Flow injection amperometry and associated studies

A satisfactory method was obtained for the determination of phosphate, arsenate and germanium as heteropoly acids by flow injection amperometry at a sessile mercury drop electrode held at 0.22Vf 0.23V and 0.18V vs SCE, respectively, without deoxygenaticn of the eluent or the sample solutions. Previous workers had shown that germanium could not be determined at a glassy carbon electrode owing to adsorption, and that arsenate could cnly be determined down to the 10-5M level owing to deterioration of the baseline. The sessile mercury drop electrode was found to be particularly useful at low negative and low positive potentials where oxygen is not reduced and therefore does not interfere, or at more negative potentials in alkaline solutions, where sulphite can be added to remove oxygen traces. This was illustrated by the determination of nitrofurantoin at -0.7V in pH 7.5, with added sulphite. The effect of electrochemical pretreatment of a glassy carbon electrode by holding it at about +2. OV for 2 min and then at about -1. OV for 2 min on the electrochemical oxidation of ascorbic acid, dopamine, dopac, HVA, arterenol and 5 HIAA was studied. The pretreatment of the electrode produced a substantial improvement in the shape of voltammograms, and in the reproducibility of both peak potential and peak current. Considerable shifts of peak potentials to less positive values, and substantial increase in peak heights were obtained by the electrode pretreatment. In the case of HVA different oxidation products were obtained at pretreated and untreated electrodes. Ascorbic acid and dopamine peaks were partially separated at the pretreated glassy carbon electrode. Online determination of ascorbic acids, dopamine and dopac is improved with the use of a pretreated glassy carbon electrode. More reproducible and higher signals are obtained with a shift of the hydrodynamic voltammograms to less positive potentials, than when a newly polished untreated electrode is used. (Continues...).