Potentiometric determination of soil pH 2/2
1.5 Analysis Step 1: Instrument Calibration
The use of different pH meters and potentiometers may vary, but each device should be handled according to the manufacturer’s instructions. Before calibration, ensure that the test solution and standard buffer solution are at the same temperature. Adjust the temperature compensator accordingly. When calibrating with a standard buffer solution, insert the electrode into a solution whose pH is no more than 2 units different from the sample. Turn on the readout switch and adjust the positioner until the displayed pH matches the standard solution. Repeat this process several times to achieve a stable reading. After calibration, remove the electrode, rinse it thoroughly, and dry it with filter paper. Then place it in a second standard buffer solution. The difference between the two readings should not exceed 0.1 pH unit. If it does, check the electrode or the standard solution for issues. Once properly calibrated, the instrument can be used to measure the sample.
2. Determination of pH in Soil Aqueous Extract
Weigh 20 g (to 0.1 g) of air-dried soil passing through a 2 mm sieve into a 50 mL beaker. Add 20 mL of deionized water to remove CO₂, stir for 1 minute, and let it stand for 30 minutes to allow full dispersion. Insert the electrode into the solution, ensuring the glass bulb is below the soil-liquid interface and the calomel electrode is in the clear upper layer. Gently shake the beaker to remove any water film on the electrode, allowing it to equilibrate quickly. Read the pH value when the display stabilizes. After recording, remove the electrode, rinse it, and dry it with filter paper before proceeding to the next sample. Every 5–6 samples, verify the electrode using a standard solution.
3. Determination of pH in Potassium Chloride Extract
When the soil pH is less than 7, the pH of the potassium chloride extract should be measured. The procedure is similar to the water extraction method, except that 1 mol/L KCl solution is used instead of COâ‚‚-free water, and the soil-to-water ratio is 1:1.
1.6 Calculation of Results
When using a pH meter, the pH value can be directly read from the display without further calculation.
1.7 Precision
The allowable absolute difference between parallel results is ≤ 0.1 pH units for neutral and acidic soils, and ≤ 0.2 pH units for alkaline soils.
1.8 Notes
1. Glass electrodes that are not used for long periods should be soaked in distilled water for 24 hours to activate them. If not in use immediately, keep them in water; if unused for a long time, store them dry. Clean the electrode surface if contaminated. The calomel electrode chamber must be filled with saturated KCl solution, with some crystals present to ensure proper contact. Avoid air bubbles between the inner electrode and the ceramic junction.
2. Proper electrode placement is crucial. The calomel electrode should be placed in the clear supernatant to avoid contact with the slurry, reducing liquid junction potential.
3. Shaking the beaker multiple times may cause low readings; let the solution rest slightly after shaking.
4. Avoid exposure to acid or base vapors during the process.
5. Standard solutions can be stored for 1–2 months at room temperature, or longer in a refrigerator at 4°C. Do not return used solutions to the original bottle. Discontinue use if turbidity or precipitation occurs.
6. Temperature affects the ionization balance of the electrode and the solution. Ensure the temperature of the standard buffer, test solution, and instrument's temperature compensator are the same. pH values of standard solutions vary slightly with temperature—refer to Table 1 for accurate calibration.
7. When measuring a batch of samples, separate those with large pH differences to prevent electrode response delays.
8. For low-density samples, adjust the water-to-soil ratio as needed, but record the changes carefully.
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