OBJECTIVESChromium is usually found as Cr(Ⅲ) and Cr(Ⅵ) in environmental water samples. The different valences of chromium produce different physiological function, thus it is necessary to analyze the different valences of chromium accurately. At present, the contents of Cr(Ⅲ) and Cr(Ⅵ) are mostly determined after separation or the content of Cr(Ⅲ) or Cr(Ⅵ) is measured first, then the total content of Cr is determined after oxidation or reduction. The content of another valence of chromium is then calculated by the subtraction method. The available method needs a complex procedure. Moreover, the valence of Cr is easily modified during the sample treatment, resulting in large error and low precision.

OBJECTIVESTo find a simple and accurate method for determination of Cr(Ⅲ) and Cr(Ⅵ).

METHODSThe first derivative spectrophotometric method was used for simultaneously determining of Cr(Ⅲ) and Cr(Ⅵ), which eliminated the interference of Cr(Ⅲ) on Cr(Ⅵ).

RESULTSChromogenic reagent EDTA-2Na was added to the mixed water sample in a 70℃water-bath at pH 3-3.5 for 15min, and the absorbance was measured. When the derivative interval factor was 10nm, the first derivative value of absorption for Cr(Ⅵ) was the maximum at 330nm wavelength and the value for Cr(Ⅲ)-EDTA was zero. The concentration of Cr(Ⅵ) can be obtained by the first derivative spectrophotometric method, whereas Cr(Ⅲ) can be determined directly at the maximum wavelength of 543nm. Under the optimal conditions, the concentration range was 0-100mg/L for Cr(Ⅵ) and 0-120mg/L for Cr(Ⅲ). The equation of linear regression for Cr(Ⅲ) was A=0.0036ρ-0.0002 (r²=0.9999), for Cr(Ⅵ) was D=0.00072ρ-0.00013 (r²=0.9996). and the limit of detection was 0.005mg/L for Cr(Ⅵ) and 0.006mg/L for Cr(Ⅲ). The recoveries for Cr(Ⅲ) and Cr(Ⅵ) were 97.8%-102.6%.

ConclusionThe method meets the requirements for analyzing waste water.