Electrochemical investigations of the effect of N-substituted aminosulfonic acids with a piperazinic ring pH buffers on heavy metal processes which may have implications on speciation studies

Abstract

The influence of piperazine- N, N′-bis(2-ethanesulfonic acid) (PIPES), N-2-hydroxyethylpiperazine- N′-2-ethanesulfonic acid (HEPES) and N-hydroxyethylpiperazine- N′-2-hydroxypropanesulfonic acid (HEPPSO) on the redox processes of cadmium, lead and zinc was investigated by differential pulse anodic stripping voltammetry (DPASV) and differential pulse polarography (DPP) (only for lead). The studies showed that these pH buffers did neither complex cadmium and zinc nor modify to a significant extent the reversibility of the heavy metals electrochemical DPASV and DPP responses, under the experimental conditions used, indicating that the three buffers might be suitable for speciation studies of those metal ions. Depression of lead peak current, recorded by DPASV and DPP, in the presence of all the pH buffers was observed due to surface coverage of the electrode by those compounds, which was confirmed by alternating current voltammetry (ACV). A slight complexation was also verified for lead–PIPES and lead–HEPES systems, this phenomenon being more marked for lead–HEPPSO system (log βPb–HEPPSO=2.56±0.06 ( n=5), determined by potentiometric titrations (pH measurements)). These results indicate that these pH buffers are not suitable for lead speciation studies, unless the extent of its complexation by the buffer is considered.