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2 Determination of pH of Soil Aqueous Extract: Weigh 20g (accurate to 0.1g) of air-dried sample passed through a 2mm aperture sieve into a 50ml high beaker, add 20ml of water to remove CO2, and stir for 1min with a stirrer to make the soil particles full. Disperse and place after 30 minutes. Insert the electrode into the test solution (note that the lower part of the glass bulb is under the soil-liquid interface, and the calomel electrode is inserted into the upper clear solution). Gently shake the beaker to remove the water film on the electrode to promote rapid equilibration. Read the switch and note the pH value when the reading is stable. Open the reading switch, take out the electrode, wash it with water, dry it with filter paper, and measure the second sample. After every 5 to 6 samples are measured, the standard solution is used to check the positioning.
3 Determination of pH of potassium chloride extract of soil: When soil pH<7, the pH value of soil salt extract should be determined. The determination method was as follows: except that 1 mol of K 1 potassium chloride solution was used instead of CO 2 -free water, the ratio of water to soil was 1:1, and the other measurement steps were the same as the determination of water pH.
1.6 Calculation of Results When pH is measured with a pH meter, the pH can be read directly without calculation.
1.7 Precision parallel results allow absolute differences: Neutral, acid soil ≤ 0.1 pH units, alkaline soil ≤ 0.2 pH units.
1.8 Notes 1 The glass electrode that is not used for a long time needs to be soaked in water for 24 hours so that it can be activated before it can react normally. If it is not used for the moment, it can be soaked in water. If it is not used for a long time, it should be stored dry. When the surface of the glass electrode is contaminated, it needs to be treated. The calomel electrode chamber should be filled with saturated potassium chloride solution, and there should be a little potassium chloride crystal at room temperature, but the potassium chloride crystal is not easy to be too much to prevent the passage between the electrode and the tested solution. There must be no air bubbles between the inner electrode of the glass electrode and the bulb, between the inner electrode of the calomel electrode and the ceramic core.
2 The position of the electrode in the suspension has an influence on the measurement result. It is required to insert the calomel electrode into the upper clear solution to avoid contact with the slurry to reduce the influence of the liquid potential of the calomel electrode.
Shaking the beaker with 3 pH readings will cause the readings to be low and should be slightly quiescent after shaking.
4 Avoid acid-base vapors during operation.
5 The standard solution can generally be stored for 1 to 2 months at room temperature. The shelf life can be extended in a refrigerator at 4°C. The used standard solution should not be poured back into the original liquid. If turbidity and precipitation are found, it cannot be used again.
6 The temperature affects the ionization balance of the electrode potential and water. The temperature compensator, standard buffer, and the temperature of the test solution must be the same. The pH of the standard solution varies slightly with temperature. Refer to Table 1 for calibration of the instrument.
Table 1 Changes of pH buffer solution at different temperatures °C pH
Standard solution 4.01 Standard solution 6.87 Standard solution 9.18
0 4.003 6.984 9.464
5 3.999 6.951 9.395
10 3.998 6.923 9.332
15 3.999 6.900 9.276
20 4.002 6.881 9.225
25 4.008 6.865 9.180
30 4.015 6.853 9.139
35 4.024 6.844 9.102
38 4.030 6.840 9.081
40 4.035 6.838 9.068
45 4.047 6.834 9.038
7 When measuring a batch of samples, it is best to separate the samples with large differences in pH value according to the type of soil, etc. to avoid measurement errors caused by the influence of the electrodes being unresponsive.
8 If the density of the sample is small, the water-soil ratio may be changed moderately, but it must be noted.
Potentiometric determination of soil pH 2/2
1.5 Analysis Step 1 Instrument Calibration: The use of various pH meters and potentiometers is not consistent. The electrode treatment and instrument use are performed according to the instrument instructions. Set the test solution to the same temperature as the standard buffer solution, and adjust the temperature compensator to this temperature value. When calibrating the instrument with a standard buffer solution, insert the electrode into a standard buffer solution that does not differ from the pH of the test sample by more than 2 pH units. Turn on the readout switch and adjust the positioner so that the reading is just the pH of the standard solution. Repeat several times. Make the reading stable. Take out the electrode and wash it. Dry it with filter paper and then insert it into the second standard buffer solution. The allowable deviation between the two standards is 0.1 pH units. If it exceeds, check the instrument electrode or standard solution for problems. After the instrument is calibrated correctly, it can be used to determine the sample.