydrolysis and adsorption of copper oxides rather than discharge of cuprous ion. For these reasons, then, it does not appear likely that discharge of cuprous ion plays a more important part in baths containing gelatin than those that are free of gelatin. One further point bearing upon the matter of excess weights is the fact that the composition of the excess weight is not constant, but depends upon several factors. While this would probably be true no matter what the cathode processes might be, in case of discharge of complex ion the proportion of the excess copper should show a tendency to become less as the gelatin content increases. That this is not so, can be seen from the following table which is computed from the data of Table VIII.

TABLE XI11 Ratio of excess Cu to excess weights Conc. of copper

(yo)

Gelatin content

sulphate

2%

1.25

-

0.5%

0.06:;

12.3

23.7

9.5

16.j

I.

11.5

0.75

12.6

IO.

18.2

0.50

12.2

0.25

IO.

8.3 10.7*

15.7

8.7

With the exception of the starred value the magnitude of the ratio shows a minimum value a t a gelatin content of o.gyc. The polarization data which we have found tend also to confirm our belief as to the mechanism of the electrode process, altho some explanation and confirmatory evidence are required. An examination of Figs. 8 and 9 shows that a t a current density of approximately 0.8 amperes per square decimeter the polarization values increase quite sharply. This rapid increase of polarization, might a t first thought be indicative of a change in the nature of the cathode process. That is, if the process below this value had been discharge of cupric ions solely, this new level might indicate discharge of cuprous ion, of complex ion, of hydrogen ion, etc. If any one of these processes occurred however, a marked change in the mass of the cathode deposit as compared t o that of the coulometer deposit, should occur if compared a t current densities above and below 0.8 amperes. We, therefore, determined the magnitude of the excess weight as a function of current density for a

2610

ROBERT TAFT AND HAROLD E. MESSYORE

0.5 M solution of copper sulfate containing one-half of one per cent of gelatin at zs0C. The values for seven current densities are given in Table XIV and are shown graphically in Fig.

IO.

EJCESS WEIGHT / N rj’ILLIGRAMJ

FIG.I O TABLE XII’ Current density ampidm2

0.32 0.64 0.88 1.28 I .84 2.40 3.20

Deposit grams

Excess weight grams

0.0010 0.0026

0.2710

0.0481 0.0968 0.1336 0.1948 0.2852

0.3536 0.4720

0.3730 0.4921

Coulometer deposit grams

0.0471 0,0942 0.1296 0.1883

0.0040

0.0065 0.0142 0.0194 0.0241

ELECTRO-DEPOSITION OF COPPER I N PRESENCE OF GELATIN

261 I

Inspection of Fig. I O shows that the marked increase in polarization can scarcely be attributed to a change in the type of electrode process and that further search must be made for its origin.' One of the first explanations to suggest itself to us, was that the increased polarization results from mechanical causes. For example, following the suggestion of Frolich, there is the possibility that the increasing content of gelatin in the film of liquid immediately contiguous to the electrode reaches such a value that gelation sets in. A large increase in gelatin content of the film would not be necessary, as we have found that solutions containing copper sulfate tend to lower the setting point of gelatin, for example a I M solution of copper sulfate containing 1 7 ~ gelatin, sets to a gel in a few hours a t 25'. If the polarization is due to this cause, however, it should be function of time alone provided the current density were kept constant a t 0.8 amperes. To check this point, several polarization measurements were made upon 0.5 M solutions of copper sulfate containing 0.5% of gelatin a t 2 5 ' and a t a constant current density of 0.8 amperes per square decimeter, for periods of time as long as three hours. No increase in polarization was observable as an inspection of Fig. 11 will show. We, therefore, concluded that the increased polarization was not due to mechanical causes.

