ELECTRODEPOSITION OF ANTIMONY
BY JNANENDRA CHANDRA GHOSH AND A. N. KAPPANA
Very little literature is available on the electrodeposition of antimony. Only two baths for this purpose appear to be quite well known for a very long time, viz: (I) the 'tartrate bath' or a hydrochloric acid solution of tartar emetic, and (2) the 'sulphide bath' which is a solution of thioantimonate of sodium or potassium. Very recently Mathers and Means' have published two papers on the subject. One of these deals with the examination of certain solutions containing antimony salts and the other with a new bath. This latter is a solution of antimonyfluoride in water mixed with an excess of hydrofluoric acid; the authors recommend this bath as being the best one. The present investigation was undertaken with a view to make a scientific examination of the solutions already known for the deposition of antimony, to determine the exact conditions governing the formation of a satisfactory electro-deposition of the metal, and to devise a bath on a theoretical basis.
Experimental Details The electrolytic vessel consisted of a beaker of 300 cc. capacity fixed in the middle of a constant temperature water bath. By the regulation of the size of the gas flame heating the temperature could be kept constant to within o'.5-1'. Constant and vigorbus stirring of the electrolyte was maintained throughout in all the experiments, by means of a stirrer (glass) which rested on the bottom of the beaker over a ground glass plate, and was connected to an electric motor by a thread, the rate of rotation of which was regulated by means of a high resistance rheostat put in series with the mot'or and connected to the mains. The discharge potential was measured in all cases against a decinormal calomel half-element, while the electrolytic current was passing through the main circuit, no commutator arrangement being made. For this purpose the cathode used was a small strip of polished platinum zcmX rcm and only one antimony anode was used. While measuring current efficiencies a silver voltameter was interposed in the circuit and was taken as standard. Copper plates of size jcm X 3 cm were used as cathodes to test the nature of the deposits. These were highly polished and thoroughly cleansed before being introduced into the bath. While taking the deposits two antimony 1
Trsns. Am. Electrochem. SOC. 31, 289 (1917).
JNANENDRA CHANDRA GHOSH AND A. N. KAPPANA
150
anodes were used, one on each side of the cathode. These anodes were sticks about I.5cm in width of chemically pure antimony supplied by Kahlbaum. In all cases of antimony deposition it is a very necessary condition that the anodes must be of the purest possible metal; for it was found in the course of this investigation that the anodes which were once used in the sulphide bath, when used again even after repeated cleaning, in the bath recommended at the end of this paper, hindered considerably the formation of satisfactory deposits, while fresh anodes gave an entirely good deposit. This is in all probability due to some slight tracesof sulphur carried from the sulphide bath which could not be removed by cleaning the surfaces of the anodes. The chemicals used in the experiments described in this paper were a11 cheniically pure and were those supplied by Merck and Kahlbaum. I n what follows, the term ‘critical current-density’ is used to mean the current density beyond which the deposit turns black.
The Examination of Some Baths The Tartrate Bath:-Barclay and Hainsworth, in their treatise on ‘Electro-plating’ recommend the following bath as being a very reliable one. Tartar emetic. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4 lbs. Hydrochloric acid. . . . . . . . . . . . . . . . . . . . . . . . . . z lbs. Water . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I lb. A solution of the same composition is also recommended by Gore’. Since tartar emetic is the starting point for this solution, it was thought necesary to examine a solution of this salt alone in water to find the difference in the’ nature of the deposit, caused by the addition of such a large amount of hydrochloric acid. A half-normal solution was used and deposits taken a t different temperatures. The results are summarised in Table 11.
TABLE I Temp 2 2’.
5
400, 5OO
70
90°
Quality of deposit Very black and spongy Greyish white Whiter than at 40 Resembles dull aluminium surface Perfectly white
Crit. C. D.
Current efficiency, . . . . . . . . . . . . . . . . 87 .OI% I 5 Milli-Amps . . . . . . . . . . . . . . . . 87.45%
................ 3 5 Milli-Amps
87.26% 93 .OI%
Fig. I gives the curves obtained by plotting the values for discharge potentials against current. The solution prepared in accordance with the formula given by Barclay and Hainsworth (loc. cit.) was found to be unstable as it threw down a heavy 1
McMillan: “Electro-metallurgy” p. 251.
