Improvement Method of HJ 1082-2019 for Determination of Hexavalent Chromium in Agricultural Soil with High Organic Matter

In view of some disadvantages of HJ 1082-2019 for the determination of hexavalent chromium in agricultural soil with high organic matter, the method of this standard was improved. The main improvement measures mainly included the following five aspects. Firstly, ethanol was added into the alkaline extract, and the co-solvent effect of ethanol was used to make the extract fully contact with the organic matter and minerals in the soil in homogeneous phase. Secondly, sodium carbonate-sodium bicarbonate instead of phosphate as buffer solution was used to avoid the inclusion of hexavalent chromium by the associative precipitation with phosphate and ions of calcium and magnesium leached from soil. Thirdly, magnesium chloride was added, and the strong ion exchange ability of magnesium ion was used to exchange with hexavalent chromium in soil. Fourthly, double frequency orthogonal ultrasonic instead of stirring was used to fully decompose and degrade organic matter, especially humic acid. Fifthly, the use of conical bottle and reflux pipe instead of polyethylene film sealing was used to reduce the loss of hexvalent chromium through reflux. It was shown that when the weighing sample was 3.000 0 g, the addition amount of extract was 40 mL, the addition amount of magnesium chloride was 0.4 g, and the addition of sodium bicarbonate-sodium bicarbonate buffer solution was 0.6 mL, the extraction effect was better. The linear range of the standard curve of hexavalent chromium was within 5.00 mg·L^-1, with detection limit (3.143s) of the improved method of 0.09 mg·kg^-1, RSDs (n=7) of the determined values for the actual samples less than 0.30%, and the recoveries in the range of 94.0%-100%. By comparing the improved method with HJ 1082-2019, the determined values of hexavalent chromium obtained by the improved method were higher than those obtained by HJ 1082-2019, and the recoveries of the improved method were better, indicating that the improved method was accurate and feasible.