Potentiometric determination of soil pH 2/2
1.5 Analysis Step 1: Instrument Calibration
The use of different pH meters and potentiometers may vary, so it is important to follow the manufacturer’s instructions for electrode preparation and instrument operation. Ensure that the test solution and standard buffer solution are at the same temperature, and adjust the temperature compensator accordingly. When calibrating with a standard buffer solution, insert the electrode into a buffer that differs from the sample pH by no more than 2 units. Turn on the readout switch and adjust the positioner until the displayed pH matches the buffer value. Repeat this process several times to ensure stability. 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 units. If it does, check the electrode or the standard solution for issues. Once the instrument is properly calibrated, it can be used for sample analysis.
2. Determination of pH in Soil Aqueous Extract
Weigh 20 g (accurate to 0.1 g) of air-dried soil through a 2 mm sieve into a 50 mL beaker. Add 20 mL of CO₂-free water and stir for 1 minute to fully disperse the soil particles. Let it stand for 30 minutes. 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, which helps achieve faster equilibrium. Read the stable pH value. After measurement, remove the electrode, rinse it, dry with filter paper, and proceed with the next sample. After every 5–6 samples, recheck the electrode using a standard solution.
3. Determination of pH in Potassium Chloride Extract
For soils with pH < 7, determine the pH of the potassium chloride extract. The procedure is similar to the aqueous extract 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 without additional calculations.
1.7 Precision
Parallel results should have an absolute difference of ≤ 0.1 pH units for neutral and acidic soils, and ≤ 0.2 pH units for alkaline soils.
1.8 Notes
1. Glass electrodes not in use for long periods should be soaked in water for 24 hours before use to activate them. If not used immediately, they can remain in water. For long-term storage, keep them dry. Clean the electrode surface if contaminated. Fill the calomel electrode chamber with saturated KCl solution, leaving a small amount of crystals at room temperature. Avoid excessive crystals that might block the ceramic junction. Ensure no air bubbles exist between the inner electrode and the bulb of the glass electrode or between the inner electrode and the ceramic core of the calomel electrode.
2. The electrode placement in the suspension affects accuracy. The calomel electrode must be placed in the clear upper solution, avoiding contact with the slurry to minimize liquid junction potential.
3. Shaking the beaker three times may cause lower readings; allow the solution to settle 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, and up to 6 months in a refrigerator at 4°C. Never return used standards to the original bottle. Discard if turbid or precipitated.
6. Temperature significantly affects electrode response and water ionization. Ensure the temperature of the standard buffer, the test solution, and the temperature compensator are all the same. Refer to Table 1 for temperature-adjusted pH values of standard buffers.
7. When measuring multiple samples, separate those with large pH differences to prevent electrode response delays.
8. If the soil density is low, adjust the water-to-soil ratio as needed but document the change.
Aluminium Angles,Powder Coated Aluminium Angle,Customized Angle Aluminium Profile,Wood Grain Aluminium Angle
FOSHAN WINKAI ALUMINIUM CO.,LTD , https://www.winkaialu.com