Influence des substances organiques dissoutes sur le dosage du plomb en milieu marin par redissolution anodique

Abstract

Lead is usually determined at pH ∼ 2 in seawater by differential pulse anodic stripping voltammetry. At such a pH however, the peak of lead is not well resolved at the hanging mercury drop electrode. Batley and Florence (1976) attribute this phenomenon to the organic matter dissolved in sea water, whereas Acebal and De Luca Rebello (1983) ascribe it to the copper dissolution with oxidation to CuCl 2 −. The objective of this work is hence to know: (1) which of these two hypotheses is right and (2) if a hanging mercury drop electrode can be used for the determination of lead in coastal seawaters. In the first section, we have studied the effects of physicochemical (concentration, acidity) and electrochemical (deposition potential) parameters on the stripping voltammetry of lead (Figs 1–3). The anodic stripping peak at pH ∼ 2 is highly modified by the presence of an interference peak at nearly the same potential; on the other hand when the sample is irradiated under ultraviolet light this “double wave” disappears to give rise to a well resolved peak (Figs 4 and 5). We have shown that the phenomenon was related to the influence of dissolved organic matter. Anodic stripping voltammetry must be used with care because, in some pH conditions, natural organic compounds like humic and fulvic acids give rise to adsorption waves which may cause an increase or a decrease in the peak heights introducing errors on the lead concentration. In the last part, the best experimental conditions to determine lead concentrations have been investigated. They are obtained when the samples are acidified and u.v. irradiated. In these conditions, anodic stripping voltammetry and atomic absorption spectrophotometry are in good agreement. It appears that if the sample is not treated as proposed, anodic stripping voltammetry at a hanging mercury drop electrode, at pH 2, gives erroneous results in excess for lead dissolved in seawater.