Off-line and on-line differential pulse anodic stripping voltammetric techniques for the determination of antimony(III) and antimony(V) in zinc plant electrolyte

Antimony, in both its trivalent and pentavalent states may be detrimental to the electrochemical deposition of zinc. Ideally, therefore antimony should be monitored in both oxidation states throughout an electrolytic zinc plant. At hydrochloric acid concentrations ( > 4 M) both antimony(III) and (V) present in zinc plant electrolyte can be reduced at a hanging mercury drop electrode (HMDE) to form an antimony amalgam, Sb(Hg). Consequently a very sensitive analytical signal is obtained for the determination of total antimony by differential pulse anodic stripping voltammetry (DPASV) using a 1:1 mixture of plant electrolyte and concentrated hydrochloric acid. In contrast, at low acid concentrations (0.1 M) predominantly only antimony(III) can be reduced and therefore determined in zinc plant electrolyte by DPASV at a HMDE by use of a 1:1 mixture of plant electrolyte and 0.2 M hydrochloric acid. The antimony(V) concentration is calculated by the method of subtraction of antimony(III) from total antimony. Consequently, simple addition of high and low concentrations of hydrochloric acid to zinc plant electrolyte can be used to provide a suitable electrolyte for both off-line and on-line methods for determining the concentration of total antimony, antimony(III) and antimony(V). The method described in this article has been applied to a wide range of zinc electrolyte samples and an on-stream analyzer technique has been successfully used for several years at the Pasminco Metals-BHAS electrolytic zinc plant in Port Pirrie, Australia.