ELECTRODEPOSITION OF ANTIMONY
151
precipitate after standing for some time. A few trials showed that a solution having the following composition was quite stable and gave a brilliant white deposit. Hydrochloric acid (S. G. 1,16.Merck). . . . . . . . .200 cc. Tartar emetic. ............................. IOO gms IOO cc. Water .................................... The discharge potential and current efficiency were next determined at various temperatures up t o goo. Fig. 2 gives the discharge potential curves. The current-efficiencies are given in Table I.
TABLE I1 Temperature
20O.5
Current Efficiency
99.9%
The current efficiency fell very rapidly with increase in temperature. When attempts were made t o obtain a thick deposit on a copper plate, it was found that although the deposit came out uniformly for some time, big crystals began to form on the surface and prevented uniform deposition. This could not be remedied by varying current density. The deposit became worse with increase of temperature and turned somewhat black. The Sulphide Bath:-Barclay and Hainsworth’ give the following formula for the preparation of a sulphide bath. Fig.
I
Antimony trisulphide . . . . . . . . . . . . . . 2 5 0 gms Sodium carbonate. . . . . . . . . . . . . . . .500gms Water .......................... 5 litres. The sodium carbonate was first dissolved in water and boiled. Antimony sulphide was next added in small quantities to the boiling solution and the whole boiled for about half an hour. The solution had to be worked while 1
“Elcctro-plating” p. 335.
JNANENDRA CHANDRA GHOSH AND A. N. KAPPANA
152
boiling; but the deposit obtained on a copper plate was dark grey and did not take a good polish. The current efficiency and discharge potential were not therefore determined for this solution.
A solution of thioantimonate, prepared by dissolving a mixture of antimony trisulphide and sulphur in a saturated solution of sodium sulphide, when electrolysed a t 70' and above, with the addition of some potassium cyanide, also did not give satisfactory deposit. Only thin deposits of a dark grey colour could be obtained. This solution was then diluted twice, thrice 2 7and four time8 and deposits taken. In no case was a satisfactory deposit ob24tained. Hence the current efficiency and discharge potential measurements 2 1were not made for this solution either. The Sulphate Solution:-In the presence of a. large amount of sulphuric acid, a small quantity of antimony sulphate goes into solution. This solution when electrolysed while boiling gave a fairly good deposit which took a good polish; but hydrogen was evolved very copiously a t the cathode even a t very low current densities and only a very thin deposit was obtained after passing the current for a long time. This solution is evidently unsuitable for purposes of electro-plating, for the current efficiency is very low.
t 8-
2
15:-
$
123U
F
96-
5-
4
Solutions of Antimony Trichloride in Organic Solvents :-Antimony trichloride dissolves in various organic solvents Fig. 2 forming complex ions in solution. It was thought worth while to see if the deposit obtained from any of these solutions would be of a saitsfactory nature. Solutions were made with the following solvents :-acetone, glacial acetic acid, nitrobenzene and benzaldehyde. The best deposit was obtained from the acetic acid solution; the colour of this deposit was bluish white. Next was the one from acetone Both these deposits peeled off after three days. The other solutions gave very bad deposits. I n general these solutions are unsatisfactory for use both because of their high resistance and because of the unsatisfactory nature of the deposits got from them. .(
-
,c
.08
.lo
.t
ELECTRODEPOSITION OF ANTIMONY
153
The Fluoride Bath:-Finally the fluoride bath of Mathers (loc. cit.) was examined. The solution prepared for this purpose was not exactly of the same composition as the one given by Mathers and Means. Solutions of hydrofluoric acid of different concentrations ranging from one percent t o twenty percent were prepared and saturated with freshly precipitated antimony oxide. The solutions were electrolysed in a glass beaker coated with paraffin; the glass stirrer was also paraffined. Deposits were first taken without the addition of a n y extra free HF. and then with the addition of the acid in different proportions. It was found that the fineness of the deposit was not necessarily dependent upon the concentration of antimonyin the solution. In fact some of .the deposits from the more dilute solutions were far finer in structure than some from more concentrated solutions. Deposits taken without the a d d i t i o n of free acids were in general rough and hard and took a polish with great difficulty; on the addition of free acid however the deposits became far finer. The best deposit was obtained from a solution which was first made by dissolving antimony oxide in a solution of 17% HF and to which 6y0of extra acid was subsequently added. Further addition of acid to this solution did not improve the deposit, although the addition of excess of acid rendered the deposits much finer. They were quite as hard as beFig. 3 fore, for the same difficulty in polishing was experienced. The addition of small quantities of aqueous solutions of citronella and bergamot oils rendered the deposits far finer and smoother. Fig. 3 gives the discharge-potential curve for the solution of composition (17% HF solution neutralised with antimony oxide, 6y0 free HF). The current efficiency was found to be 99.0%. Inferences from the above Examination The deposit from tartar emetic solution alone at room temperature was found t o be quite black, while the solution containing both tartar emetic and HCl gave a brilliant white deposit which was even far superior to the deposit from tartar emetic solution at go'. This difference was evidently due to the influence of HCI.
