A study of the interaction of Cu2-xS coatings with S-4(2-) ions by the method of cyclic voltammetry

The interaction between Cu2-xS (1 < (2-x) < 2) coatings without elemental s ulphur and 0.01 M Na2S4 has been studied by cyclic voltammetry in 0.05 M H2 SO4. It has been shown that elemental sulphur forms in the Cu2-xS coating expose d to Na2S4 solution. The highest S-o amount (2.10(-8) mol cm(-2) during the first 30 s) has been found to be formed when the coating held in Na2S4 sol ution was Cu1.25S. This is explained by the fact that during interaction of a Cu2-xS coating with Na2S4 solution the unstable mixed polysulphides of C u(I) and Cu(II) can be formed e.g. in the case of Cu1.25S (Cu5S4) the compo und Cu5S7 can arise: Cu-I Cu-2(II) S-3(4) + Na2S4 --> Cu-I Cu-2(II) S-3(7) + Na2S which is again decomposed, initially into Cu5S4 and S-o: Cu-I Cu-2(II) S-3(7) --> Cu-I Cu-2(II) S-3(4) + 3S(o) It is supposed that this process can recur and due to this four modificatio n of S-o arise as the time of treatment is increased. Stoichiometric sulphi de Cu1.99S does not interact and CuS interacts insignificantly with Na2S4 ( after 30 s 0.4.10(-8) mol cm(-2) of S is formed). In a real process the contribution from the coating is supposed not to exce ed several per cent.