Redoxspeciation of selenium in water samples by cathodic stripping voltammetry using an automated flow system

A method for the determination of Se(iv) and Se(vi) species in water samples is described based on the cathodic stripping voltammetry of copper selenide at the mercury electrode using an automated flow system. The use of a voltammetric flow-through cell facilitates the automation of the measurements. It is shown that the applied electrolyte exchange is reducing the matrix effect during the voltammetric determination step. The system was optimized systematically with respect to the electrochemical and hydrodynamic parameters. The prior reduction of Se(vi) to Se(iv) was carried out after 1 h of UV-irradiation in alkaline medium (pH 11.0). The content of Se(vi) was calculated from the difference between the content of Setotal and Se(iv), previously determined. The linear range for the measurements was from 1.0 to 150.0 mg l21 with a detection limit calculated as 0.35 mg l21 and a RSD of 1.4% (n = 5) for 5.0 mg l21 of Se(iv) measured. During the UV-irradiation of water samples not only the reduction of Se(vi) takes place but also the decomposition of the organic matter present. The combination of both enables the speciation of Se(iv) and Se(vi) as well as the determination of total selenium in water samples with a high content of organic interferences. A mechanism for both photolytic processes, the reduction of selenium and decomposition of the organic matter is proposed, showing that both phenomena occur under the conditions used. The interferences of some organic compounds and inorganic ions were tested. Recoveries between 93 and 109% were obtained using the standard addition method for spiked samples of purified water, model water, drinking water and lake water. Additional measurements were carried out on a certified water sample.