Electrochemical studies on sulphonephtalene part 3. Kinetics of electrochemical reduction of Xylenol orange and square wave adsorptive catodic stripping voltammetry of its lanthanum complex.

The electrochemical reduction behaviour of xylenol orange, XO, in aqueous medium was studied over a pH range of 2.3–9.6 at a hanging mercury drop electrode, HMDE, on the basis of cyclic voltammetric, and double-potential step chronoamperometric and chronocoulometric data. The experimental results indicate that in acid medium (pH 2.28) the reduction of XO proceeds via an ECEC, first-order mechanism giving a single two-electron diffusion-controlled cyclic voltammetric wave. It was concluded that the rate-determining step is the protonation of the protonated anion intermediate to the final product, with rate constant k₂ of 0.26 s⁻¹. At pH 7.25 the reduction follows an ECE kinetics along two cyclic voltammetric waves in which the first wave was attributed to an EC, first-order process and the second wave to an irreversible electron transfer step, E. On addition of lanthanum(III) to xylenol orange, it forms 1:1 and 1.2 La(III)–XO chelates, which are adsorbed and reduced on the HMDE at more negative potentials than the peak potential of free XO. The adsorptive cathodic stripping voltammetry, ACSV, of these chelates was studied using the square-wave, SW, method. It was found that the SW-ACSV of La(III)–XO can be applied to the determination of lanthanum at the nanomole level. Optimum conditions and the analytical method of determination were presented and discussed. Keywords: electrochemical, reduction mechanism, xylenol orange, lanthanum complex, double potential step chronoamperometry and chronocoulometry, stripping voltammetric determination.