QUANTIFICATION OF TOTAL CHROMIUM AND HEXAVALENT CHROMIUM IN WATER BY ELECTROTHERMAL ATOMIC ABSORPTION SPECTROMETRY

Abstract

The two primary oxidation states of chromium in natural waters, Cr(Ill) and Cr(VI), differsignificantly in biological, geochemical and toxicological properties. Whereas Cr(Ill) isconsidered essential for human in glucose, lipid and protein metabolism, Cr(VI) is toxicbecause of its ability to oxidize other species and its adverse effects on the lung, liver andkidney. Because of the different toxicities and bioavailability of Cr(Ill) and Cr(VI),determination of the total chromium content does not give full information about possiblehealth hazard. Hence monitoring of the concentration of the separate chromium species isof great importance. Many different techniques have been in use for Cr containing samplespreparation and metal ions speciation: ion chromatography, flow injection analysis, andatomic absorption spectrometry (AAS).

Procedures for the quantification of total chromium and hexavalent chromium in watersamples arc presented. For the quantification of total chromium and hexavalent chromiumin water Chrornabond NH2 columns (arninopropyl phase with a 3ml volume and 500mg ofsorbent) obtained from Machery- Nagel (Duren, Germany) were used.

The pH value of the water sample was adjusted to 5.5 using acetic acid or sodium acetateand sample was aspirated through the previously conditioned column. The column contentswere dried under vacuum and the hexavalent chromium selectively linked was eluted withnitric acid and quantification was performed by Electrothermal Atomic AbsorptionSpectrometry (ET AAS). For the detection of total chromium, Cr(Ill) was transformed intoCr(VI) by oxidizing the sample with 1% K2S20S solution and AgNOJ at 100°C for 15 min.Oxidized solution was eluted through a Chromabond column and total Cr level wasquantified by ET AAS using the same instrumental conditions as for hexavalent chromium.Total chromium was also quantified directly in the water samples using ET AAS. Thetemperature programme of the graphite furnace, the use of chemical modifiers, the atomictechnique employed and the effectiveness of deuterium background correction wereinvestigated. Chromium was reliably determined by without chemical modifiers orbackground correction.

The detection limits were 0.4 and 0.5~gll for total chromium and hexavalent chromiumrespectively. The linearity changed under the optimized conditions was 0.4 - 50j.lgll and0.5-50j.lgll and the relative standard deviation was less than 3.5%. The validation of bothprocedures was performed by the standard addition method and the recoveries were higherthan 96% in all cases. It is proved that the method can be successfully employed as analternative to the commonly used preconcentration and speciation analytical techniques.The direct procedure was adopted for the estimation of total chromium in water samples becauseboth procedures applied for total chromium gave similar results. The methods were applied to thedetermination of total chromium and hexavalent chromium in 40 water samples.