SELECTIVE AND QUANTITATIVE-DETERMINATION OF COBALT USING SOME ARSENATE COMPOUNDS

A very simple, highly sensitive and selective spectrophotometric method for the rapid determination of cobalt (II) in aqueous media containing 30\% (v/v) ethanol is described. The method is based on the formation of green colour complex by the reaction of cobalt (II) with 1'-(2-arsenito-phenylazo) 2'-naphthol (I-a) and its 3'-carboxylic acid (I-b) in basic media. The effects of amount of reagent, pH, time and temperature have also been studied. Beer's law is obeyed over the range 0.1-4.0 mu g ml(-1) of cobalt with a minimum photometric error of 1.33\%. A fair degree of selectivity for cobalt is achieved by measuring the absorbance of green colouration at 580 and 582 nm for I-a and I-b, respectively. The molar absorptivities for the green complexes are 3.65 and 4.1 x 10(4)-1 mol(-1) cm(-1) sensitivities are 8.1 and 7.2 ng cm for complexes of reagent I-a and I-b, respectively. The proposed method has been applied to determination of vitamin B-12 (cyanocohalamine phosphate) in pure form and in various pharmaceutical formulations.