Differential-pulse cathodic stripping voltammetric determination of sodium nitroprusside at a hanging mercury drop electrode aided by copper(II) and poly-L-lysine modification

The nitroprusside anion, Fe(CN)5NO2–, can be accumulated from pH 5 Britton–Robinson buffer solution on to the surface of a hanging mercury drop electrode (HMDE), which has been modified by means of adsorbed poly-L-lysine and which is held on open circuit. This was shown by closing the circuit at –0.2 V versus the Ag–AgCl electrode, after accumulation, and scanning cathodically by use of differential-pulse voltammetry. The voltammetric peak that was observed at about –0.6 V, however, was that for the adsorbed product of the first reduction step of nitroprusside, reported to be Fe(CN)4NO2–, which is the product of the one-electron reduction of the nitroprusside ion [to Fe(CN)5NO3–] followed by the elimination of CN–. When accumulation was carried out at the peak potential of the first polarographic reduction peak of nitroprusside (–0.35 V), the peak current at –0.6 V was enhanced 4-fold, which indicated that the Fe(CN)4NO2– was accumulated on poly-L-lysine more extensively than was the nitroprusside ion. From pH 2 (0.01 mol dm–3 hydrochloric acid) solution, nitroprusside can be accumulated very efficiently between 0 and –0.1 V at an unmodified HMDE in the presence of excess of copper(II). The cathodic stripping peak at about –0.35 V is due to the reduction of an adsorbed copper/reduced nitroprusside species, which has not been fully identified, but is most probably either the copper(I) or copper(II) salt of Fe(CN)4NO2–. This signal was enhanced slightly when accumulation was effected at a poly-L-lysine-modified HMDE. Linear calibration graphs were obtained for up to at least 1 × 10–7 mol dm–3 for the determination of nitroprusside at both pH 5 and pH 2, by using cathodic stripping voltammetric methods based on the above electrode processes, and the limit of detection was calculated to be about 5 × 10–10 mol dm–3 in both instances. Relative standard deviations were typically <2% at the 1 × 10–8 mol dm–3 level.