Applicability of potentiometric stripping analysis to the speciation of lead–humic acid complexes using potassium permanganate as oxidant

Abstract

Potentiometric stripping analysis (PSA) using potassium permanganate as an oxidising agent during the stripping step was applied to the measurement of the lead complexation properties of pyridine-2,6-dicarboxylic acid and humic acids (HA) at pH 6.2 and 0.1 mol l^–1 ionic strength (KNO₃). Values of the conditional stability constants (β′) of the well defined 1:1 and 1:2 lead(II)–pyridine-2,6-dicarboxylic acid complexes, which are labile within the time-scale of the technique, were determined by the DeFord–Hume method. The study was repeated by using differential-pulse anodic stripping voltammetry (DPASV) for comparison. The results obtained by both techniques are consistent among themselves and in accordance with the literature, which shows that permanganate was suitable as an oxidant during the stripping step of PSA measurements. A study of the lead(II)–HA (6.6 and 39.7 mg l^–1 total organic carbon) complexation properties was performed using both PSA and DPASV. It was observed that the average stability constant (K) obtained by DPASV was much higher than that determined by PSA. This result suggests that HA adsorption phenomena at the mercury electrode interfere more markedly with lead(II)–HA stability constants determined by DPASV than with those obtained by PSA.