ONLINE MONITORING OF COBALT IN ZINC PLANT ELECTROLYTE BY DIFFERENTIAL-PULSE ADSORPTIVE STRIPPING VOLTAMMETRY

Two methods that have now been used routinely for several years for on -line monitoring of cobalt in zinc plant electrolyte by adsorptive str ipping voltammetry (ADSV) are described in which interference from the very high background zinc concentration is overcome. The first method combines ADSV of cobalt as its dimethylglyoxime complex at a hanging mercury drop electrode with in situ matrix exchange. This method utili ses a bottom-drain flow-through cell to overcome problems associated w ith the high density of zinc electrolyte and enables cobalt to be dete rmined down to 9 mug l-1 in plant electrolyte solutions which contain 150 g l-1 zinc. This method is therefore suitable for low-purity elect rolyte. However, in highly purified zinc electrolyte the matrix exchan ge technique lacks the required sensitivity for cobalt determination a nd a second technique known as catalytic differential pulse ADSV is em ployed which has a detection limit of 0.25 mug l-1 cobalt in zinc plan t electrolyte. The catalytic technique again utilises a bottom-drain f low-cell, but involves formation of the alpha-benzil dioxime complex r ather than that with dimethylglyoxime. Addition of nitrite enhances th e cobalt signal and eliminates the need for matrix exchange. Both meth ods have been adapted for use in on-line voltammetric analysers at Pas minco Metals-EZ. The conditions used for efficient removal of cobalt d uring the purification process almost invariably result in removal of nickel to levels where possible interference from the presence of high nickel concentrations does not occur in the practical situation.