JNANENDRA CHANDRA GHOSH AND A. N . KAPPANA
I54
To see if this property of improving the nature of the deposit wa.s a peculiar characteristic of HC1, or whether acids in general could exercise this influence, the effect of the addition of a number of acids on the nature of the deposit was tested, the results of which are summarised in Table 111.
TABLE 111 I.
2.
3. 4.
5.
Acid T N. Tartaric acid
Temr, 22O
-do-do2.N. -do-doN. formic acid
60'
-do2N. Formic acid -doMonochlor-acetic
60'
-do-
90' 22'
60' 22'
22'
701 22
7' 0
Q,ualitv of deDosit Dark grey and smooth. Dull white and smooth. Perfectly white Dull white and smooth. Slightly better than at 22'. Whiter than the deposit taken from N. tartaric acid at 60". Perfectly white. Quite white and smooth. Better than at 22'. Better than the deposit with N. Formic acid and brighter. Bluish whi+,eand lust,rous.
The acid solutions were first prepared and then saturated with tartar emetic. These experiments proved beyond doubt, that the acids in general exercised a beneficial influence on the nature of the deposit while added to the tartar emetic bath. It was further observed that the deposit became better as the strength of the acid and the quantity added increased. To see if the potassium ion in tartar emetic was in any way responsible for the dark colour of the deposit obtained from this solution without the addition of any acid, a deposit was taken from a solution of antimony tartrate which was prepared by dissolving antimony oxide in a boiling solution of tartaric acid. This deposit was greyish white resembling the one taken from a solution of tartar emetic in normal tartaric acid. This meant that the elimination of potassium radicle was as efficacious as the addition of extra tartaric acid t o tartar emetic in improving the colour of the deposit. On the addition of hydrochloric acid to this solution of antimony tartrate the deposit became brighter and whiter; the addition of thirty percent by volume of the acid gave a brilliant mirror of antimony on a polished copper cathode. This solution appeared therefore to be superior to the tartar emetic solution containing hydrochloric acid, in so far as it required a very much lesser quantity of acid to give a deposit of even better quality. The solution was made as follows: 500 grams of tartaric acid were dissolved in a litre of water and boiled. To the boiling solution antimony oxide was added in small quantities until no more could go into solution. TOthe filtered solution 30% of hydrochloric acid was added. (HCl.1.16 S.G) solutions of lesser concentrations did not give good-looking deposits. The cathode potential and current-efficiency were next determined. Fig. 4 gives the current potential curves at various temperatures. The current efficiency varied
ELECTRODEPOSITION OF ANTIMONY
I55
between 95.4% and 97.1% when the temperature varied between 25Oand 65'. The potential measurements showed that very low voltages were sufficient for the deposition of the metal from this solution. The critical.current density was found to be 75 milli-amperes per cm?; but current densities above 50 milli-amperes per cm were found to produce very rough surfaces. When attempts were made to obtain thick deposits from this bath, it was found that big crystals began to form in an irregular manner on the surface. Resides it was found that the deposits on keeping for a day or two began to peel off spontaneously. Attempts were next made t o remedy this defect. Addition agents such as glue, gum arabic, gelatine, essential oils and some other organic as well as inorganic compounds have been well known to produce beneficial results in electro-plating baths. These addition agents not only improve the nature of the deposit, but also render certain non-adhesive deposits adhesive. This has been to be the case in the case of tin deposits obtained from an alkaline bath, by Mathers and Bell.' It was therefore hoped that by the choice of a suitable addition agent, it would be possible to obtain thick, uniformly smooth and adhesive deposit. The following ,substances were examined. ( I ) . Gelatine, ( 2 ) glue, ( 3 ) gum arabic, (4) glucose, (5) hydrosilicofluoric acid, (6) hydrofluoric acid, ( 7 ) salicylic acid, (8) borosilicic acid, ( 9 ) resorcinol, (Io) pyrogallol, ( I I ) sodium laurate, (I 2 ) sodium oleate, (13) sodium stearate, Fig. 4 (I 4) safrol, ( I s) eugenol, ( I 6) geraniol, (17) cinnamic aldehyde, (18) citronella oil, (19) anise oil, (20) anethol, ( 2 1 ) bergamot oil, ( 2 2 ) castor oil, (23) oil of turpentine. One per cent aqueous solutions of substances (I) to (4) and (7) (9) and (IO) to (13) were made and added to the bath in small measured quantities. The three acids ( 5 ) , ( 6 ) , and (8) were added directly to the bath in small quantities. Substances (14) to (23) were all shaken very well for nearly six
.
