SILVER CYANIDE PLATING SOLUTION

F. H. Nordman

THEORY
The fundamental reactions in silver plating solutions when subjected to electrolysis are comparatively simple. The double potassium silver salt in solution is ionized according to the equation:

KAg(CN)2 = K+ + Ag(CN)2- (l)

This primary ionization is undoubtedly accompanied by a secondary slight ionization of Ag(CN)2-.

Ag(CN)2- = Ag+ + 2 CN- (2)

The entire ionization reaction may then be stated:

KAg(CN)2 = K+ + Ag+ + 2(CN)- (3)

The free cyanide is ionized according to the following reaction:

KCN = K+ + CN-

On electrolysis the selective neutralization of the positive ions present is dependent upon the voltage impressed. That ion which is most easily neutralized will be deposited and will continue to be deposited as long as any of that type ion remains, provided the voltage impressed is such as to allow a selective neutralization of charge. Any voltage impressed that is sufficiently high as to allow the neutralization of the most difficult neutralizable ion will likewise effect the deposition of the two metals simultaneously. In the case of silver cyanide plating solutions, it is probable that the silver ion is the most easily neutralized and therefore one would expect the deposition of the silver molecule at the cathode without any change in the potassium ion present. At the cathode then the reaction is as follows:

Ag+ + (-) = Ag

At the anode, which in silver plating is silver, the CN- is neutralized by a positive charge.

2(CN)- + 2(+) = 2(CN)

The metallic silver anode is attacked by the liberated (CN) radical resulting in the formation of Ag(CN)2.

Ag + 2(CN) = Ag(CN)2

This compound is immediately ionized according to reaction (2), and the cycle of electrolysis repeats itself.
It would be well to consider here the first objection to the use of sodium cyanide in place of potassium cyanide, namely, that of causing increased “spotting-out” and difficulties in burnishing. Although the question of spotting-out has not been systematically investigated, the general opinion is that this phenomenon may be attributed to the occlusion of cyanide in the pores of the plate. With inefficient washing these cyanides remain in the pores, and being deliquescent attract moisture and corrode the base metal. If spotting out is more pronounced with the use of sodium cyanide, the most logical conclusion is that the deposits from sodium cyanide baths possess a physical structure different from deposits obtained from potassium cyanide baths.

The factors that ultimately affect the type of deposit obtained may be classified as follows:

1. Current density.
2. Temperature.
3. Metallic ion concentration.
4. Type of metal deposited on.
5. Presence of colloids, etc.

If the current density, temperature, base metal, and type of solution are maintained constant, the only difference in the physical structure of the deposit would seem to be due to different metallic ion concentrations in the two solutions. The metallic ion concentration of baths of this type has never, to the writer’s knowledge, been determined. An examination of the deposits from a bath made from C. P. sodium cyanide and one made from C. P. potassium cyanide should show if there exists a difference in the metallic ion concentration, at least to the degree that affects commercial operation.

Inasmuch as commercial cyanides are not, in general, exceptionally pure, especially sodium cyanide, the effect on the character of the deposit of such impurities as chlorides, hydroxides and carbonates should be considered. Chlorides are often added to sodium cyanide to decrease the total cyanogen content to one hundred per cent potassium cyanide equivalent; hydroxides may be present from the original preparation; carbonates are formed as a decomposition product of cyanides exposed to air. An examination of the fundamental reactions, previously given, seems to indicate that, under given conditions of electrolysis there should be no difference in the type of deposit obtained in the presence or absence of these impurities. The only minor reaction that might take place would be between these salts and metallic silver; it is doubtful whether this is sufficiently great to affect the character of the deposit. Frary and Porter (2) investigated the effect of potassium chloride and potassium hydroxide on the simple potential of various metals used as cathodes. No appreciable different results were obtained in the presence of potassium chloride and potassium hydroxide. One would therefore expect no adverse effect on the type or adhesion of the deposit.

A theoretical examination of the fundamental reactions of electrolysis, previously stated, shows that regeneration of the bath should be unnecessary, since there occurs only dissolution and deposition of the silver. However, this is not the case. Decomposition of the free cyanide occurs with the formation of carbonates, formates, ammonia, etc. But little investigation as to the products and reactions of decomposition has been attempted, so that the reactions themselves are very little known. Theoretically, the three factors that may account for the decomposition of the solution are as follows:

1. Hydrolysis.
2. Presence of air.
3. Electrolysis.

A resume of the literature shows that very little has been done to determine the proportion of decomposition that may be due to each of the above factors. Provided that these factors are equal in magnitude in the case of baths made from potassium cyanide and sodium cyanide, the rate of decomposition in each case would in all probability be the same.

The reactions of hydrolysis may be briefly stated as follows:

1. The potassium cyanide is hydrolyzed in neutral solution.

KCN + H2O = KOH + HCN

2. The hydrocyanic acid in the presence of an alkali and water breaks up, forming ammonia and formic acid.

HCN + 2 H2O = H2CO + NH3

3. In the presence of an alkali, the formate of the alkali is formed.

KOH + H2CO2 = HCOOK + H2O

The above reactions may be represented by a single equation which shows the hydrolysis of potassium cyanide.

