ON THE SPECTROPHOTOMETRIC FLOW-INJECTION DETERMINATION OF CHROMIUM(VI) IN NATURAL-WATERS AFTER ONLINE PRECONCENTRATION ON ACTIVATED ALUMINA

The feasibility of chromium(VI) preconcentration on to activated alumi na in a continuous flow system with spectrophotometric detection was i nvestigated. Chemical and flow variables, and the influence of concomi tant species were studied both with and without preconcentration syste ms. The best results were obtained by using a 2.5 cm long, 1.6 mm i.d. alumina minicolumn, and selecting 1 x 10(-4) M nitric acid as the pre concentrating medium and 0.1 M ammonium hydroxide as the eluent. The e luted chromium(VI) was mixed with diphenylcarbazide in acidic medium a nd the absorbance of the colored complex was measured at 540 nm. Linea r calibrations for 5, 25 and 50 mi sample volumes were established ove r the concentration ranges 10-50 mu g l(-1), 2-10 mu g l(-1) and 1-5 m u g l(-1) with sensitivity enhancements of 44, 196 and 392 and detecti on limits (3 sigma) of 3.0 mu g l(-1), 0.3 mu g l(-1) and 0.2 mu g l(- 1), respectively. The method is relatively fast and cheap. Natural wat ers were analyzed with use of the developed procedure.