Platinum Metals Rev., 1983, 27, (1), 26
Cyclic Voltammetry of Cyanide Melts Containing Platinum Group Metals
Progress in Understanding Plating Anomalies
The use of molten alkali metal cyanides as solvents during the electrodeposition of the platinum group metals has been reported in this journal on a number of occasions (1,2,3,4). Additionally, the platinum group metals may be recovered from cyanide melts by solvent extraction into liquid metals such as tin or bismuth, but this work is possibly less well known (5,6).
In aqueous solution reactions, cyanides are reducing agents and also form stable complexes with the platinum group metals. This situation also occurs with the fused salts, but is modified by side reactions which reach greater prominence in the melt than in aqueous solution.
This was revealed in a paper presented at a meeting of the American Electrochemical Society by E. Th. van der Kouwe and D. J. Muller, who are part of the team working with K. F. Fouché at the Atomic Energy Board, South Africa. It was reported that high temperature solvent extraction chemistry of the platinum group metals had resulted in an extensive study of oxidation/reduction reactions of the metal ions and of the cyanide melt itself, and their latest paper described cyclic voltammetry of cyanide melts containing most of the Group VIII metals, as well as copper, silver and gold.
The basic melt used was the sodium cyanide-potassium cyanide eutectic. In the melt, the cyanide ion can be oxidised, either electrochemically at the anode, or chemically by reaction with high oxidation state metal ions which results in the formation of cyanogen
Cyanogen reacts with more cyanide ions to produce the dicyanamide ion N(CN)2− which is an oxidant in its own right
The cyanamide ion, CN2=, on the other hand is a reducing agent, and has been added to melts containing dicyanamide to obtain control of the redox potential of the melt.
Metal-containing melts were made either by adding the appropriate metal cyanides or by anodic dissolution of the metal. Cyclic voltammetry was carried out over the range 0 to 1.9V with respect to a sodium reference electrode, these limits being imposed by the generation of sodium or potassium on the one hand and by the oxidation of cyanide ions on the other.
Of all the metals tested titanium was found to be the most resistant to attack with the potential at the anodic limit; dissolution of the other metals was apparently increased by the dicyanamide which formed under these conditions. Some of the metals were dissolved at potentials less than 1.9V, notably palladium, rhodium and gold.
Most platinum group metals showed more than one oxidation state when dissolved in the melt; rhodium and iridium occurred as M(I) and M(III) and platinum and palladium as M(II), M(I) and M(O); the M(O) state being produced by a two electron transfer reaction with soluble reactants and products:
Reduction of rhodium and iridium is a two stage process:
which is reversible, followed by:
However, for iridium a stripping peak showing this reaction in reverse was not found, the only dissolution reaction for iridium being a direct oxidation to Ir(III) at the anodic limit.
Gold and silver peaks were also obtained by cyclic vo tainmetry but the cyanide medium is too reducing and both metals are precipitated. Nickel and cobalt showed differences in behaviour from the noble metals in that the potential of the reversible redox reaction and the metal deposition potential are very close together. Cobalt also shows an unusual peak shape.
This work is of special interest to those actively engaged in fused salt plating with platinum metals and goes a long way towards explaining some of the apparent anomalies that exist in the published literature of this electroplating art.
- 1 D. Schlain,, F. X. McCawley and G. R. Smith, Platinum Metals Rev., 1971, 15, ( 2 ), 58
- 2 Platinum Metals Rev., 1975, 19, ( 1 ), 15
- 3 D. Schlain,, F. X. McCawley and G. R. Smith, Platinum Metals Rev., 1977, 21, ( 2 ), 38
- 4 J. H. F. Notton, Platinum Metals Rev., 1977, 21, ( 4 ), 122
- 5 K. F. Fouché,, J. G. V. Lessing and P. A. Brink, Proc. Int. Solvent Extraction Conf., Lyon, Sept. 1974, p. 2685
- 6 E. Th. van der Kouwe and A. van Gruenewaldt, J. Appl. Electrochem., 1977, 7, 407
- 7 E. Th. van der Kouwe and D. J. Muller, The Electrochem. Soc., Ext. Abst. Fall Meeting, Denver, Colorado, Oct. 11–16, 1981, Abst. No. 588, p. 1411