Chromium speciation in environmental samples using a solid phase spectrophotometric method

A solid phase extraction technique is proposed for preconcentration and speciation of chromium in natural waters using spectrophotometric analysis. The procedure is based on sorption of chromium(III) as 4-(2-benzothiazolylazo)2,2′-biphenyldiol complex on dextran-type anion-exchange gel (Sephadex DEAE A-25). After reduction of Cr(VI) by 0.5 ml of 96% concentrated H2SO4 and ethanol, the system was applied to the total chromium. The concentration of Cr(VI) was calculated as the difference between the total Cr and the Cr(III) content. The influences of some analytical parameters such as: pH of the aqueous solution, amounts of 4-(2-benzothiazolylazo)2,2′-biphenyldiol (BTABD), and sample volumes were investigated. The absorbance of the gel, at 628 and 750 nm, packed in a 1.0 mm cell, is measured directly. The molar absorptivities were found to be 2.11 × 107 and 3.90 × 107 L mol−1 cm−1 for 500 and 1000 ml, respectively. Calibration is linear over the range 0.05–1.45 μg L−1 with RSD of <1.85% (n = 8.0). Using 35 mg exchanger, the detection and quantification limits were 13 and 44 ng L−1 for 500 ml sample, whereas for 1000 ml sample were 8.0 and 27 ng L−1, respectively. Increasing the sample volume can enhance the sensitivity. No considerable interferences have been observed from other investigated anions and cations on the chromium speciation. The proposed method was applied to the speciation of chromium in natural waters and total chromium preconcentration in microwave digested tobacco, coffee, tea, and soil samples. The results were simultaneously compared with those obtained using an ET AAS method, whereby the validity of the method has been tested.

Graphical abstractThe log–log plot of the distribution ratio vs. BTABD concentration; conditions, pH 7.5, 0.6 mg L−1 Cr(III), 75 mg Sephadex DEAE A-25, stirring time 20 and 30 min for 500 and 1000 ml samples, respectively.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Chromium can be determined efficiently with BTABD using SPS. ► The proposed method has several advantages to warrant its use. ► The proposed method has been applied to determine chromium in natural waters. ► The proposed method is simple and more sensitive than other methods commonly used at microgram level.