ELECTROCHEMICAL STUDIES OF AMMONIUM TETRASULFIDE IN SOLUTION IN LIQUID-AMMONIA

An electrochemical study of solutions of ammonium tetrasulphide ((NH4)2S4) in liquid ammonia has been performed using the cyclic voltammetry and the electrochemical impedance methods. The data are very different from those found for lithium tetrasulphide solutions. It is shown that they can be regarded as resulting from a one-electron transfer mechanism between the radical anion S3- and the polysulphide S3(2-), but this reaction is strongly perturbed by homogeneous equilibrium reactions involving S3(2-) and NH4+. Both species of the redox couple S3-/S3(2-) are present in the solution, in the equilibrium state. They originate from the disproportionation of S4(2-). Evidence for disproportionation was previously provided by spectroscopic techniques. The impedance data have been analyzed quantitatively by the coupling of the electrochemical reaction with a reversible reaction (Gerischer profile). A simulation of voltammograms was done with the parameters deduced from analysis fo the impedance diagrams. The agreement between the cyclic voltammetry and impedance data can be considered satisfactory.