1-(2-pyridylazo)-2-naphthol modified carbon paste electrode for trace cobalt(II) determination by differential pulse cathodic voltammetry

A differential pulse cathodic voltammetric method was developed for the sensitive and selective determination of Co-II at a 1-(2-pyridylazo)-2-naphthol modified carbon paste electrode. The analysis procedure consisted of an open circuit accumulation step in stirred sample solution. This was followed by medium exchange to a 'clean' solution where the accumulated Co-II was oxidized and subsequently a cathodic potential scan was effected to obtain the voltammetric peak. The precisions obtained for 10 successive determinations each of 1.00 x 10(-6) mol l(-1) 1.00 x 10(-7) mol l(-1) and 1.00 x 10(-8) mol l(-1) Co-II were 2.80, 5.14 and 9.67% (relative standard deviations), respectively. The detection limit was estimated to be 5.9 x 10(-9) (0.35 ppb) (S/N = 3), for 180 s accumulation. Of the 35 selected substances examined for interferences, only V-II, Ce-III, EDTA and Triton X-100 interfered. These interferences could be easily eliminated. The developed method was tested using a certified sample (USEPA WP 386) and then applied to the determination of Co-II in human hair, pig liver and spinach samples.