The estimation of cobalt

Abstract

A critical survey of the methods and primary standards used in the estimation of macro quantities of cobalt has been made. This indicates that even the most recommended methods contain unsatisfactory features. Further, it shows that not only have a wide variety of cohalt compounds been used as primary standards, hut many stock solutions prepared for use in investigating some methods, have been standardised using other methods, very often of doubtful accuracy. A number of cobalt primary standards have been compared by analysis under strictly controlled conditions. The results indicate that many standards, including Co(ll) sulphate and electrolytic cobalt should he rejected is recommended as a good standard. A number of cobalt analytical methods have been examined experimentally. The electrolytic and sulphate methods have been rejected for the accurate estimation of cobalt. In the former a contaminated deposit was obtained under all conditions used. The a nitroso B naphthol, ferricyanide, cyanide, E.D.T.A., and perborate methods have likewise been rejected for accurate work. The anthranilate method has been found to give slightly high results, probably due to contamination by the reagent. Its use is not recommended. The methods chosen for the most detailed examination were the phosphate, electroytic, anthranilate and carbonate complex methods. It has been hoped that a comparison might be made of the accuracy and precision of these methods. Where a potentially good method has not been acceptable previously because of some defect, an attempt has been made to remedy this, and where methods have been thought capable of further development, this has been attempted. The phosphate method is an example of the former; a drawback in this method, the solubility of cobalt ammonium phosphate, has been overcome by incorporating a rapid spectrophotometric method of estimating the filtrate cobalt with Nitroso-R-salt. As an example of the latter, the now standard ferrous iron-dichromate titration has been used in a volumetric method based on initial oxidation of cobalt by hydrogen peroxide in the presence of potassium bicarbonate. Both these methods, in the forms developed, have been found to give high accuracy. Where this is desired, their use is recommended in place of other methods shown to be unsatisfactory in the present work.