Separation and Determination of Cr(III) and Cr(VI) with Cation-Exchange Chromatography and Atomic Absorption Spectroscopy. An Experiment for Quantitative Methods of Analysis

Abstract

This article reports on the separation and determination of Cr(III) and Cr(VI) with cation-exchange chromatography and flame atomic absorption spectroscopy. A sample containing the two ions is added to a column containing a cation-exchange resin; the Cr(VI) ions are not retained by the resin and are collected in the effluent. The Cr(III) ions are retained by the resin and eluted with an HCl solution. The separated ions are aspirated into an air/acetylene flame and the absorbance measurements are made at λ = 357.9 nm and 0.2-nm slit width. The experiment has been implemented in a quantitative methods of analysis course. The results of the analysis of synthetic water samples obtained by students majoring in various areas of science are presented. The data showed that 58% of the Cr(VI) and 39% of the Cr(III) values had relative errors in the range of ±5%. The evaluation of the precision of the student results revealed that 48% of the Cr(III) and 81% of the Cr(VI) reported values had RSD ≤ 4%. This experiment introduces the students to the concept of chemical speciation and gives experience in combining analytical techniques to solve chemical problems.