Ion chromatographic determination of sulfide and cyanide in real matrices by using pulsed amperometric detection on a silver electrode

Abstract

The determination of free sulfide and cyanide by pulsed amperometric detection (PAD) at a silver-working electrode was improved through a deep de-oxygenation (at least 10 min) of both standard and real solutions containing the two analytes and adopting a two-potential waveform able to eliminate Ag working electrode fouling. The waveform stepped around the oxidation of Ag in the presence of 0.1–0.4 M hydroxyl ion, from −0.1 to 0.1 V versus saturated calomel electrode (SCE). The eluent composition (0.4 M NaOH plus 7.5 mM oxalate solution) allowed a very good column efficiency and selectivity. The presence of a polysulfide species was hypothesized in sulfide solutions that had not been de-oxygenated and aged. The polysulfide eluted just before sulfide and was confirmed by a chemical test with SO 3 2− producing the elimination of the polysulfide peak. Detection limits, according to the Hubaux–Vos method, were 1.0 and 2.0 μg/l for S 2− and CN −, respectively. We demonstrated good performance of the optimized method by repeatedly injecting standard solutions and by analyzing different real matrices. The method exhibited very good accuracy and repeatability (10 μg/l and a 500 μl injection loop, had a repeatability better than 3% for sulfide and 100 μg/l had a repeatability better than 1% for cyanide). The two-potential waveform ensured long-term stability of the electrode surface that required no manual polishing procedure for at least 1 month (20 analysis per day).