Method Optimization for the Determination of Soil Organic Carbon and Its Components by Automatic Titrator
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摘要:
土壤有机碳及其组分(颗粒有机碳、矿物结合态有机碳等)是反映土壤质量的关键性指标，开展准确高效地测定这些指标对相关研究具有重要意义。自动滴定仪与传统人工滴定相比，人员工作强度低，检测准确，但检测效率不及人工滴定的30%。为解决此问题，本文探讨了4种氧化剂加入量对测定有机碳的影响，研究了提前预加滴定液对检测效率提高的效果。最终优选氧化剂加入量为2mL，预加滴定液量以空白样滴定量的1/3计，建立了用于测定土壤有机碳的自动滴定仪法。采用有机碳不同水平的土壤样品和标准物质对方法进行验证，并与人工滴定进行比较，结果表明，自动滴定仪法与人工滴定法无显著性差异，方法的相对标准偏差(RSD，n=6)在2.10%～12.96%，加标回收率在93.37%～98.06%，标准物质相对误差为4.31%～4.79%；优化后的自动滴定法单个试样滴定用时由11min缩短到3.5min，整体检测效率高于人工滴定，而试剂耗用量仅是人工滴定法的40%。自动滴定仪法显著提升了有机碳的检测能力。
Abstract:The rapid and accurate detection of soil organic carbon and its components (such as particulate organic carbon and mineral-associated organic carbon) is of great significance because they are the key indicators reflecting soil quality. Compared with manual titration, the automatic titrator has a low work intensity and accurate detection, but its detection efficiency is less than 30% of that of manual titration. To solve this problem, the influence of the addition amount of four oxidants on the determination of organic carbon was explored, and the effect of pre-adding titrant in advance on the improvement of detection efficiency was studied. Finally, the optimal addition amount of oxidant was 2mL, and the amount of the pre-added titrant was one-third of the titration amount of the blank sample. An automatic titrator method for the determination of soil organic carbon was established. This method was verified by using soil samples with different levels of organic carbon and reference substances and was compared with manual titration. The results showed that there was no significant difference between the automatic titrator and the manual titration. The relative standard deviation (RSD, n=6) of the method was 2.10%−12.96%, the recovery rate of standard addition was 93.37%−98.06%, and the relative error of the reference material was 4.31%−4.79%. The titration time of a single sample was shortened from 11min to 3.5min, and the overall detection efficiency was higher than that of manual titration. The reagent consumption was only 40% of that of manual titration. The automatic titrator significantly improved the detection ability of organic carbon.
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表 1 氧化剂不同加入量的比较(n=10)
Table 1. Comparison of different addition amounts of oxidants (n=10)
氧化剂加入量 有机碳含量(g/kg) 空白耗用硫酸亚铁量
(mL)滴定用时
(min)终点颜色稳定性 显著性 含量范围 5+5 0.054 3.65～48.91 20 10 稳定 3+3 3.78～47.17 12 6 稳定 2+2 3.61～48.56 8 3.5 稳定 1+1 3.44～48.98 4 2.5 不稳定 注：显著性为有机碳含量主体间效应检验结果。 
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表 2 不同称样量的可测有机碳含量比较
Table 2. Comparison of measurable organic carbon content with different sample weighing amounts
称样量
(g)可测有机碳
含量最高值
(g/kg)称样量
(g)可测有机碳
含量最高值