It is essential, of course, that an explanation of this marked increase in polarization be made. Our data apparently indicates that it is not due to a change in the nature of the electrode process, nor to mechanical causes. It has long been known that cathode polarization in cells containing complex ions is far greater than in cells containing supposedly simple metal ions. A number of explanations of this increased polarization have been advanced and we have turned to such explanations to account for the increase in polarization in this case. Thus Cady and Graening2 have suggested that the formation of complex ions would cause a change in the rate of increase of electrode potential with current. Their argument is briefly as follows: In a solution of a simple salt, such as silver nitrate, the cathode process is solely Ag+

+

E

= Ag

a t low potentials. As the current is increased, the supply of silver ion in the immediate neighborhood of the cathode is diminished, and if not replenished l As can be seen from the data of Table XIV different masses of copper are obtained in each trial, i.e. the time of electrolvsis was the same in each case. I t would be more logical

to compare the excess weights fdr the same mass of copper deposited. However, if the deposits in the cells containing gelatin are reduced to slmilar conditions (by multiplying the mass of the deposit by the ratio of the coulometer deposits) the same effect is still noted. Thus converting the mass of the deposits to the deposit equivalent to 0.1296

grams of coulometer deposit gives in the first five cases Mass of deposit 0.1324gms. 0.1332 0.1336 1.za 0.1341 1.84 0.1364 J. Phys. Chem., 30, 1597 (1926). Current density .32 .64 .88

Excess weight 2.8 mgms. 36 4.0 4.5 6.8

2612


by migration, convection, or dissociation of complex ions, may result in some other electrode process occurring such as the liberation of hydrogen for example. Let it be assumed that an equilibrium such as

Ag+

+ nHpO$[Ag

(HzO)n]+

has taken place, but that the reverse process is not an instantaneous one but requires an appreciable fraction of time to occur; increased potential (which is

P O L M U A T f O N IN @LTJ

FIG.I I

polarization) must therefore be applied to obtain the same electrode process. The additional energy involved would be assumed to compensate the energy required for the reversal of the above process. In our particular case the increased polarization is explainable by modifying the assumption of Cady and Groening to fit our particular case. Let it be assumed that gelatin forms complex cations with copper; this is certainly justified when one considers the hydroxy and amino-like character of gelatin and the known ability of copper to react with hydroxy and amino (ammonia,

ELECTRO-DEPOSITIOS

O F COPPER IX PRESENCE OF GELATIK

2613

for example) bodies.' In the case of electrolysis at lower current densities than 0.8 amperes per square decimeter the discharge of cupric ion will care for the current passing from solution to cathode. Above this current density, cupric ions are not supplied at a sufficiently rapid rate by convection and migration and demand must therefore be made upon the reserve supply of copper ions available in the complex cation. If this process, i.e. (Cu (Gel)n)++$CuL+

+ &el.

is not instantaneous and requires energy, an increase in potential must occur if the same electrode process is to continue.? Such a view is in accord with the experimental facts a,! we know them at present. It should be noted that there is a distinction between formation of complex ions and discharge of complex ions. K e are assuming and grant that complex ions do occur in solutions of copper sulfate containing gelatin, but that the electrode process is primarily one of discharge of cupric ions. Fuseya and Murata, and Fuseya and Xagano advocate the view that addition agents function as the result of discharge of complex cations formed from addition agent and nietal ion, but present no direct evidence for this difficult problem. To our minds, it seems very unlikely that such comples ions would be discharged when it is recalled that deposits of very pure copper can be obtained from baths containing such complex ions as (Cu(Ch-2)- and (Cu!NHs))d'+. The processes here doubtless take place by discharge of simple ion and dissociation of complex ion; it is also to be noted that the complex ion formation in the two cases cited above is probably far more complete than they would be in cases of solutions containing a large supply of copper ions and very small concentrations of gelatin.3 Of the various views which have been advanced to account for the presence of gelatin in the deposit, there apparently remains only that of adsorption. The positive evidence of this explanation as the correct one lies in our data involving the excess weight and the form of our deposits. The form of our excess-weight-gelatin-content curves resemble those of familiar adsorption phenomena. That these diagrams are directly comparable to adsorption curves will be evident from the following considerations. In each of n series of runs upon which these curves arc based, the copper deposited electrolytically is approximately the same. \Ve are assuming in these cases that copper is the adwrbent and therefore the difference bptween thc weight There seems to he additional evidence for complex ion formation from determinations of the transport number of copper ion in the presence of gelatin, as has been done by Mutscheller tChem. Met. Eng., 13, 353 i191j),)hlutscheller's interpretation of his data is, to our minds, entirely erroneous, no account being taken of chnnyas in t h e concentration of gelatin before nnd after electrolysis. 2 Isgarischew: Kolloidchem. Beihefte, 14, 2 j (1921), has advanced a somewhat similar explanation of this polarization phenomenon. While the results of these investigators are based upon other addition agents than gelatin, they imply that their assumptions hold for this material as well.

