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2025
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Determination of sulfate content in soil by fully automated potentiometric titration
I. Introduction
The sulfate ion content in soil not only affects crop growth due to excessive soil salinity, but also causes severe corrosion and aging of underground pipelines, leading to major leaks. Therefore, detecting the sulfate content in soil is of great significance.
This method uses the JH-T6 fully automated potentiometric titration method from Shanghai Jiahang to determine the sulfate content. First, excess barium chloride is used to completely precipitate the sulfate ions in the solution. The excess barium ions, along with the calcium and magnesium ions already present in the leachate, are titrated with EDTA standard solution at pH=10 until the endpoint of the potential jump. To ensure a clear endpoint, a certain amount of magnesium ions is added. The sulfate content can be calculated from the net amount of barium ions consumed.
II. Instruments and Reagents
2.1 Instruments
JH-T6 fully automatic potentiometric titrator, calcium ion selective electrode, analytical balance, etc.
2.2 Reagents
Barium-magnesium mixture: (1.22 g barium chloride (BaCl2 · 2H2O) and 1.02 g magnesium chloride (MgCl2 · 6H2O) dissolved in water and diluted to 500 mL. The concentration of barium and magnesium ions in this solution is 0.01 mol/L each, and approximately 1000 µg of sulfate can be precipitated per mL), EDTA standard solution (0.02 mol/L), hydrochloric acid solution (1+4), 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 that has 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, and shake for 3 minutes.
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 conical flask. After complete filtration, thoroughly mix the filtrate, stopper, and prepare for analysis.
Easily filtrationable soil suspension: Filter using filter paper on 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.
Difficult-to-filterable soil suspension: Filter repeatedly using a tightly folded double layer of filter paper on a 10 cm diameter funnel. For suspensions of alkalized soils and heavy clay soils with very low total salt content, suction filtration with a plain porcelain filter candle can be used. If suction filtration is not used, centrifugation can also be employed; the separated solution must also be clear and transparent.
3.1.2 Determination of Sulfate in Samples
Take 5.0 mL of soil leachate and place it in a 2 cm diameter test tube. Add 2 drops of 1:1 hydrochloric acid and 5 drops of 50 g/L barium chloride solution. Mix immediately and observe the turbidity. Compare the turbidity with the sulfate standard turbidity solution using the line method to estimate the approximate sulfate content. Based on this, select the appropriate sulfate determination method and the volume of leachate to be taken during the determination, as well as the amount of barium-magnesium mixture used in the EDTA method, according to Table 1.
Table 1 Selection of Prediction and Determination Methods for SO₄²⁻ in Leachate and Control Conditions for Each Method
Based on the predicted results, pipette 10.00–25.00 mL of the leachate into a 150 mL Erlenmeyer flask. (If the amount of leachate is small, dilute with water to approximately 25 mL). Add 8 drops of 1:4 hydrochloric acid and heat to boiling. Accurately add the specified amount of barium-magnesium mixture as specified in the table, and record the volume added (the barium-magnesium mixture should be 50%–100% excess to ensure that the barium ion concentration in the solution reaches above 0.0025 mol/L after barium sulfate precipitation). Continue to boil gently for 5 minutes, cool, and let stand for 2 hours (or overnight).
Add 2 mL of ammonia buffer to this solution, shake well, and immediately titrate with EDTA to the potential jump endpoint. Record the volume of EDTA standard solution used, V3.
Take approximately 25 mL of pure water, add 8 drops of 1:4 hydrochloric acid and the same volume of barium-magnesium mixture as above. Then add 2 mL of ammonia buffer and titrate with EDTA solution. Record the volume of EDTA solution used, V4.
Set the titrator parameters as shown in Table 2:
Table 2 Titration Parameter Settings
3.1.3, Example of Test Spectrum
IV. Results and Discussion
4.1 Experimental Results
The experimental results are shown in Table 3:
Table 3 Test Results
Note: V2 is the volume of 0.02 mol/L EDTA consumed when determining the total calcium and magnesium content.
4.2 Conclusion
It is feasible to use a potentiometric titrator to detect sulfate ions in soil. However, it should be noted that since the calcium electrode has a small response to barium ions,
appropriate magnesium ions should be added to improve the breakthrough.
References
[1] LY/T 1251-1999 Analysis of water-soluble salts in forest soils [S]
Keyword:
automated potentiometric titration
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