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 0.5ml of 96% concentrated H(2)SO(4) 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 750nm, packed in a 1.0mm cell, is measured directly. The molar absorptivities were found to be 2.11×10(7) and 3.90×10(7)Lmol(-1)cm(-1) for 500 and 1000ml, respectively. Calibration is linear over the range 0.05-1.45μgL(-1) with RSD of <1.85% (n=8.0). Using 35mg exchanger, the detection and quantification limits were 13 and 44ngL(-1) for 500ml sample, whereas for 1000ml sample were 8.0 and 27ngL(-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. |
