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Determination of Soil Organic Matter (SOM) Content by Fully Automated Potentiometric Titration
1. Introduction
Soil is an important natural resource. Due to its complex spatiotemporal variability, accurate soil information is urgently needed for current agricultural production and environmental assessment. The level of soil organic matter content is a key indicator of soil quality. Soil organic matter refers to carbon-containing organic compounds existing in various forms in the soil; it refers to substances in the soil derived from life and includes all substances in the soil other than soil minerals. Generally, under similar conditions, within a certain range, the content of organic matter is positively correlated with the level of soil fertility.
There are many methods for testing soil organic matter. This paper mainly follows the national standard NT/Y 1121.6-2006. In the pretreatment, a graphite digestion apparatus was used instead of oil bath treatment. Finally, a potentiometric titrator was used to determine the excess potassium dichromate. The calculated results were within the range of 11.5-12.7 g/kg described in the quality control sample, showing good accuracy and meeting the precision requirements of the national standard, making it worthy of promotion.
2. Instruments and Equipment
2.1 Instruments
JH-T6 fully automatic potentiometric titrator, 10mL burette, Hamilton platinum composite electrode, SH220F graphite digester
2.2 Reagents
Ferrous sulfate solution (0.1mol/L), ultrapure water, potassium dichromate-sulfuric acid solution (0.4mol/L), potassium dichromate standard solution (0.1mol/L), HTSB-3 Ningxia silt
3. Experimental Methods
3.1 Experimental Procedure
Accurately weigh 0.5 g of the sample (accurate to 0.0001 g), place it in a hard glass tube, accurately add 10.00 mL of potassium dichromate-sulfuric acid solution (0.4mol/L), and shake well. Insert the glass tube into an SH220F container preheated to 185℃. Start timing when the solution in the test tube boils. To prevent violent boiling, the temperature can be lowered to 180℃. After 5 min ± 0.5 min, remove the tube and allow it to cool briefly. Transfer the digestion solution and soil residue from the test tube into a titration cup without damage. Rinse the test tube with water, adding the washings to the titration cup, ensuring the total volume of solution in the cup is 50-60 mL. Titrate with ferrous sulfate solution to the endpoint. Two blank tests are required for each batch of analysis. No substitutes were used in this blank test; the other steps are the same as for the soil sample test.
3.2 Parameter Settings
4. Results and Discussion
4.1 Results
The concentration of ferrous sulfate solution was 0.09845 mol/L, and the blank volume was 44.417 mL.
Calculation formula:
Where:
X -- Mass fraction of soil organic matter, g/kg;
V0 -- Volume of ferrous sulfate solution consumed in the titration of the blank, mL;
V -- Volume of ferrous sulfate solution consumed in the titration of the soil sample, mL;
c -- Concentration of ferrous sulfate solution, mol/L;
0.003 -- Millimolecular mass of 1/4 carbon atom, g;
1.724 -- Coefficient for the conversion of organic carbon to organic matter;
1.10 -- Oxidation correction coefficient;
m -- Mass of the dried sample, g;
1000 -- Conversion to mass per kilogram.
4.2 Spectrum
4.3 Conclusion
This experiment used a graphite digester in the sample pretreatment process and finally tested it with a potentiometric titrator. Results with good repeatability and clear peaks were obtained. The soil sample was HTSB-3 Ningxia irrigation soil quality control sample, with an average content of 12.095 g/kg. The test results were consistent with the SOM content range of the quality control sample (11.5-12.7 g/kg).
References
[1] NY/T 1121.6-2006 Soil Testing Part 6: Determination of Soil Organic Matter.
Precautions
The preparation and standardization of ferrous sulfate solution should refer to the national standard NY/T 1121.6-2006. The carbon dioxide bubbles generated during heating are not true boiling; time should only be calculated when true boiling occurs. A small amount of soil sample may stick to the test tube wall after being transferred to a dry test tube; it is recommended to rinse with a small amount of water.
Keyword:
Automated Potentiometric Titration
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