Spectrophotometric determination of selenium with o-phenylenediamine

2 Spectrophotometric determination of selenium with o-phenylenediamine General remarks 

In acid solution selenium (IV) reacts with aromatic o-diamines forming yellow compounds. The intensity of the colour of the compounds is measured directly, or following extraction with toluene, at 334 nm. In the method described here the colour reaction is performed with o-phenylenediamine  

The extinction maximum of 3,4-benzo-l,2,5-selenodiazole lies in the region of 330 - 335 nm ( = 17 750), Beer s law applies in the range from 0.13 - 

High selenium contents cannot be expected in natural waters and interference must be anticipated from other substances contained in the water. Consequently, it is advisable to concentrate the selenium and carry out distillation before photometric determination. Alkaline concentration of 1 - 2 litres of water in a vacuum rotation evaporator and subsequent distillation of the concentrate with hydrobromic acid containing bromine leads to 

The SeBrz is decomposed in aqueous solution (in the distillation receiver) according to SeBrz + 3 H2O—H2Se03 + 4 HBr to give selenious acid which reacts with o-phenylenediamine. Excess bromine is bonded with hydroxyl-ammonium chloride. Any Se(VI) which is present is reduced to Se(IV) by HBr. The detection therefore always encompasses total selenium. 

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