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    水洗除氯-重铬酸钾滴定法测定新疆盐碱地土壤中有机质的含量

    Determination of Organic Matter in Saline Alkali Soil from Xinjiang by Potassium Dichromate Titration with Water Washing for Chlorine Removal

    • 摘要: 针对新疆盐碱地土壤样品中氯含量高,直接使用重铬酸钾滴定法检测时有机质测定结果偏高的问题,探究了氯离子对有机质测定的影响,提出先水洗除氯再采用重铬酸钾滴定法测定的方法。此外,采用离子色谱仪和总有机碳测定仪分别测定了水洗后上清液中氯离子和水溶性有机质的含量,并比较了水溶性氯校正系数扣除法的测定效果以及水溶性有机质对实际测定的影响。样品经风干、过筛后,称取0.100 0~0.500 0 g,加入约5 mL去离子水,于室温振荡3 min,离心3 min,上清液收集于25 mL容量瓶中,重复该过程1次,合并上清液,用水定容,用于水溶性有机质和水溶性氯含量的测定。将残渣于105 ℃烘2 h,加入10 mL含0.4 mol·L−1重铬酸钾的50%(体积分数)硫酸溶液,摇匀后置于185 ℃可控温油浴锅中,沸腾5 min。冷却,将消煮液及土壤残渣转移至150 mL锥形瓶中,加入3滴含14.9 g·L−1邻菲啰啉的10 g·L−1硫酸亚铁铵溶液,用0.100 0 mol·L−1硫酸亚铁标准溶液滴定剩余的重铬酸钾,当溶液变至棕红色时停止滴定,计算非水溶性有机质的含量。结果表明:当水溶性氯质量分数不小于7.00 g·kg−1时,存在氯干扰;水洗样品2次或采用水溶性氯校正系数扣除法均可消除不大于49.05 g·kg−1水溶性氯产生的干扰;水溶性有机质含量占非水溶性有机质的不足5.0%,对检测结果的影响可忽略不计;对由氯化钠和土壤有效态成分分析标准物质GBW07459a组成的标准样品进行重复测定,测定值的相对标准偏差(n=7)不大于2.0%,测定值与认定值的相对误差为−0.59%~2.3%。

       

      Abstract: To address the issue of elevated organic matter determination results caused by high chloride content in saline alkali soil samples from Xinjiang when using the potassium dichromate titration directly, the effect of chloride ions on organic matter determination was investigated. A method was proposed, which involved washing the samples to remove chloride prior to organic matter determination using the potassium dichromate titration. Additionally, ion chromatograph and total organic carbon analyzer were employed to determine the chloride ion and organic matter in the supernatant after washing, the effectiveness of the water-soluble chlorine correction factor deduction method was compared, and the effect of water-soluble organic matter on the actual determination was explored. After air-drying and sieving the samples, an aliquot (0.100 0-0.500 0 g) was taken, and approximately 5 mL of deionized water was added. The mixture was oscillated at room temperature for 3 min, and centrifuged for 3 min, and the supernatant was collected in a 25 mL-volumetric flask. This process was repeated once, and the supernatants were combined and their volume was made up with water for determination of the water-soluble organic matter and the water-soluble chloride. The precipitate was dried at 105 °C for 2 h, and 10.00 mL of 50% (volume fraction) sulfuric acid solution containing 0.4 mol·L-1 potassium dichromate was added. After shaking, the mixture was placed in a temperature-controlled oil bath pot at 185 °C and boiled for 5 min. After cooling, the digested solution and soil residue were transferred to a 150 mL-conical flask, and 3 drops of a 10 g·L-1 ammonium ferrous sulfate solution containing 14.9 g·L-1o-phenanthroline were added. The remaining potassium dichromate was titrated with a 0.100 0 mol·L-1 ferrous sulfate standard solution until the solution turned reddish-brown, at which point the titration was stopped. Non-water-soluble organic matter was then calculated. It was shown that when the mass fraction of water-soluble chloride was not more than 7.00 g·kg-1, chloride interference was present. Washing the samples 2 times or applying the water-soluble chloride correction factor deduction method could eliminate interference from water-soluble chloride up to 49.05 g·kg-1. The content of water-soluble organic matter accounted for less than 5.0% of that of the non-water-soluble organic matter, and its effect on detection results was negligible. Repeated determinations of standard samples composed of sodium chloride and the soil effective component analysis reference material GBW07459a were conducted, giving RSDs (n=7) of the determined values not more than 2.0%, and relative errors between the determined values and certified values in the range of −0.59%-2.3%.

       

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