Trans. Am. Electrochem. Soc. 38, 135 (1920).
156
JNANENDRA CHANDRA GHOSH AND A. N. KAPPANA
hours with water and allowed to stand in separating funnels for more than a day and then the aqueous layers were separated. Small quantities of these squeous portions were added to the bath. The following were the results:( I O ) Gelatine gave a good and adhesive deposit only once; but when the experiments were repeated a number of times the same result could not be reproduced. The deposits were in general bad and not uniform. ( 2 ) Glue when added to the bath even in very small quantities produced fern-like figures. Addition of larger quantities did not improve the deposit in any way. (3) Gum arabic. This gave a fairly good deposit when 8 C.C. of its solution was added to 250 C.Q. of the bath. This deposit peeled off on gently striking the plate on the ground. Further additions of the solution to t,he bath rendered the deposits worse. (4)Glucose. Very thin but fairly uniform deposits were obtained with this addition agent which peeled off after four days. ( 5 ) Hydro-fluosilicic acid and (6) hydrofluoric acid did not improve the deposit. This was also the case with (7) salicylic acid. (8) Borosilicic acid. The deposit in this case is very good but not adhe-
sive. (9)and (IO) Resorcinol and pyrogallol gave fairly smooth deposits. These are again nonadhesive and do not take a good polish. (11) Sodium laurate (12) oleate and (13) stearate give excellent deposits which accept a vey good polish; but these deposits fall off after a very short time. (14) Safrol (IS) eugenol and (16)geraniol made the deposit rough and crystalline. (17)Cinnamic aldehyde gave deposits which were fairly smooth but these were not adhesive. (18) Citronella oil. When only 2 . C.C. of this solution was added to 150 C.C. of the bath, a smooth and uniform deposit was obtained which took an excellent polish; but this deposit peeled off in a day. (19)Anise oil and (20) anethol. With these addition agents the deposits were very smooth and uniform but not adhesive. (2 I) Bergamot oil. The deposit with this addition agent was the best one obtained so far and was found to be quite adhesive. However after about two months those deposits which were thick peeled off spontaneously; but the deposits, which were not very thick, adhered firmly and did not come off even on striking the deposited plates hard on the ground. The deposit improved slightly when deposited at 50'. ( 2 2 ) Castor oil gave a deposit which was very bright the first few seconds it was deposited but which rolled up like paper even while the deposition was in progress.
ELECTRODEPOSITION OF ANTIMONY
1.57
(23) Turpentine oil did not improve the deposit in any way.
