Subspeciation of NiS in sulfidic nickel by carbon paste electrode voltammetry

A new procedure is described for the determination of NiS in a carbon paste electrode (CPE) by cathodic stripping voltammetry in pH 5 acetate. The method was developed with both synthetic NiS and millerite. A cathodic scan from +0.3 to −0.8 V with accumulation at +0.3 V for 60 s gave rise to a cathodic peak at ca. −0.5 V for NiS measurement. This cathodic peak was traced to the reduction of a sulfur-rich sulfide layer or sulfur, which could result from applying a positive potential to NiS. The current plot showed a linear range of 90–1180 μg and a detection limit of 20 μg NiS. The interference by Ni3S2 was significant under these conditions, but was minimized by lowering the initial accumulation potential to 0.0 V. For validation, two series of known mixtures of NiS with either Ni3S2 or NiS2 were prepared in the following ratios: 9:1, 7:3, 5:5, 3:7, and 1:9. Recoveries of NiS from both series of mixtures were 93–109%, except for the 1:9 mixtures of NiS/Ni3S2 or NiS/NiS2, where recoveries of NiS were 152.0 and 120.0%, respectively. This method was applied to three millerite samples with pretreatment of CPE at −0.8 V for 30 s, which showed 34.80, 64.50 and 43.74 wt.% of NiS for the three samples. They correlated with 35.20, 60.80 and 39.70 wt.% of NiS, respectively, obtained by total nickel analysis of the acid digests of the mineral.