Selenium is an essential trace element in the human body. A lack or excess of selenium leads to several disease. A sensitive method of determining selenium is therefore necessary for studying diseases involving a lack selenium or selenium poisoning. The determination of nanogram levels of selenium in biological material is usually difficult. Analysis by atomic absorption spectroscopy has been shown to be a quick and simple method using the hydride vapor generation technique.
1,2Although there are several references to this method, there are also a number of factors generally overlooked, such as interferences and losses through volatilization at high temperatures. The detection limit is also low compared to that of voltammetric methods.Modern voltammetric methods have been used for determining trace amounts of selenium. Of these, anodic stripping voltammetry (ASV) 3-5 , cathodic stripping voltammetry (CSV) 6-8 , adsorptive stripping voltammetry (Ads. SV) 9,10 and differential pulse polarography (DPP) 11,12 are the most popular. In our previous studies 3, 13 we observed a phenomenon during the anodic stripping voltammetric determination of selenite in the presence of copper. When selenite was added to a solution containing copper (in order to determine selenium, standard additions were made) the peak height decreased and a new peak appeared at a more negative potential than that for selenium oxidation. This new peak was attributed to an intermetallic compound between selenium and copper. This fact was thoroughly investigated using cyclic voltammetry 13 and it was shown that there was intermetallic compound formation on the hanging mercury-drop electrode, and that the new peak during stripping was due to a reduction of this compound. A similar behavior was stated [6][7][8] to happen between copper and selenium during the cathodic and anodic stripping voltammetric determination of selenium.During our DPP studies we observed that selenite and some ions, such as copper, lead and cadmium, diminished the peak of each other. This unusual behavior may not be due to the formation of metal selenites, since it was also observed in 2 M acid solutions, where selenite formation was not possible. Because of this phenomenon, most of the electroanalytical determinations in solutions which contain both selenite and at least one of these ions would be wrong. It was thus found that it is important to investigate this behavior thoroughly. The aim of this study was to determine the kind of interaction taking place between selenite and the mentioned ions.
Experimental
ApparatusA PAR Model 174A polarographic analyzer system, equipped with a PAR mercury-drop timer, was used. The drop time of the mercury electrode was approximately 2 -3 s (2.75 mg s -1 ). A saturated calomel electrode (SCE) was used as the reference, and a platinum wire as the counter electrode. The polarograms were recorded with a Linseis LY 1600 X-Y recorder (Linseis, Selb, Germany). The pH values were measured with a Hanna HI 8521 pH meter. Differential pulse polarogr...
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