Abstract

Automatic procedures for the determination of chromium in liquors, tannery effluent and leathers have been investigated. Two methods of oxidation have been considered, perchloric acid in the Technicon Digestor Unit and ammonium persulphate at 95°c. The first can be followed either by direct colorimetric determination as the chromate ion, provided con centrations are high enough, or after reaction with diphenylcarbazide, the second only by reaction with diphenylcarbazide. Oxidation with perchloric acid was difficult to control, careful matching of standards was required and separation of samples was not very good. The persulphate oxidation procedure was, therefore, preferred. Although the persulphate can only deal with a limited amount of organic matter, the high sensitivity of the diphenylcarbazide method generally allows for dilution to a suitable level before oxidation. The method covers a concentration range of 0—03 mg Cr203 per litre and higher concentrations by the intro duction of appropriate manual or automatic dilution steps. The method is applicable to leathers containing 03 °/ C023r and up wards; these can be conveniently brought into solution by heating in acid. Chromium in Kjeldahl digests and acid hydrolysates can be determined under carefully controlled conditions, thus making possible simultaneous determinations of chromium and nitrogen (hide substance and chromium and hydroxyproline (collagen). For many purposes it would be more logical to express chrome contents on a hide substance or collagen rather than a weighs basis and such procedures would enable this to be done relatively ea%ily without the necessity to weigh the sample.