2614


of the deposit in any one case and of the copper deposited electrolytically represents the weight of the material adsorbed by the copper.’ If these differences are due to adsorption, plotting their values against the gelatin content of the solution should give curves similar in form to adsorption curves if the process is actually a surface phenomenon. There is this difference to be noted, however; as the gelatin content increases, the size of the copper crystals deposited tends to decrease and hence for the same mass of copper, the specific surface is greater in those solutions containing greater It is recognized that the excess weight is not entirely due to gelatin. A small part of it is due to “excess” copper as we have already pointed out. The form of the curves obtained, however, are the same whether excess weight is plotted as a function of gelatin content, or whether excess weight minus excess copper is used as the ordinate. It should also be stated that in the cathode deposits from baths containing gelatin, sulfate is found, a fact which Marie and Buffat have already pointed out. Analysis for small quantities of sulfates in the presence of gelatin we found very difficult t o carry out. However, a number of attempts were made, the analyses being conducted by dissolving the cathode deposit with nitric acid, evaporating to dryness, and cautiously igniting to destroy the gelatin. The residue was then redissolved in nitric acid and the usual sulfate determinations made upon this solution. The following table gives the results of such a series of analyses where duplicate cells were run; the cathode from one of a pair of duplicate cells being used to determine excess copper, and the second, sulfate.

TABLE XV Copper and sulfate analyses from electrodes prepared from 0.5 31 CuSOl a t I ampjdm2, and a t 30’C. z.oc; gel. 0 . 2 5 % gel. 0.03% gel. Deposit-grams 0.3952 0.3944 0.3930 0,3938 0.3825 0.3823 Exc. wt., mgm. 17.5 16 7 15 3 16 I 4 8 4 6 dnalyses for Cu BaSOl Cu BaSOa Cu BaSO, gave, grams o 3800 o 0053 o 3789 0 0051 0 3787 0 0024 Excess copper, mgm. 2 3 I 2 I O CuSO, corresponding t o excess copper 5 7 3 0 2.5 C u S 0 4 corresponding to BaSOl 3.6 3.5 1.6 Same conditions as above, but a t 2 amps/dm* Deposit, grams Exc. weight, mgm. Analyses for

ave, grams BCuS04 xcess copper, mgm. corresponding

0 4024 24 4 Cu o 3804 2 4

to excess copper CuSOl corresponding to BaSOl Coulottrs~ers

0.4025 24 5 BaSOI o 0107

6 0

0 3956 0 3956 17 6 17 6 Cu BaSO, o 3794 o 0096 1 4

3 5

7 3

3829 0 3829 4 9 4 9 Cu BaSO o 3787 o 0050 0 7

0

1 7

6.6

3.4

Amp. C.D. 2 Amps.. C.D. ,3780 ,3780 ,3779 ,3775 As will be seen the amounts of sulfate present are of the same magnitude as the excess copper. This sulfate presumably finds its way into the deposit as a result of occlusion of copper sulfate or complex formation with the gelntin. Possibly the adsorption, which n e postulate, is one of neutral particles of gelatin sulfate or of copper gelatin sulfate. I t might be of interest in passing to state that we have made analyses of sulfate upon the coulometer deposits but the sulfate found therein is negligibly small, amounting in t h e maximum case t o less than one-fiftieth of one per cent; which small amount may have come from the reagents employed in making the analyses. I

