Determination of Hydrolysable Nitrogen in Soil Samples by Alkaline Hydrolysis Diffusion Separation Acid-base Titration Based on a Polypropylene Diffusion Dish
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摘要:
传统的碱解扩散分离-酸碱滴定法测定土壤样品中的水解性氮，通常采用玻璃扩散皿进行碱解扩散分离。但是使用该法对样品进行前处理和碱解扩散分离过程中，操作者常会面临如下三种困扰：一是玻璃扩散皿内室溶液极易被氢氧化钠溶液和碱性胶液污染；二是玻璃扩散皿和盖子之间容易出现氨气泄漏，而且发现时往往无法解决；三是玻璃扩散皿比较笨重易碎，实验操作不方便；最终导致测定结果不稳定性和操作者对该法体验不佳。本文采用聚丙烯扩散皿，通过改进扩散皿清洗方法和提升扩散皿密封性、统一还原剂加入量和氢氧化钠溶液浓度(1.8mol/L)、适当增加氢氧化钠溶液的加入体积和降低盐酸标准溶液浓度，提高了水解性氮测定的稳定性和准确性。该方法中发生的污染明显降低，避免了发生氨气泄漏，操作便捷。应用于分析土壤有效态标准物质的测定值与标准值一致，如水解性氮测定值< 50mg/kg的标准物质GBW07416a，其绝对偏差为0.2~1.8mg/kg；水解性氮测定值在50~200mg/kg的标准物质GBW07415a、NSA-1、NSA-4、NSA-5、NSA-6，其绝对偏差为0~4.0mg/kg。还原剂对硝态氮转化为铵态氮的加标回收率在89.6%~96.4%之间。该方法可满足测定土壤样品中水解性氮含量的要求。
Abstract:BACKGROUND The traditional alkaline hydrolysis diffusion separation acid-base titration method is used to determine the hydrolysable nitrogen in soil samples. Usually, a glass diffusion dish is used for alkaline hydrolysis diffusion separation. However, during sample pretreatment and alkaline hydrolysis diffusion separation, the operator often faces the following three problems. First, the solution in the inner chamber of the glass diffusion dish is very easily polluted by sodium hydroxide solution and alkaline glue solution. Second, ammonia leakage occurs easily between the glass diffusion dish and the cover, and it is often not possible to remedy when it is found. Third, the glass diffusion dish is bulky and fragile, and the experimental operation is inconvenient, all of which lead to the instability of measurement results due to inexperience of the operator.
OBJECTIVES To establish a new method for the determination of hydrolysable nitrogen in soil samples by alkaline hydrolysis diffusion separation acid-base titration based on polypropylene diffusion dish.
METHODS A polypropylene diffusion dish was used instead of a glass diffusion dish in the alkaline hydrolysis of hydrolysable nitrogen. The cleaning method and the sealing of the diffusion dish were improved. The addition amount of reducing agent and the concentration of sodium hydroxide solution (1.8mol/L) were unified. The addition volume of sodium hydroxide solution was appropriately increased, and the concentration of hydrochloric acid standard solution was reduced.
RESULTS The absolute deviation of reference materials GBW07416a with the measured value of hydrolysable nitrogen < 50mg/kg was 0.2-1.8mg/kg. The absolute deviation of reference materials GBW07415a, NSA-1, NSA-4, NSA-5 and NSA-6 with the measured value of hydrolysable nitrogen of 50-200mg/kg was 0-4.0mg/kg. The recovery rate of nitrate nitrogen converted to ammonium nitrogen by reducing agent was 89.6%-96.4%. The measured value of soil available reference materials was consistent with the standard value.
CONCLUSIONS The stability and accuracy of hydrolysable nitrogen determination are improved. The pollution in this method is significantly reduced, ammonia leakage is avoided, and the operation is convenient. The method meets the requirements for determining the content of hydrolysable nitrogen in soil samples.
