Photometric titrations—V A selective, chelometric determination of cobalt

Cobalt can be titrated with good selectivity by converting it with hydrogen peroxide in alkaline medium to the cobalt^III EDTA complex and back-titrating photometrically in acid medium the excess EDTA with bismuth solution, using pyrocatechol violet as indicator. No interference is observed with alkalies, alkaline earths, Ni, Cu, Cd, Zn, Pb, Al, U, and W. Bi, Zr, In, Ga and other ions forming very stable EDTA complexes are co-titrated. Mo, Fe, Ag and Hg interfere and should be separated. The interference of Mn can be excluded by addition of ascorbic acid. Th can be masked with sulphate. Cr^III is co-titrated, but by applying a difference method operating with two aliquots, both Cr and Co can be determined. Complexing anions interfere, as does chloride, which causes a turbidity with bismuth.