Specific extraction of chromium as tetrabutylammonium-chromate and spectrophotometric determination by diphenylcarbazide: Speciation of chromium in effluent streams

A very specific, selective, simple and inexpensive procedure was developed for the speciation of Cr-VI and Cr-III. This method is based on the quantitative extraction of chromate and Cr-III (previously oxidized to Cr-VI) as a tetrabutylammonium-chromate ion-pair in methyl isobutyl ketone (MIBK), and then back extraction and preconcentration with an acidic diphenylcarbazide (DPC) solution. Back extraction was applied to achieve further preconcentration by a final factor of 20. The Cr-VI-DPC complex was determined in back-extract by a spectrophotometer at 548 nm. Under these extraction conditions, most of the probable concomitant cations and anions remained in the first inorganic phase. The calibration curve was linear up to 0.14 mug L-1 of Cr-VI with a detection limit of 2.22 ng L-1. The developed procedure was found to be suitable for the determination of the Cr-VI and Cr-III species in various natural water samples with a relative standard deviation of better than 1.6%. The method was successfully applied to the speciation of chromium in spiked natural water samples, and also samples of effluent from a leather treatment plant.