RAPID PHOTOMETRIC DETERMINATION OF COBALT IN COBALT-BASE ALLOY PLATING SOLUTION

In an ammoniacal buffer solution of pH 11, a stable complex is formed between 2-(1-hydroxy-4,6-dinitro-2-phenylazo)-1,8-dihydroxynaphthalene-3,6-disulfonic acid (or named briefly as picramazo chrom) and cobalt(Ⅱ) ion. The absorption maxima of this complex was found at the wavelength of 650nm. Beer′s law holds in the range of 0～60μg of cobalt per 25ml of solution, with a molar absorptivity of (e_(650)) 3.1×10~4L·mol~(-1)·cm~(-1). This color reaction has the special feature of allowing the presence of relatively large amount of nickel (Co∶Ni=1∶52). Magnesium(Ⅱ) interferes seriously to the color reaction of cobalt (Co∶Mg=1∶0.5). This color reaction has been successfully applied to the direct determination of cobalt in cobalt-base alloy plating solution, since co-existing cations in this alloy plating solution do not interfere with the color reaction of cobalt. RSD′s obtained in the sample analysis were less than 1%.