Photometric Determination of Trace Amount of Cr(Ⅲ) Based on the Colour Fading of the Association System of Cr(Ⅲ) Potassium Thiocyanate-Methylene Blue

In acetate buffer of pH 5.7, Cr(Ⅲ) reacts with potassium thiocyanate and methylene blue (MB) to form a stable ion-association complex of ［MB］3［Cr(SCN)6］, leading to colour fading of methylene blue as measured at its absorption maximum of 664 nm. Linear relationship between values of decreasin absorbance and concentration of Cr(Ⅲ) was obtained in the range of 0.006-0.30 mg·L-1. Detection limit (3s/k) found was 5.4 μg·L-1. Apparent molar absorptivity was found to be 1.04×105L·mol-1·cm-1. The proposed method was applied to the determination of Cr(Ⅲ) in electroplating waste water samples, giving values of average recovery of 98.1% and RSD′s (n=5) less than 2%.