鉴于用HJ 1082-2019测定高有机质农业土壤中六价铬含量时存在的一些弊端,对该标准方法进行了改进。改进措施主要有以下5个方面:一是向碱性提取剂中加入乙醇,利用乙醇的共溶剂效应使提取剂与土壤中的有机质、矿物质等在均相中充分接触;二是以碳酸钠-碳酸氢钠代替磷酸盐作为缓冲溶液,避免六价铬被磷酸根与土壤中浸出的钙、镁离子缔合成的沉淀包裹;三是加入氯化镁,利用镁离子较强的离子交换能力,使其与土壤中六价铬进行交换;四是采用双频正交超声代替搅拌,以充分分解、降解有机质,特别是腐殖酸;五是采用锥形瓶和回流管代替聚乙烯膜封口,利用回流以减少六价铬的损失。结果表明:当称样量为3.000 0 g,提取剂加入量为40 mL,氯化镁加入量为0.4 g,碳酸钠-碳酸氢钠缓冲溶液加入量为0.6 mL时,可达到较好的提取效果。六价铬标准曲线的线性范围在5.00 mg·L^-1以内,改进方法的检出限(3.143s)为0.09 mg·kg^-1,实际样品测定值的相对标准偏差(n=7)小于0.30%,回收率为94.0%~100%。将改进方法与HJ 1082-2019进行比对,改进方法得到的六价铬测定值均高于HJ 1082-2019的,且前者的回收率更好,说明改进方法准确度较高,方法可行。

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.