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Determination of carbonate and bicarbonate content in soil by fully automated potentiometric titration
I. Introduction
Soil acidity and alkalinity have a significant impact on plant growth and development. The main sources of soil acidity are carbonic acid formed from CO2 dissolved in water, organic acids produced by the decomposition of organic matter, inorganic acids from the oxidation of minerals, and residual inorganic acids from applied fertilizers, such as nitric acid, sulfuric acid, and phosphoric acid. Therefore, detecting carbonate and bicarbonate ions in soil is of great significance for soil acidity investigation.
This method uses the JH-T6 fully automated potentiometric titration method from Shanghai Jiahang to determine the content of carbonate and bicarbonate ions. The pH of the titration endpoint is determined using an indicator. The endpoint titration mode of the potentiometric titrator was used, and the endpoint pH was set to 8.3 and 3.8, respectively. Sulfate was used for titration to obtain the content of carbonate and bicarbonate ions.
II. Instruments and Reagents
2.1 Instruments
JH-T6 fully automated potentiometric titrator, pH composite electrode, analytical balance, etc.
2.2 Reagents
Sulfuric acid standard solution (0.02 mol/L).
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 be employed; the separated solution must be clear and transparent.
3.1.2 Determination of Carbonate and Bicarbonate Content in Samples
Pipette 25.00 mL of the leachate into a 150 mL Erlenmeyer flask and add 1 drop of phenolphthalein indicator. If the solution does not turn pink, it indicates the absence of carbonate ions; continue measuring bicarbonate. If a red color appears, add sulfuric acid standard solution using a potentiometric titrator until pH = 8.3. Record the volume of sulfuric acid solution used, V1. Continue titrating with 0.01 mol/L sulfuric acid standard solution until pH = 3.8. Record the volume of sulfuric acid standard solution used in this titration, V2.
The titrator parameters are set as shown in Table 1:
Table 1 Titration Parameter Settings
IV. Results and Discussion
4.1 Experimental Results
The experimental results are shown in Table 2:
Table 2 Test Results
Note: The sample tested did not turn red after adding phenolphthalein indicator, so it does not contain carbonate.
4.2 Conclusion
The JH-T6 fully automatic potentiometric titrator is used for the detection of carbonate and bicarbonate in soil. It is simple to operate and efficient, and is suitable for use with an autosampler to complete a large number of soil testing experiments. However, it should be noted that since the titration mode used is endpoint titration, the slope of the motor must be calibrated before each test to ensure the accuracy of the electrode pH.
References
[1] LY/T 1251-1999 Analysis of water-soluble salts in forest soils [S]
Keyword:
automated potentiometric titration
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