Application of automatic soil organic matter analyzer in detection

Yuan Suzhen, Huang Zhiying, Chao Chuyue, Xu Yang

(Jiangxi Province Yichun Ecological Environment Monitoring Center, Yichun 336000, Jiangxi Province)

1 Research Background

1.1 Necessity Analysis

Soil organic matter is an important component of the solid part of the soil and one of the main sources of plant nutrition. It can promote the growth and development of plants, improve the physical properties of soil, promote the activities of microorganisms and soil organisms, and promote the decomposition of nutrient elements in the soil. Improve soil fertility and buffering properties. It is closely related to soil structure, ventilation, permeability, adsorption, and buffering properties. Usually, under the same or similar conditions, within a certain range of content, the content of organic matter is positively correlated with the level of soil fertility. Therefore, accurate measurement of organic matter content in soil has certain guiding significance for understanding soil fertility, fertilizing and improving soil.

1.2 Insufficient advance method

The center’s previous method of measuring soil organic matter was based on NYT 1121.6-2006 Soil Testing Part 6: The determination of soil organic matter used the potassium dichromate volumetric method-external heating method. This method uses paraffin solution to heat, digest, and oxidize organic matter to determine organic matter. The oil bath method is a classic method that has been used for many years, but there are the following deficiencies in actual use.

(1) The digestion temperature fluctuates greatly, it is difficult to control the temperature, and it is mostly adjusted by experience.

(2) When the sample is put into the oil bath in the digestion tube, it is easy to be unevenly heated, and the result is easy to fluctuate, and the error is large.

(3) The paraffin wax liquid must not only maintain a slightly boiling state to prevent the decomposition of potassium dichromate, but also accurately calculate the boiling time of 5 minutes. At the same time, the paraffin wax releases gas during high temperature heating, polluting the air in the laboratory, and poisoning the experimental work. the body of the person.

(4) Oil stains are easy to adhere to the outer wall of the test tube, which is difficult to clean.

The above shortcomings make the potassium dichromate volumetric method-oil bath method difficult to carry out in large batches and multiple batches in practical applications, and the monitoring task of soil organic matter in our center is heavy and time-consuming. Analytical efficiency and accuracy.

2 Methods and materials

2.1 Experimental method and principle

This article chooses to use the automatic soil organic matter analyzer (ST308G) to replace the traditional oil bath method. The automatic soil organic matter analyzer has four modules: automatic liquid addition module, heating and digestion module, intelligent manipulator module and automatic titration module. The robot three-axis motion system is used to realize automatic sample transfer, automatic sample addition, automatic digestion, automatic titration, automatic end point judgment, automatic calculation results and data output. The heating module adopts an aluminum alloy module with an acid and alkali corrosion-resistant coating, and is heated by far-infrared radiation.

2.2 Experimental Instruments

Instruments used: automatic soil organic matter analyzer (ST308G type. Jinan Shengtai Electronic Technology Co., Ltd.), glass digestion bottle.

2.3 Experimental reagents

1. Standard substance

The certified standard samples of organic matter in soil produced by Steel Research Nanoco Testing Technology Co., Ltd. were used. The standard substance numbers were NCS204005 and NCS204007, and the concentrations were (4.5±0.6) g/kg and (24.6±2.3) g/kg, respectively.

2. Experimental reagents

(1) Standard solution of 0.4 mol/L K2Cr2O7-H2SO4. Accurately weigh 40 g of analytically pure potassium dichromate (K2Cr2O7) and dissolve in 500 mL of distilled water, dilute to 1 L after cooling, then slowly add 1000 mL of concentrated sulfuric acid (H2SO4) with a specific gravity of 1.84, and keep stirring, adding 200 mL , should be placed for 10 min to 20 min to cool the solution, and then add the second part of concentrated sulfuric acid (H2SO4). After adding acid, store it in a reagent bottle after cooling for later use.

(2) 0.1 mol ferrous sulfate standard solution. Accurately weigh 28.0 g of analytically pure ferrous sulfate (chemically pure) or 40.0 g of ferrous ammonium sulfate (chemically pure), dissolve it in 600 mL to 800 mL of distilled water, add 20 mL of concentrated sulfuric acid (chemically pure), and then dilute with water to 1 L.

(3) O-phenanthroline indicator. Weigh 1.49 g of analytically pure phenanthroline, dissolve it in 100 mL of aqueous solution containing 1.0 g of ferrous sulfate, and store it in a brown dropping bottle (this indicator should be prepared just before use).

(4) Potassium dichromate standard solution (0.250 mol/L potassium dichromate standard solution): Accurately weigh 12.258 g of the standard reagent K2Cr2O2 (baked at 130 °C for 3 hours to constant weight), dissolve it in pure water, and carefully transfer to 1 In the L volumetric flask, add water to the mark, which is the standard 0.250 mol/L K2Cr2O2 solution.

