A new colorimetric determination of diquat produced with several moderate reductants

Abstract

A new colorimetric method is described for the quantification of diquat. A red colored compound is produced on the reduction of diquat with either 2-mercaptoethanol, dl-dithiothreitol or l-cysteine in 0.2 N NaOH solution. The absorption maximum of the red colored compound is 495 nm and the molar absorption coefficient is 2.072 × 10 4 (0.1125 in 1 μg diquat/ml with 1 cm light path). The red color is clearly visible in 5.43 × 10 −6 M solution (= 1 μg/ml) suggesting that this reaction is applicable as a color test of diquat. The absorption at 495 nm shows a linear concentration dependence for the range 0.1 – 10 μg diquat/ml. The fading of the color is less than 5% after 1 h. Under the same condition, paraquat does not produce any colored products. Therefore, in a mixed solution containing both diquat and paraquat, the concentration of diquat can be determined at first from the absorption at 495 nm after reduction with 2-mercaptoethanol. Then by adding sodium dithionite to the solution, the concentration of paraquat is calculated by subtracting the contribution of diquat from the total optical density at 600 nm of the mixture.