Multi-wavelength spectrophotometric determination of Cr(VI) in water with ABTS

A new method for Cr(VI) (0.025-8.00 mg/L) determination based on the reaction of Cr(VI) and 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonate) (ABTS) in aqueous solutions was developed. The colorless ABTS reacted with Cr(VI) under strong acidic conditions ([H⁺] = 3.0 or 6.0 M) producing a stable colored radical ABTS⁺, which could be measured spectrophotometrically at 415, 649 or 732 nm. The absorbance increase at these three wavelengths for ABTS⁺ generation were all linear (less than 1.0% deviation) with respect to the amount of added Cr(VI) and the sensitivity were 1.099 × 10⁵, 3.720 × 10⁴, and 4.150 × 10⁴ M^-1 cm^-1 of added Cr(VI) at 415, 649 or 732 nm, respectively. Cr(VI) of 0.025 mg/L below the discharge standard of drinking water could be detected with the method quantitatively and 0.002 mg/L Cr(VI) was determined qualitatively. The molar absorptivity of ABTS⁺ generated was determined to be (3.69 ± 0.01) × 10⁴ M^-1 cm^-1 at 415 nm and the reaction between Cr(VI) and ABTS had a stoichiometric factor of 1:3 in excess of ABTS. The absorbance of generated ABTS⁺ was found to be stable in deionized water or wastewater and Cr(VI) spiked in wastewater could be determined accurately. The ABTS method also had a good anti-interference performance against Co(II) ions. Moreover, the ABTS method could be successfully used in the Cr(VI)-S(IV) system for Cr(VI) determination.