Optimization of Cation Exchange Capacity Determination in Soil by Hexamminecobalt Trichloride Method
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摘要:
土壤阳离子交换量(CEC)是评价土壤保肥能力和改良土壤、合理施肥的重要依据，同时也能影响土壤中污染物的迁移转化。与传统的乙酸铵交换法、乙酸钙交换法测定CEC的方法相比，三氯化六氨合钴法的操作步骤简洁，检测效率高，试剂消耗量小，对大批量样品的分析有明显优势。但该方法的适用范围较小，对中性和碱性样品检测结果好，而对酸性样品的检测结果偏低至50%。为解决以上问题，本文以2mol/L氢氧化钠溶液调节pH至碱性，样品的CEC测定值有显著提高，酸性和碱性样品测定结果均可达到标准值范围内。通过绘制pH调节曲线，得到不同pH范围的土壤样品中加入2mol/L氢氧化钠溶液体积，以此调节未知样品的pH值，实现CEC的准确测定。在原有的方法基础上调节样品pH至碱性，优化后的方法精密度范围为1.02%～3.82%(n=6)。
Abstract:Compared with the traditional ammonium acetate exchange method and calcium acetate exchange method to determine cation exchange capacity (CEC), hexamminecobalt trichloride method has obvious advantages in the analysis of large quantities of samples. However, the application scope of this method is limited, and the detection results of neutral and alkaline samples are good, but the detection results of acidic samples are as low as 50%. In order to solve these problems, the pH was adjusted to alkaline with 2mol/L sodium hydroxide solution, the CEC value of the sample was significantly improved, and the determination results of both acidic and alkaline samples reached the range of standard values. By drawing the pH adjustment curve, the volume of 2mol/L sodium hydroxide solution was obtained in soil samples with different pH ranges, so as to adjust the pH value of unknown samples and realize the accurate determination of CEC. On the basis of the original method, the pH of the sample was adjusted to alkalinity, and the precision of the optimized method ranged from 1.02% to 3.82% (n=6). After optimization, the application scope of the method is expanded, the precision and accuracy are improved, and the detection efficiency of a large number of samples is effectively improved.
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表 1 标准物质和实际样品详细信息
Table 1. pH and CEC in reference materials and samples
标准物质 pH值 CEC标准值
(cmol+/kg)标准物质和实际样品 pH值 CEC标准值
(cmol+/kg)GBW07412 6.8 16.0±2.1 GBW07414a 8.18 17.0±1.0 GBW07413 8.15 13.0±1.1 GBW07460 8.5 9.6±1.3 GBW07414 8.18 22.4±1.7 样品1 6.23 — GBW07415 6.08 19.6±2.2 样品2 7.80 — GBW07416 4.71 11.2±1.5 样品3 5.51 — GBW07417 6.8 6.0±0.5 
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表 2 标准物质的pH值、标准值及不同pH条件下CEC的测定结果
Table 2. The pH, standard values of national reference materials and the determination results of CEC at different pH.
标准物质
编号调节pH前
CEC测定值
(cmol+/kg)调节pH后
CEC测定值
(cmol+/kg)CEC
标准值
(cmol+/kg)pH
标准值GBW07412 8.6 18.4 16.0±2.1 6.80 GBW07413 5.5 8.2 13.0±1.1 8.15 GBW07414 19.1 23.4 22.4±1.7 8.18 GBW07415 9.8 16.0 19.6±2.2 6.08 GBW07460 6.9 7.7 9.6±1.3 8.50
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表 3 不同土壤pH值范围下，2mol/L氢氧化钠溶液用量
Table 3. Dosage of 2mol/L NaOH solution under different soil pH ranges
土壤样品pH值范围 取用2mol/L氢氧化钠溶液
体积(mL)4～5 0.35 5～6 0.25 6～7 0.20 ＞7 0.15
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表 4 方法准确度和精密度实验结果
Table 4. Precision and accuracy tests of the method
标准物质编号 CEC标准值
(cmol+/kg)CEC单次测定值
(cmol+/kg)CEC测定平均值
(cmol+/kg)RSD
(%)GBW07412 16.0±2.1 16.3 16.6 15.7 16.2 2.09 16.5 16.1 15.9 GBW07413 13.0±1.1 10.7 11.0 10.6 10.9 2.81 11.0 11.4 10.7 GBW07414 22.4±1.7 21.1 21.2 21.1 21.3 1.02 21.5 21.4 21.6 GBW07415 19.6±2.2 17.0 17.3 17.3 17.3 1.06 17.2 17.5 17.4 GBW07416 11.2±1.5 11.4 10.6 11.4 11.3 3.82 11.9 11.4 11.2 GBW07417 6.0±0.5 6.5 6.5 6.5 6.4 1.61 6.4 6.2 6.5 GBW07414a 17.0±1.0 17.0 15.2 16.0 16.1 3.82 16.2 15.7 16.3 GBW07460 9.6±1.3 7.6 7.7 7.6 7.6 1.32 7.5 7.4 7.5 样品1 — 11.9 11.6 11.7 11.7 1.93 11.6 11.9 11.3 样品2 — 22.6 23.1 22.5 22.5 1.50 22.1 22.6 22.3 样品3 — 8.8 8.4 8.2 8.4 3.01 8.6 8.2 8.2
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