Rapid determination of chemical oxygen demand by a semi-automated method based on microwave sample digestion, chromium(VI) organic solvent extraction and flame atomic absorption spectrometry

In the present work, a new flow injection analysis (FIA) manifold for the rapid determination of chemical oxygen demand is presented and evaluated. The FIA manifold combines sample digestion by means of an oxidant solution (i.e., potassium dichromate and mercury sulfate) and an acid solution (sulfuric acid and silver sulfate) in a domestic microwave oven with the liquid-liquid extraction of Cr(VI) with tributyl phosphate (TBP). Finally, the remaining (non-reduced) Cr(VI) is determined in the organic phase by atomic absorption spectrometry. The digestion of the organic matter is carried out in the stopped-flow mode. Potassium hydrogenphthalate has been used for optimization of the method and as a standard to check its performance. The optimized conditions (oxidation) are: digestion time, 7 min; acid concentration, 67% (v/v). The optimized conditions for the extraction of the remaining Cr(VI) are: acid concentration, greater than or equal to 30% (v/v); TBP, 1 ml. The proposed method has been successfully applied, with the exception of pyridine, to several organic compounds and waters. A semi-micro method has been used for comparison. The application range extends from 40 to 12 000 mg O-2 l(-1), whereas the lowest Limits of detection and quantification are 13.5 and 45 mg O-2 l(-1), respectively. Following this method, the interference of Cl- can be compensated for at least up to 10 000 mg l(-1) Cl-. Up to nine samples per hour can be analyzed. (C) 1998 Elsevier Science B.V. All rights reserved.