A sensitive method for the spectrophotometric determination of osmium at the ppb level is described. The method is based on the formation of a brown-coloured complex by heating the reaction mixture containing Os(VIII), pyrocatechol and a hydroxyamidine at pH 8.5 over a boiling water bath, with subsequent extraction of the coloured species into chloroform. The molar absorptivity of the coloured species with N-hydroxy-N,N#x2032;-diphenylbenzamidine is 3.95 #xD7; 1061 mol#x2212;1cm#x2212;1at #x3BB;max= 410 nm. The method is free from interferences for almost all ions tested.
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