Electrode processes of chloramines in aqueous solutions

Electroreduction of chloramines (H2NCl, HNCl2 and NCl3) at the platinum electrode was investigated in aqueous solutions of varying pH. The two former compounds are reduced irreversibly and their cathodic peak potentials are pH dependent E-pc (H2NCl) = -0.25 to -0.15 V (pH 7-4), E-pc (HNCl2) = 0.35-0.45 V (pH 4.5-2.5), at scan rate nu = 0.05 V s(-1). Trichloramine is reduced at the same potential at which Cl-2 is formed, and the whole system is reversible. From the dependence of cathodic peak potentials on pH the protonation equilibrium constants of H2NCl and HNCl2 were determined. They were found to be (pK) 6.4 +/- 0.1 and 3.31 +/- 0.05, respectively. The extinction coefficients of protonated species are lower than that of non-protonated ones. The formal potential of the electrode reaction: NCl3 + 4H(+) + 6e(-) = NH4+ + 3Cl(-) extrapolated to pH 0 equals 1.44 V (pH 0, SHE), thus the Gibbs energy of formation of NCl3, Delta G(f) = 362.1 kJ mol(-1). (C) 1998 Elsevier Science S.A. All rights reserved.