(g/kg)0.0200 176.00 0.1000 35.20 0.0400 88.00 0.2000 17.60 0.0600 58.67 0.3000 11.73 0.0800 44.00 0.4000 8.80 注：以上可测有机碳含量最高值均按空白样滴定量8.40mL、最小滴定量2.8mL进行计算。
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表 3 两种检测方法的配对样本t检验结果
Table 3. Paired sample t-test of the two detection methods
测定方法 有机碳含量测定平均值
(g/kg)样本数量
N标准差 成对样本相关性
(双侧p)t 显著性
(双侧p)自动滴定仪法 16.56 57 14.59 r=0.997 −1.780 0.081 人工滴定法 16.83 57 14.48
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表 4 两种方法的精密度
Table 4. Precision of the two methods
测定方法 土壤样品
编号有机碳含量6次测定值(g/kg) 有机碳含量平均值
(g/kg)RSD
(%)1 2 3 4 5 6 自动滴定仪法 12 13.01 12.62 12.85 12.94 12.88 12.29 12.76±0.27 2.10 15 8.48 8.75 8.36 8.08 9.18 8.96 8.63±0.41 4.70 24 46.61 47.75 49.85 46.63 49.04 50.27 48.36±1.60 3.30 52 4.96 3.66 3.58 3.86 4.25 4.50 4.13±0.54 12.96 人工滴定法 12 13.19 14.10 11.17 13.18 13.70 12.40 12.96±1.13 8.72 15 8.50 8.58 6.79 8.81 9.67 8.79 8.52±1.05 12.29 24 48.41 52.27 48.21 50.72 50.78 48.67 49.84±1.65 3.31 52 3.87 3.54 3.29 4.46 4.40 3.94 3.92±0.52 13.20
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表 5 两种方法的加标回收率
Table 5. Recovery of standard addition for the two methods
测定方法 土壤样品
编号称样量
(g)碳含量本底值
(mg)加标量
(mg)加标测定值(mg) 加标回收率(%) 加标回收率
平均值(%)1 2 3 1 2 3 自动
滴定仪法12 0.2053 2.62 2 4.49 4.45 4.55 93.67 91.38 96.54 93.37±1.85 0.2053 2.62 4 6.38 6.33 6.3 94.05 92.68 91.93 15 0.2640 2.28 2 4.12 4.15 4.14 92.00 93.50 93.00 94.29±1.78 0.2640 2.28 4 6.15 6.11 6.07 96.75 95.75 94.75 24 0.0448 2.17 2 4.03 4.11 4.16 93.00 96.80 99.55 94.92±2.72 0.0448 2.17 4 5.94 5.89 5.88 94.34 93.00 92.85 52 0.3559 1.47 2 3.53 3.4 3.37 102.78 96.29 94.79 98.06±2.72 0.3559 1.47 4 5.37 5.42 5.4 97.45 98.83 98.20 人工
滴定法
12 0.2022 2.62 2 4.48 4.39 4.5 92.97 88.74 93.75 92.56±2.05 0.2022 2.62 4 6.36 6.3 6.4 93.55 91.93 94.43 15 0.2679 2.28 2 4.06 3.98 4.1 89.00 85.00 91.00 91.88±4.46 0.2679 2.28 4 6.17 6.08 6.04 97.25 95.00 94.00 24 0.0435 2.17 2 3.99 4.11 4.18 90.81 96.80 100.55 93.72±4.32 0.0435 2.17 4 5.93 5.74 5.8 94.09 89.23 90.85 52 0.3742 1.47 2 3.51 3.38 3.35 101.78 95.29 93.79 97.63±2.81 0.3742 1.47 4 5.39 5.43 5.39 97.95 99.03 97.95
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表 6 自动滴定仪方法测定土壤中有机碳含量的准确度
Table 6. Accuracy for the determination of organic carbon content by the automatic titrator
标准物质编号 有机碳含量标准值
(g/kg)有机碳含量测定结果(g/kg) 有机碳含量平均值
(g/kg)相对误差
(%)1 2 3 4 GBW07493 9.05±0.99 8.48 8.75 8.36 8.96 8.64±0.27 4.53 GBW07415 19.32±0.58 19.16 18.39 18.03 18.00 18.40±0.54 4.79 GBW07412b 27.15±1.10 28.84 28.72 27.93 27.77 28.32±0.47 4.31
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表 7 两种方法检测效率及试剂耗用量的比较
Table 7. Comparison of detection efficiency and reagent consumption with the two methods
工效参数 人工滴定法 自动滴定仪法 称样和消煮用时 10h 10h(有9h工作与滴定同步) 滴定用时 10h 18h(9h的滴定与称样和消煮同步进行) 器皿清洁用时 4h 4h(4h的清洗工作与滴定同步进行) 人工操作用时 24h 14 检测总用时 24h 19 人员工作特点 连续操作 非连续操作 人员工作强度 大 小 重铬酸钾+硫酸用量 5mL+5mL 2mL+2mL 0.2mol/L硫酸亚铁溶液用量 2400～7500mL 800～2400mL 邻菲啰啉用量 30mL 0
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