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)2,20-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)2,20-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 lg 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.
|