ELECTRO-DEPOSITION

OF COPPER IN PRESESCE OF GELATIN

26 I 5

proportions of gelatin. This may account for the very rapid rise of the adsorption curves as shown in Figs. 3, 4 and 5 as compared t o the more familiar rounded curves so typical of adsorption. Very likely, too, this matter of specific surface in a large measure accounts for the differences between excess weights found in solutions containing the same concentration of gelatin but different concentrations of copper sulfate. (See Fig. 6). As is well known, the grain size of the deposit is a function of the metal ion concentration and hence of the salt content. Variations in salt content would therefore tend to produce masses of different specific surfaces. Further it is to be noted that limiting values of the excess weight as a function of gelatin content are obtained under all conditions of current density and concentration of copper sulfate which we have employed. The influence of temperature upon the mass of the excess weight also is in line with adsorption data, as the mass of material adsorbed per unit mass of adsorbent diminishes, as is well known, in all cases involving surface adsorption, with rising temperature. Still another line of evidence tending to support the adsorption theory is the fact that the mass of the excess weight is not an easily reproducible quantity. That is, in two cells which are exact duplicates of each other as nearly as is experiment'ally possible, the masses of the deposits are rarely the same when electrolyzed under the same conditions. While the same is true of coulometer deposits, the magnitude of the differences is generally far greater in cells containing gelatin. An examination of those cases in our data where duplicate determinations were carried out, shows this very clearly. For example in two cells containing 0. j 11 CuSO, and 0.2 jyc gelatin, electrolyzed a t a current density of 0.5 amperes per square decimeter a t 3ooC, the masses of the two deposits were 0.3872 and 0.3896 grams. The two coulometer deposits in the same run were 0.3783 and 0.3787 grams (an unusually large variation for duplicate coulometer deposits). The difference between the deposits in the baths containing gelatin is 2.4 milligrams and between the coulometer deposits is 0.4 milligram. This lack of exact reproducibility we ascribe to the possibility that different faces and positions of the copper crystals occur as they are deposited and hence different effective adsorbing areas are produced in the two cells. Bearing upon the matter of addition-agent-action in general, it should be pointed out that the fact that the mass of the deposit increases to a limiting value with increasing content of addition agent is by no means restricted to the case of gelatin. The data for such evidence in the literature is meager but Datta and Dharl have unwittingly provided several cases. These investigators compared copper coulometers containing varying quantities of cane sugar and of tartaric acid with silver coulometers. Each concentration of cane sugar (or of tartaric acid) was carried out as a separate experimental procedure and consequently different quantities of electricit,y were employed. 1

J. Am. Chem.

SOC.,38, 1156 (1916).

2616

ROBERT TSFT AND HAROLD E. YESSMORE

If the masses of the cathode deposit are a11 recomputed to the same mass of silver’ (and therefore t o the same quantity of electricity) by multiplying the maas of the cathode deposit by the ratio of the silver deposits in the two cases, it can be seen that limiting values of the deposit are reached with increasing qiiantities of cane wg;ar or of tartaric acid. We have recomputed the value5 of Datta and Dhar in this way and have shown the deposits as

0

3

Cao

e

0

12-

/5

,e

o f Cane J v g o r r n Gr0-3

Per

2,

2-7

24

12550

of

m

So/u/ion

FIG.1 2

C o n c o f Torfanc Asidm Gromr Per 12s e c of Solvir-

FIG. 13

functions of the cane sugar and tartaric acid content graphically in Figs. 1 2 and 13, respectively. I t will be noted that the last experimental point in the case of both the sugar and the tartaric acid solutions is somewhat lower than the limiting values. These two cases could be attributed either to the liberation of hydrogen or the electrolytic reduction of the addition agent, as these concentrations of cane sugar and tartaric acid are quite large (30 and 32 grams of water respectively). 1 The mass of silver obtained \\hen no cane sugar for tartaric acid) was added to the copper sulfate solution \vas chosen as the litisis for the computatioIi.

ELECTRO-DEPOSITION OF COPPER IN PRESEXCE OF GELATI?;