KCN + 2 H2O = HCOOK + NH3

Worley and Brown (3) investigated the hydrolysis of sodium cyanide in connection with the dissolution of gold and found that at 20°C the value of the hydrolysis constant multiplied by 104 was equal to 0.27. To the writer’s knowledge, no work has been done on the hydrolysis of potassium cyanide solutions at ordinary temperatures. There is, therefore, no data available on which to base a determination of the different amounts of hydrolysis of free sodium and potassium cyanides in cyanide plating solutions.

The presence of such impurities as chlorides and carbonates should have no effect on the extent of hydrolysis. Hydroxides would decrease it due to the common ion effect.

Many practical discussions on electroplating note that the formation of the carbonates in silver cyanide plating solutions is probably due to the absorption of carbon dioxide from the air. Jordis and Stramer (4) noted that a silver cyanide plating bath, when not being used, increased in carbonate content on standing in contact with air. A number of authorities on general chemistry mention the fact that when moist air comes in contact with potassium cyanide, potassium carbonate is formed and hydrocyanic acid gas is liberated. One would, therefore, conclude that in the presence of air, the plating solution would take up carbon dioxide, with the subsequent removal of two molecules of potassium cyanide for the formation of one molecule of carbonate.

2 KCN + H2CO3 = 2 HCN + K2CO3

Atmospheric air might have an oxidizing action on potassium cyanide, forming potassium cyanate, and this on hydrolysis would yield potassium and ammonium carbonates.

2 KCN + O2 = 2 KCNO
2 KCNO + 4 H2O = (NH4)2CO3 + K2CO3

Thus for every two molecules of cyanide decomposed there should be formed two molecules of carbonate.
The effect of impurities such as chlorides, carbonates, and hydroxides on the decomposition of cyanide due to the presence of air may now be noted. Theoretically there is no apparent reason why chlorides should have an effect on the rate of decomposition. Hydroxides might aid in the absorption of the carbon dioxide from the air, giving an increase in carbonic content, without the same proportional increase in the decomposition of the cyanide.

2 KOH + H2CO3 = K2CO3 + H2O

The presence of carbonates should have no decided effect on the decomposition of the cyanide.
Jordis and Stramer (4) also noted that the decomposition of the free cyanide in a silver- cyanide plating solution was greater when the solution was being electrolyzed than when it was idle. Just why this decomposition would be different in a bath in use is not at once apparent. The reactions of electrolysis have been previously stated. Theoretically, there seems to be no explanation of why the decomposition should increase on electrolysis. However, when the mechanical details of electrolysis are investigated, the loss of KCN may be accounted for. For instance, the silver anode is hung on an iron wire, which extends into the solution. There is therefore every possibility of the liberation of the (CN)2 radical where it cannot attack the silver; it perhaps does not attack iron, so that it is liberated. It might combine with any base present to form the cyanide and the cyanate according to the equation:

(CN)2 + 2 KOH = KCN + KCNO + H2O

The cyanate might then be hydrolyzed, according to the reactions given above, to produce ammonium and potassium carbonates. This decomposition would not decrease the free cyanide content; it would however result in the format on of carbonates at the expense of the decrease in the combined cyanide present.
Then, too, if the voltage was such as to cause the neutralization of a potassium ion simultaneously with the deposition of silver, the effect would be the electrolysis of potassium cyanide.

K Ag (CN)2 = K+ + Ag+ + 2(CN)-
KCN = K+ + CN-

The potassium, liberated at the cathode, would combine with the water present to form potassium hydroxide and hydrogen.

2K+ + 2H2O = 2KOH + H2

The (CN) liberated at the anode would attack the anode according to the reactions previously given. Such a series of reactions would result in the decomposition of free cyanide with the subsequent formation of a molecule of potassium hydroxide.

The presence of such impurities as hydroxides, carbonates and chlorides would tend to cause an oxidation reaction, similar to that caused by potassium cyanide provided the deposition voltage of the chloride, hydroxyl and carbonate ions was reached. Jordis and Stramer (4) noted that the formation of carbonates was greater when the cyanide plating solution contained chlorides than when it was free from chlorides. This can only be accounted for by assuming that such a voltage was being used that the deposition voltage of the chloride ion was reached. The oxidation reactions discussed above would then account for the increase in carbonate formation.

A consideration of the above reactions when comparing plating solutions made from sodium cyanide and potassium cyanide shows that there are few reasons why there should be any difference in the rates of decomposition. It may be noted here that sodium carbonate is much less soluble than is potassium carbonate, a fact that might have an influence on the reactions of carbon dioxide of the aid on the solution. On the equilibrium between carbonates and cyanides; also, (To be continued)

TOLEDO BRANCH

Hooray: Coming to 1927 Convention

Toledo, Ohio, the city selected for this year’s convention of the National Electro-Platers Association, to be held June 29 and 30 and July 1 and 2, is known as the Gateway City to the Great Lakes.

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