Of all the addition agents that were tested, as has been stated above, only bergamot oil gave satisfactory results. Even this addition agent could not make thick deposits stick fast. It was therefore thought necessary to see if by any other suitable manipulation it would be possible to make the deposit more adhesive. It was believed that adhesiveness could be improved if the bright copper surface be given a thin coating of some metals which help in the formation of a solid solution at the interface which is responsible for the adhesiveness. To see if such a preliminary coating would improve the adhesiveness in any way, thin coatings of mercury, tin and cadmium were given on to the polished copper surface and antimony deposited on these. In the case of tin and cadmium, even before the current was put on, a black spongy layer of antimony began to form; however by switching on the current immediately after introducing the plates into the bath thick deposits could be obtained which were quite smooth and uniform. But these peeled off spontaneously. With mercury the deposits were quite good but peeled off after about a month. Deposits taken on rough, instead of smooth polished surfaces were found to be adhesive even after the three months; but these also came off after striking gently. In this case also thin deposits adhered firmly. Bismuth has almost the same discharge potential as antimony. It is therefore possible to deposit both these simultaneously from a solution containing the salts of these metals. To see if small quantities of bismuth when deposited along with antimony would make the deposit adhesive, small quantities of bismuth chloride (varying in concentration from 0.5%-0.5’%) were added to’the bath and deposits taken. These deposits were somewhat dark and peeled off after about a fortnight. On analysing qualitatively no traces of bismuth could be detected in the deposit. Deposits taken from the antimony fluoride bath containing hydrofluoric acid were found to be very adhesive. It was therefore thought that adhesiveness could be imparted to the deposit from the tartrate bath while at the same time retaining the softness of the deposit, by using hydrofluoric acid in place of hydrochloric acid. Deposits were taken with various concentrations of hydrofluoric acid (1%-25%); the best deposit was obtained with 17 per cent of acid. These deposits appeared to be quite adhesive at the beginning; but, contrary to expectations they peeled off after a fortnight. Interpretation of Results An electrodeposit of antimony might be obtained (I) in an inhomogeneous form (powder or sponge) ( 2 ) in a coarsely crystalline form, (3) as a tough microcrystalline deposit and (4) as a bright mirror. Beilbyl has shown that the fine crystals on the surface layer of any specimen of antimony, under the action of the polishing lathe, pass through a liquid condition which sets to a hard enamel-like protecting coating when the polish1
“Aggregation and Flow of Solids,” p. 87 (1921).
.
JNANENDRA CHANDRA GHOSH AND A. N . KAPPANA
15%
ing is finished. A bright mirror of antimony is then a congealed liquid surface. This mirror is obtained from the acid tartrate bath, only a t the initial stages of electrodeposition, for small or moderate current densities. On continuing the electrolysis, the superimposed layers become minutely crystalline and finegrained. The ions in solution resemble the gaseous state; at the beginning of electrodeposition t,hey form a liquid layer at the cathode, which at once sets to a vitreous solid. With time crystalline forces operate to produce devitrification; microcrystalline bodies are formed in the initial vitreous layer, and the forces of orientation, thus brought into play, produce a finely crystalline structure in the superimposed layers. I n this connection it is interesting to recall the experiments of Beilby which demonstrated the parallel growth of sodium nitrate crystals on a polished calcite surface. Though polishing has developed over the crystal surface a true vitreous skin, still the orienting influence of the isomorphous calcite crystals beneath could make itself felt. Varieties (3) and (4) only are suitable from a plater’s point of view, and the beneficial influence of addition agents is obvious. They are as a rule protective colloids and we have the familiar explanation that ions are deposited in a colloidal state in presence of these bodies. Mathers and Leible’ have attempted to find a relation between the adsorption of these colloids by the plating metal and their efficiency as addition agents. It was found in most cases, that larger adsorption and greater efficiency go hand in hand. If the surface tension of liquid antimony initially deposited be diminished by the adsorption of these addition agents, the formation of small globules will be hindered. These latter possibly devitrify into crystalline specks and destroy the smoothness of the deposit. It is thus probable that addition agents produce a smooth uniform matrix of the plating metal by producing a lowering of surface tension. Large current densities produce inhomogeneous deposits-black and spongy. Bancroft2 suggests that large currents produce strong polarisation, the cathode film becomes dilute, resulting in the precipitation of an oxide or basic salt. Smee reached a generalisation long ago which has been supported by McMillan3 that a deposit by a current strong enough to produce hydrogen simultaneously is dark in colour and powdery. I n the case of antimony deposits, it appears however, that for moderate current densit,ies, a black sponge is obtained a t the cathode’wheneverit is produced there as a result of secondary chemical action and not by the primary discharge of the antimony ion, A consideration of the current potential curves will make this point clear. Pure antimony dipped in a hydrochloric acid solution of tartar emetic has been found to have an electrode potential of 0.26 volt against a decinormal calomel electrode. I n Figs. 2 and 3 it will be noticed that the discharge potential plotted against electrolysing current, at first increases very rapidly and then remains constant-a curve which is characteristic of the discharge of metal ions. The discharge potential has a value varying from 0.34 to 0.37 which is Trans. Am. Electrochrm. SOC.3 1 , 2 7 (1917).