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表 1 不同类型土壤有效态标准物质中硝态氮的加标回收率(n=6)
Table 1. Recovery rate of nitrate nitrogen in available reference materials of different types of soil (n=6)
样品名称 水解性氮标准值(mg/kg) 未加标样品水解性氮测定值(mg/kg) 加标量(以硝酸钾中氮元素含量计，μg) 加标样品水解性氮测定值(mg/kg) 加标回收率(%) GBW07415a(水稻土) 165±10 168 166 321 92.2 NSA-1(黑土) 166±7 166 166 326 96.4 NSA-4(紫色土) 64±4 63.9 69.2 130 95.5 NSA-6(红壤) 96±7 95.5 111 195 89.6 
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表 2 不同浓度盐酸标准溶液的滴定结果及对应的方法特性指标
Table 2. Titration results of different concentrations of hydroric acid standard solutions and corresponding method characteristic indexes
盐酸标准溶液浓度(mol/L) 可滴定水解性氮最大含量wN(mg/kg) 方法检出限MDL(mg/kg) 测定下限(mg/kg) 0.002 476 0.74 2.94 0.005 1190 1.84 7.35 0.01 2380 3.68 14.7
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表 3 土壤有效态标准物质水解性氮测定结果的绝对偏差和允许偏差
Table 3. Absolute deviation and allowable deviation of determination results of hydrolysable nitrogen in soil available standard materials
标准物质编号 土壤类型 水解性氮标准值(mg/kg) 水解性氮测定值(mg/kg) 水解性氮测定平均值(mg/kg) 绝对偏差(mg/kg) LY/T 1228—2015允许偏差(mg/kg) GBW07415a 水稻土 165±10 169 167 2.0 绝对偏差10~2.5 GBW07415a 水稻土 165±10 166 1.0 绝对偏差10~2.5 GBW07415a 水稻土 165±10 171 4.0 绝对偏差10~2.5 GBW07415a 水稻土 165±10 164 3.0 绝对偏差10~2.5 GBW07415a 水稻土 165±10 167 0 绝对偏差10~2.5 GBW07415a 水稻土 165±10 165 2.0 绝对偏差10~2.5 GBW07416a 红壤 44±4 42.9 43.4 0.5 绝对偏差 < 2.5 GBW07416a 红壤 44±4 42.7 0.7 绝对偏差 < 2.5 GBW07416a 红壤 44±4 45.2 1.8 绝对偏差 < 2.5 GBW07416a 红壤 44±4 43.8 0.4 绝对偏差 < 2.5 GBW07416a 红壤 44±4 43.2 0.2 绝对偏差 < 2.5 GBW07416a 红壤 44±4 42.5 0.9 绝对偏差 < 2.5 NSA-1 黑土 166±7 163 163 0 绝对偏差10~2.5 NSA-1 黑土 166±7 167 4.0 绝对偏差10~2.5 NSA-1 黑土 166±7 165 2.0 绝对偏差10~2.5 NSA-1 黑土 166±7 162 1.0 绝对偏差10~2.5 NSA-1 黑土 166±7 159 4.0 绝对偏差10~2.5 NSA-1 黑土 166±7 163 0 绝对偏差10~2.5 NSA-4 紫色土 64±4 62.5 62.2 0.3 绝对偏差10~2.5 NSA-4 紫色土 64±4 64.2 2.0 绝对偏差10~2.5 NSA-4 紫色土 64±4 61.7 0.5 绝对偏差10~2.5 NSA-4 紫色土 64±4 61.4 0.8 绝对偏差10~2.5 NSA-4 紫色土 64±4 62.2 0 绝对偏差10~2.5 NSA-4 紫色土 64±4 60.9 1.3 绝对偏差10~2.5 NSA-5 水稻土 180±10 174 176 2.0 绝对偏差10~2.5 NSA-5 水稻土 180±10 178 2.0 绝对偏差10~2.5 NSA-5 水稻土 180±10 176 0 绝对偏差10~2.5 NSA-5 水稻土 180±10 175 1.0 绝对偏差10~2.5 NSA-5 水稻土 180±10 175 1.0 绝对偏差10~2.5 NSA-5 水稻土 180±10 179 3.0 绝对偏差10~2.5 NSA-6 红壤 96±7 97.3 98.2 0.9 绝对偏差10~2.5 NSA-6 红壤 96±7 99.2 1.0 绝对偏差10~2.5 NSA-6 红壤 96±7 98.8 0.6 绝对偏差10~2.5 NSA-6 红壤 96±7 100 1.8 绝对偏差10~2.5 NSA-6 红壤 96±7 94.4 3.8 绝对偏差10~2.5 NSA-6 红壤 96±7 99.5 1.3 绝对偏差10~2.5
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表 4 土壤中水解性氮测定的不同测定方法比对与综合评价
Table 4. Comparison and comprehensive evaluation of different methods used in determination of hydrolysable nitrogen in soil
测定方法名称 相对标准偏差RSD(%) 相对误差(%) 方法优势 存在问题 参考文献 基于塑料带孔扩散皿的碱解扩散分离-酸碱滴定法 1.63~3.19 0~2.28 无碱性胶液，精密度和准确度好 碱解过程中，如果发生氨气泄漏无法被发现 [24] 基于玻璃扩散皿的碱解扩散分离-酸碱滴定法 3.09~5.47 1.23~2.42 准确度好 扩散皿内室容易被污染，密封性不强，测定结果不够稳定 [20] 基于乳胶塞注射式玻璃扩散皿的碱解扩散分离-酸碱滴定法 1.97~2.84 0.61~2.46 精密度和准确度好，密封性好 扩散皿内室容易被污染，需额外操作注射器 [26] 基于FOSS Kjeltec 8400凯氏定氮仪碱解蒸馏-酸碱滴定法 1.10~2.13 3.40~7.33 操作简单，精密度较高，抗污染能力强 高温蒸馏，测定结果可能偏高 [17] 基于聚丙烯扩散皿的碱解扩散分离-酸碱滴定法(本文方法) 1.56~2.30 1.21~1.40 精密度和准确度好，密封性好 流程偏长 本文
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