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 sample s is described based on the cathodic stripping voltammetry of copper seleni de at the mercury electrode using an automated flow system. The use of a vo ltammetric 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 parame ters. 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 calc ulated from the difference between the content of Se-total and Se(IV), prev iously determined. The linear range for the measurements was from 1.0 to 15 0.0 mu g l(-1) with a detection limit calculated as 0.35 mu g l(-1) and a R SD of 1.4% (n = 5) for 5.0 mu g l(-1) of Se(IV) measured. During the UV-irr adiation of water samples not only the reduction of Se(VI) takes place but also the decomposition of the organic matter present. The combination of bo th 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 interfer ences. A mechanism for both photolytic processes, the reduction of selenium and decomposition of the organic matter is proposed, showing that both phe nomena occur under the conditions used. The interferences of some organic c ompounds and inorganic ions were tested. Recoveries between 93 and 109% wer e obtained using the standard addition method for spiked samples of purifie d water, model water, drinking water and lake water. Additional measurement s were carried out on a certified water sample.