Complex ions, deposition potentials discharge

If two metals normally have similar discharge potentials, the conditions can be altered to make them sufficiently different for separation to be possible. For example, in the case of nickel and zinc in ammoniacal solution, to which reference was made previously, the deposition potentials are similar at 20 , but differ at 90 . The two metals can thus be separated satisfactorily at the higher temperature, but not at the lower. Another illustration is provided by the copper-bismuth system, in which simultaneous deposition takes place from simple salt solutions if cyanide is added, however, the copper ions form the complex cuprocyanide and the discharge potential becomes more negative (cf. Table LXXXIII). If citric or tartaric acid is present to keep the bismuth in solution, the addition of cyanide hardly affects the deposition potential of this metal quantitative separation from copper is then possible. 

Fig.2, this influence spreads to the character of subsequent change of the potential. Currentless transport is commonly due to the presence in the melt of two different valencies of the metal deposited [3]. According to the voltammetric studies of various authors [2,4-8], in melts of different compositions Ta(V) ions are discharged in one 5-electron stage, Le. only the pentavalent form is stable. However there is both indirect [7] and direct [9,10] evidence of the presence of tantalum ions in molten alkali metal halides in a lower oxidation state. Our findings also prove the presence of low-valent tantalum ions in the melts. The different stability of low-valent tantalum complexes in melts of varying compositions results in different concentrations of tantalum ions of the lower oxidation state. The fluoride ion... 

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