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Determination of Total Calcium and Magnesium in Soil by Fully Automated Potentiometric Titration
I. Introduction
The determination of water-soluble calcium and magnesium content in soil provides a methodological basis for evaluating the supply capacity of calcium and magnesium elements to crops. It allows for early detection of problems before plant deficiency or toxicity symptoms appear, enabling timely prevention and control measures. This is of great significance for guiding farmers in scientific, economical, and rational fertilization, reducing costs and increasing efficiency, and promoting high-yield, high-quality, and high-efficiency agriculture.
This method uses the JH-T6 fully automated potentiometric titration method from Shanghai Jiahang to determine the total calcium and magnesium content. It utilizes the ability of EDTA to complex calcium and magnesium ions in soil leachate in an ammonia buffer solution at pH=10, and then determines their content using a potentiometric titrator.
II. Instruments and Reagents
2.1 Instruments
JH-T6 fully automated titrator, calcium ion-selective electrode, analytical balance, etc.
2.2 Reagents
EDTA standard solution (0.02 mol/L), ammonia-ammonium chloride buffer solution (pH=10).
III. Experimental Methods
3.1 Analytical Procedures
3.1.1 Soil Sample Preparation
Accurately weigh 50.0 g of air-dried soil sample (passed through a 2 mm sieve) using a balance and place it in a dry 500 mL Erlenmeyer flask. Accurately add 250 mL of carbon dioxide-free pure water using a graduated cylinder, stopper the flask, and shake for 3 min.
Depending on the ease of filtration of the soil suspension, use one of the following methods to obtain a clear leachate. Collect the filtrate in a dry Erlenmeyer flask. After complete filtration, thoroughly mix the filtrate, stopper the flask, and prepare it for analysis.
Easily filtrationable soil suspension: Filter using filter paper through a 7 cm diameter funnel, or use a Buchner funnel for suction filtration. Cover the funnel with a watch glass to reduce evaporation. The initial filtrate is often cloudy and must be filtered repeatedly until clear.
For soil suspensions that are difficult to filter: Filter repeatedly through a 10 cm diameter funnel using tightly folded double-layered filter paper. For suspensions of alkalized soils and heavy clay soils with very low total salt content, vacuum filtration with a plain porcelain filter candle can be used. If vacuum filtration is not used, centrifugation can also be employed; the separated solution must be clear and transparent.
3.1.2 Determination of Total Calcium and Magnesium in Samples
Take 25.00 mL of the leachate, add 1 mL of ammonia buffer, turn on the magnetic stirrer, and after stirring evenly, immediately titrate with EDTA standard solution to the second potential jump endpoint. Record the volume of EDTA used (V2).
Set the titrator parameters as shown in Table 1:
Table 1 Titration Parameter Settings
3. Test Spectrum Example
IV. Results and Discussion
4.1 Experimental Results
The experimental results are shown in Table 2:
Table 2 Test Results
4.2 Conclusion
The test showed that the calcium and magnesium content in the soil was 1.3381 kg/mol. The repeatability of the test was good, and the potential jump was also relatively obvious. It can meet the needs of customers to a certain extent and improve the efficiency of the test.
References
[1] LY/T 1251-1999 Analysis of water-soluble salts in forest soils [S]
Keyword:
Automated Potentiometric Titration
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