* J. Phys. Chem. 9,28 (1905). 3
“A Treatise on Electro-metallurgy,” p. 205.
I59
ELECTRODEPOSITION O F ANTIMONY
greater than the electrode potential of antimony in the same bath by about 0.1 volt. This difference is obviously due to what is called residual current polarisation. In HC1 solution of tartar emetic or antimony tartrate, antimony is deposited by the direct discharge of Sb"'ion, and the deposit appears at first as a bright mirror and later as a fine-grained structure. From a solution of tartar emetic only antimony is not deposited by the direct discharge of Sb"' ion as will be at once evident from Fig. I . The abscissa representing discharge potential has a magnification 1/6th that of the 2nd and 1/3 that of the other curves, but still the current potential curve is not a t all steep. Evidently this case represents the discharge of a gaseous ion on the cathode. In this case the current efficiency also did not exceed 87% up to 70' indicating that the reaction
+
C4H406K SbO 3H+C4HdOeKH
+HzO
+Sb
is not complete, but that only 87% of HZliberated is used up in decomposing tartar emetic. The high discharge potential indicates that on an antimony surface, hydrogen has a considerable over-voltage, The contention of Smee and McMillan that simultaneous liberation of HI with the deposition of the metal produces dark and powdery deposits cannot be substantiated. As will be observed from Table I1 the current efficiency in a hydrochloric acid tartar emetic bath is only 30% at 50' though the deposit, is very bright and smooth; a t 25' the current efficiencyis 100%. As the current potential curve indicates the deposit is primarily that of metallic antimony. At higher temperatures a part of the precipitated metallic antimony dissolves immediately in hydrochloric acid, liberating hydrogen. The effect however from the plater's point of view is not disastrous. Composition of the Bath The only traceable regularity that has been generally observed is that metals deposited from solutions in which they are mostly present as complex anions, generally come down in a dense, smooth, finely-grained form. The complex anion serves as reserve for a low constant concentration of the metal cation, and prevents hydrolysis and the formation of a basic precipitate. This is also true of antimony baths, but subject to the condition that the concentration of Sb"' should be sufficient to produce antimony metal as the primary product of electrolysis. I n a neutral tartar emetic solution Jordis and Meyer' have shown that the equilibrium concentration of Sb"' ion due to the reaction C4H406KOSb+3R (present in pure water) C4H,06KH+H20+Sb"' is very small . Hence it is intelligible that Sb is"' not discharged at the cathode. With addition of acid, Sb"' is obtained in increased concentration resulting in its primary discharge and producing an improved deposit. Antimony fluoride has a far greater tendency towards complex formation than the other halides, complex salts like KSbF4, NaSbR. 2NaF, are common Z. angew. Chem. 17, 169,204,236 (1904).
I 60
JNANENDRA CHANDRA GHOSH AND A. N . KAPPANA
where the antimony exists as a negative ion the equilibrium concentration of Sb”’ ion due to the dissociation of the complex SbF4 is much greater than in the neutral tartar emetic solution. Antimony is obtained as a primary deposit as the steep nature of the current potential curve in Fig. 3 B indicates. The discharge potential is much higher than the electrode potential of antimony in normal antimony chloride solution. This is partly due to residual current polarisation and to the very small concentration of Sb”’ present as such in the acid fluoride bath.
Influence of Temperature In an acid tartar emetic bath the current efficiency fell from 100% to 30% as the temperature was raised from 25O-50’; while in an acid antimony tartrate bath, the current efficiency remained pretty nearly constant a t 97Ojo. No satisfactory explanation is available unless it be that potassium has a marked catalytic action on the solution of antimony by hydrochloric acid. The advantage of the latter bath is obvious, for it does not require any regulation of temperature. Conclusion The composition of the acid tartrate bath has been arrived at on theoretical grounds after a critical examination of the baths already known as well as certain other solutions. With oil of bergamot as addition agent this bath gives a very smooth, uniform nice white deposit, which on polishing assumes a silvery appearance. Thin deposits of thickness 0.025, mm have been found to be quite adhesive; this thickness is quite sufficient for plating purposes. High current densities can be employed (50 milli-amperes per cm) and the current efficiency is as high as 977i. These and other points of advantage mentioned in the paper above, mark the bath as specially suitable for plating on a large scale.
