SUMMARY
The interaction between Cu2-xS (1<(2-x)<2) coatings without elemental sulphur and 0.01 M Na2S4 has been studied by cyclic voltammetry in 0.05 M H2SO4.
It has been shown that elemental sulphur forms in the Cu2-xS coating exposed to Na2S4 solution. The highest So amount (2·10−8 mol cm2 during the first 30 s) has been found to be formed when the coating held in Na2S4 solution was Cut1.25S. This is explained by the fact that during interaction of a Cu2-xS coating with Na2S4 solution the unstable mixed polysulphides of Cu(I) and Cu(II) can be formed, e.g. in the case of Cu1.25S (Cu2S4) the compound Cu5S7 can arise:
CuI2CuII3S4 + Na2S4 → CuI2CuII3S7 + Na2S
which is again decomposed, initially into Cu5S4 and So:
CuI2CuII3S 7 → CuI2CuII3S4 + 3So
It is supposed that this process can recur and due to this four modifications of So arise as the time of treatment is increased. Stoichiometric sulphide Cul.99S does not interact and CuS interacts insignificantly with Na2S4 (after 30 s 0.4·108 mol cm2 of S is formed).
In a real process the contribution from the coating is supposed not to exceed several per cent.