Amperometric study of some effects in the precipitation of lead and barium chromates

Amperometric titration has been applied to study parameters determining the extent of mutual co-precipitation of lead and barium in the formation of their chromates. The anomalous behaviour of dichromate due to repulsion at the dropping mercury electrode, which severely affects the precision of the measurements, has been eliminated by addition of tetrabutylammonium hydroxide. By this modification the amperometric end-point detection has been extended to higher concentrations of the reagent and a wider pH range of the medium as required for this study. Shifts in the distribution of chromium species, slow evolution of the precipitate and, in the case of lead, its varying composition, all depend on pH and ionic strength. They have been identified as limiting factors in the amperometric titration of lead and barium and optimum conditions found for their determination. The applicability of the technique to practical analysis is limited, but in the absence of ions which precipitate at the required pH, it permits precise and accurate titration of lead in the presence of barium at pH 2-3 and of their sum at pH 5-6.