Determination of soil pH value (potentiometric method)
Method summary: The potentiometric method is used to determine soil pH by inserting pH glass electrodes and calomel electrodes into soil suspension or leachate, measuring their electromotive force values, and then converting them into pH values. The pH value can be directly read after being determined by the standard solution on the acidity meter. The ratio of soil and water has a significant impact on pH value, especially on the dilution effect of calcareous soil. It is advisable to adopt a small soil to water ratio. This law stipulates a soil to water ratio with a soil pH of 1:1. At the same time, in addition to measuring the pH value of water immersed soil, the pH value of salt immersed soil should also be measured in acidic soil, which is 1mol/L ˉ Soak soil H+in a potassium chloride solution and measure it using the potentiometric method.
This method is applicable to the determination of pH values in various types of soil.
3 Main instruments and equipment
① Acidity meter;
② PH glass electrode;
③ Saturated calomel electrode;
④ Stirrer.
4 reagents
① 1molL ˉ 1. Potassium chloride solution: Weigh 74.6 ɡ Dissolve potassium chloride (chemically pure) in 800ml of water, adjust the pH of the solution batch to 5.5-6.0 with dilute potassium hydroxide and hydrochloric acid, and dilute to 1L;
② PH 4.01 (25 ℃) standard buffer solution: Weigh 10.21 potassium hydrogen phthalate, which has been dried at 110-120 ℃ for 2-3 hours, and dissolve it in water. Transfer it to a 1L volumetric flask, make up to volume with water, and store it in a polyethylene bottle;
③ PH 6.87 (25 ℃) standard buffer solution: Weigh 3.533g of disodium hydrogen phosphate and 3.388g of potassium dihydrogen phosphate, which have been dried at 110-130 ℃ for 2-3 hours, and dissolve them in water. Transfer them to a 1L volumetric flask, make up to volume with water, and store them in a polyethylene bottle;
④ PH 9.18 (25 ℃) standard buffer solution: Weigh 3.800g of balanced borax (Na2B4O7 · 10H2O) and dissolve it in CO2 free water. Transfer it to a 1L volumetric flask, make up to volume with water, and store it in a polyethylene bottle;
Balance treatment of borax: Place borax in a dryer containing saturated aqueous solutions of sucrose and salt to balance for two days and nights;
⑤ Distilled water to remove CO2.
5 Analysis steps
① Instrument calibration: The usage methods of various pH meters and potentiometers are not consistent, and the electrode treatment and instrument use are carried out according to the instrument manual. Adjust 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, first insert the electrode into a standard buffer solution with a pH difference of no more than 2 pH units from the tested sample, activate the reading switch, adjust the locator to make the reading exactly the pH value of the standard solution, and repeat several times to stabilize the reading. Take out the electrode and wash it clean. Use a filter paper strip to absorb the moisture, and then insert it into * * standard buffer solutions. The allowable deviation between the two standard solutions is 0.1 pH unit. If it exceeds the limit, check whether there is a problem with the instrument electrode or standard solution. Only after the instrument calibration is correct can it be used to determine the sample.
② Determination of pH of Soil Water Immersion Solution: Weigh 20g (* * to 0.1g) of air dried sample passing through a 2mm sieve and place it in a 50ml high beaker. Add 20ml of water to remove CO2 and stir for 1 minute with a stirrer to fully disperse the soil particles. After 30 minutes, the measurement is carried out. Insert the electrode into the test solution (note that the lower part of the glass electrode bulb is located below 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, promote its rapid balance, let it stand for a moment, press the reading switch, and record the pH value when the reading stabilizes. Release the reading switch, take out the electrode, wash it with water, and use a filter paper strip to absorb the moisture before measuring * * samples. After testing 5-6 samples, it is necessary to check the positioning with standard solution.
③ Determination of pH of soil potassium chloride extraction solution: When the pH value of soil water immersion is less than 7, the pH value of soil salt extraction solution should be measured. The measurement method is to remove 1mol L ˉ Except for replacing CO2 free water with a potassium chloride solution, the water to soil ratio is 1:1, and the other measurement steps are the same as the pH value measurement in water immersion.
When measuring the pH value using an acidity meter, the pH value can be directly read without the need for calculation.
7 precision parallel results allow for * * differences: neutral and acidic soils ≤ 0.1pH units, alkaline soils ≤ 0.2pH units.\