2617

Researches now in progress in t’his laboratory upon the deposition of copper in the presence of gum arabic, also, conform, in general, to the behavior shoivn by gelatin. That is, the mass of the deposit increases to a maximum ralue with increasing gum arabic content, provided, however, that the solubility of gum arabic in copper sulfate solutions is not exceeded. This case of gum arabic is all the more of interest as presumably the gun1 arabic is present in solution chiefly as the large and complex anion,’ which should migrate toward the anode. Evidently the direction of migration is not an essential feature of this type of addition agent action. The data obtained by Fuseya and liagano upon the masses of copper deposited in the presence of glycocoll, although interpreted by these authors to mean discharge of complex cation, more logically fit into an explanation based upon surface phenomena. For example, t’he curves of Fig. I (of Fuseya and Sagano) when taken into account with their statement “as for the size of the crystals in the deposit, the greater the amount of glycocoll added or the greater the increase in the weight t’he finer was the crystalline structure,” should more properly be regarded as evidence of surface phenomena. The steady upward slope of the curves would indicate that the limiting value of the specific surface was not reached. Fig. 3 from the paper of these authors very distinctly shows the approach to a limiting value. The very marked effect of temperature upon the mass of the deposit, (the mass diminishing with rising temperature) is similar to that found by us for deposits of copper in the presence of gelatin. In the last place the form of the deposits obtained by us can be explained by assuming that surface forces are a t work. It will be recalled (Fig. I ) that the number of raised areas diminishes as the gelatin content increases, finally giving place to well developed striae. Our explanation of this phenomenon is as follows: The original electrode (copper) adsorbs gelatin upon its surface, the amount of gelatin adsorbed per unit area becoming greater as the gelatin content increases up to a limiting maximum value. When electrolysis is begun the discharge of copper ion becomes increasingly difficult as the gelatin content becomes great’er. The cupric ions do find areas, however, which are bare of gelatin (or areas where the cupric ions are able to get by adjacent gelatin particles) and are discharged upon these areas. As the quantity of gelatin adsorbed increases, bhese bare areas diminish in extent and consequently the initial deposition of copper is restricted t o fewer points. It is our belief, then, that these raised areas represent the initial points of deposit. As a result of their formation the current tends to converge upon these areas and consequently these areas tend to grow above the base metal. The nev areas thus formed present new surfaces of copper which in turn are able to adsorb gelatin. As adsorption increases Tyith increasing current density, the relative amount of gelatin adsorbed upon these new areas is greater per unit of area and of mass then upon the basal area. The gelatin thus adsorbed upon these raised areas increases the polarization upon these a-cas T:ift and >Inlm. J. Phys. C h e m , 35, Xi.+

(1931)

2618


and the copper ions seek new points of deposit. If the polarization (upon the raised areas) is sufficient, discharge will take place upon the basal metal adjacent to any raised area. The current density is presumable less upon the basal metal (greater 1R from cathode to anode), and further, the gelatin content of the solution bathing the basal areas has been reduced as a result of adsorption upon the raised areas. Due to these two factors, the copper deposited is more crystalline (Le. coarser) than upon the raised areas (Fig. 3). As discharge of cupric ions takes place, the solution contiguous to the cathode becomes depleted in copper sulfate and tends to rise owing to its smaller specific gravity. Where the initial points of copper deposit are few, i.e., in those solutions containing greater concentrations of gelatin, the convection currents will tend to make these deposits grow upward and lengthen out into well developed striae. As the gelatin content becomes still higher, cupric ions have apparently equal difficulty in being discharged over the entire area and consequently the deposits again present a more nearly uniform appearance. It will be noted that these deposits (from solutions containing I 1/37' gelatin or greater) are nearly uniform in appearance and occur a t similar concentration of gelatin to the maximum excess weights. Somewhat similar views have already been expressed as explanations of striated deposits. Rosa, Vinal, and McDaniel' have in particular, elaborated a theory of discontinuous deposits and the explanation which we have given above is chiefly an adaptation of their theory. summary I. The effect of adding increasing amounts of gelatin upon the form of the cathode deposit obtained from solutions of copper sulfate has been observed and recorded. The effect of the presence of gelatin upon the mass of the cathode 2. deposit from solutions of copper sulfate has been determined for various concentrations of gelatin and of copper sulfate, for various current densities, and for various temperatures. 3. The data obtained is most directly explained by assuming that the copper deposited by the current adsorbs gelatin upon its surface. 4. Measurements of the magnitude of the cathode polarization in the cell Cu/CuSOa, Gelatin/Cu have been made. We interpret our results to indicate that complex cations are formed between cupric ion and gelatin but that the electro-chemical process occurring a t the cathode is primarily discharge of cupric ion. University of Kansas, Lawrence, Kansas.

Bureau of Standards, Bull. 9, 277 (1913).

of gelatin - American Chemical Society

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