A sensitive and selective method for determination of Cr(VI) has been developed using chromium-catalyzed oxidation of
o-phenylenediamine with hydrogen peroxide. Under an optimized condition, the absorbance at 430 nm and at a fixed reaction time of 10 min was proportional to the concentration of Cr(VI) up to 1 mg L
−1. A detection limit (DL) was 1 μg L
−1. On-site determinations were carried out using a laboratory-made palm-top size colorimeter with a red-green-blue light emitting diode. Measurement of the absorbance for blue light was used for the determination up to 1 mg L
−1 with a DL of 1 μg L
−1. A kinetic study derived an on-site reaction time of τ = τ
oexp[
b(1/
T–1/
To)], where the standard reaction time τ
o = 10 min,
b = 7.65×10
3, a standard reaction temperature
To = 298 K. By this equation, τ giving an absorbance value at τ
o and
To can be calculated for any on-site temperature (
T) between 20 and 40°C. On-site analyses within an error of 6% were carried out without control of reaction temperature and with a laboratory-prepared calibration curve at τ
o and
To. The proposed method was successfully applied to natural and standard river-water samples and an industrial waste-water sample.
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