3 Calculation and analysis

3.1 Detection limit verification

According to the Agricultural Industry Standard of the People's Republic of China "Determination of Soil Organic Matter in Soil Testing" NY/T1121.6-2006, and according to the requirements of Appendix A.1.2 of HJ168-2020 "Technical Guidelines for Environmental Monitoring and Analysis Method Standard Revision", the titration method adopts The theoretical detection limit (MDL) and defined lower limit (4MDL) of the method were determined using a reagent blank.

1. The calculation process is as follows:

The titration method is generally calculated according to the minimum drop volume produced by the buret used, and the detection limit calculation formula is shown in the following formula:

In the formula: λ—the molar ratio of the measured component to the titrant;

ρ—mass concentration of titrant, g/mL;

V0——The minimum droplet volume produced by the burette, mL;

M0—molar mass of titrant, g/mol;

m1 - the sampling mass of the component to be tested, kg;

M1——the molar mass of the tested item, g/mol;

k—k=1 for one-time titration, k=2 for back titration or indirect titration.

The chemical reaction equation of the standard method is: K2Cr2O7+C+2FeSO4+5H2SO4=K2SO4+Cr2(O4)3+CO2+Fe2(SO4)3+5H2O, as can be seen from the above equation, k=2, λ=1/2, ρ= 0.08 g/mL, V0=0.05 mL, M1=12 g/mol, M0=278.01 g/mol, m1=0.0005 kg.

2. Calculation results

Calculate the detection limit of soil organic matter as follows:

Method detection limit MDL=0.3 g/kg

Method lower limit = 4MDL = 1.2 g/kg.

Table 1 Determination of detection limits of instrumental methods

The detection limit of the instrument method of the automatic soil organic matter analyzer is lower than the theoretical detection limit of the titration method.

3.2 Precision Verification

Use certified standard samples of organic matter in soil produced by Steel Research Nanoco Testing Technology Co., Ltd., the standard substance numbers are NCS204005 and NCS204007, and the concentrations are (4.5±0.6) g/kg and (24.6±2.3) g/kg, respectively. The analytical precision was measured 7 times in parallel, and the verification results are shown in Table 2.

Table 2 Precision Verification Results Table

After the method verification, the standard deviation of the standard material in the laboratory of this method meets the requirements of precision determination.

Table 3 Allowable difference table of precision parallel determination results

3.3 Accuracy Verification

Use certified standard samples of organic matter in soil produced by Steel Research Nanoco Testing Technology Co., Ltd., the standard substance numbers are NCS204005 and NCS204007, and the concentrations are (4.5±0.6) g/kg and (24.6±2.3) g/kg, respectively. The analytical accuracy was measured 7 times in parallel, and the verification results are shown in Table 4.

Table 4 Accuracy verification result table

This method verified the analytical accuracy of the two concentrations of certified substances. The test results were all within the concentration range specified by the certified standard sample, and the relative error was small. The accuracy of this method verification met the requirements.

4 Conclusion and Analysis

Compared with the standard organic matter measurement technology, the ST308G automatic soil organic matter analyzer has the following characteristics:

1. Potassium dichromate volumetric method-external heating method requires the experimenter to manually put the vessel into paraffin oil for heating, and the paraffin oil will cause certain danger to the experimenter during the heating process. At the same time, the digestion solution will be lost during the transfer process, which will affect the final result. The fully automatic soil organic matter analyzer realizes automatic sample transfer, automatic sample addition, automatic digestion, automatic titration, automatic end point judgment, automatic calculation results and data output, simplifies operation, high safety, improves experimental efficiency, and reduces human factors. error, which improves the accuracy of the experimental results.

2. Potassium dichromate volumetric method-external heating method will produce volatile gas and soot pollution during the heating process, which will cause certain harm to the experimental environment and experimenters. The automatic soil organic matter analyzer uses aluminum alloy for overall heating to measure soil organic matter, which reduces the volatilization of polluted gases and oil fume pollution, and causes less physical harm to the experimental environment and experimenters.

3. The automatic soil organic matter analyzer adopts advanced visual capture technology to judge the titration end point. The titration needle adopts a professional design, does not hang liquid, and the minimum titration volume is 0.01ml. The titration accuracy, titration reaction speed and titration error are all better than the experimenter artificial.

4. The automatic soil organic matter analyzer adopts a desktop computer control system. The system automatically completes the entire experimental process except soil weighing, and automatically outputs the experimental result data, which is convenient for the experimenters to make statistics and greatly liberates the labor of the experimenters.

Combining the above characteristics, using the ST308G automatic soil organic matter analyzer to measure the content of soil organic matter is easy to operate, has high accuracy, good repeatability, and high work efficiency, and is very suitable for large-scale soil sample determination. At the same time, the working environment of the instrument is not demanding, suitable for ordinary laboratories, and worthy of promotion.