Application of ph meter in measuring soil ph acidity and alkalinity
Summary of ph meter measurement method
The pH of the soil is determined by the potentiometric method. The pH glass electrode and the calomel electrode are inserted into the soil suspension or leaching solution, and the electromotive force value is measured, and then converted into a pH value (acidity and alkalinity). Measured on the acidity meter, the pH value can be read directly after the standard solution is set. The water-to-soil ratio has a greater impact on the pH value, especially the effect of the dilution effect on calcareous soil. It is advisable to adopt a small water-to-soil ratio, and the soil pH is 1:1. At the same time, in acidic soil, in addition to measuring the ph value of the water-soaked soil, the ph value of the salt-soaked soil should also be measured, that is, the soil H+ is leached with 1molLˉ1 potassium chloride solution and then measured by the potentiometric method.
1.2 Application scope of online ph meter measurement Applicable to the determination of various soil ph values.
1.3 The main instruments and equipment for ph meter measurement
①Acidity meter; ②ph glass electrode; ③saturated calomel electrode; ④stirrer.
1.4 ph meter measurement reagent
① 1molLˉ1 potassium chloride solution: Weigh 74.6g of potassium chloride (chemically pure) and dissolve it in 800ml of water, adjust the pH of the solution to 5.5-6.0 with dilute potassium hydroxide and dilute hydrochloric acid, and dilute to 1L;
② ph4.01 (25°C) standard buffer solution: Weigh potassium hydrogen phthalate 10.21 which has been dried at 110-120°C for 2-3 hours, dissolve in water, transfer to a 1L volumetric flask, make up to volume with water, and store in a polyethylene bottle;
③ pH6.87 (25°C) standard buffer solution: Weigh 3.533g of disodium hydrogen phosphate and 3.388g of potassium dihydrogen phosphate which have been dried at 110-130°C for 2-3 hours, dissolve in water, transfer to a 1L volumetric flask, and dilute to volume with water. Store in polyethylene bottles;
④ pH9.18 (25°C) standard buffer solution: Weigh 3.800g of balanced borax (Na2B4O7·10H2O) and dissolve it in CO2-free water, transfer it into a 1L volumetric flask, dilute it with water, and store it in a polyethylene bottle;
Balance treatment of borax: put borax in a desiccator filled with saturated aqueous solution of sucrose and salt to balance for two days and nights;
⑤ Distilled water to remove CO2.
1.5 pH meter measurement and analysis steps
① Instrument calibration: The usage methods of various pH meters and potentiometers are not consistent, and the treatment of electrodes and the use of instruments are carried out according to the instrument instructions. Adjust the solution to be tested and the standard buffer solution to the same temperature, and adjust the temperature compensator to this temperature value. When calibrating the instrument with a standard buffer solution, first insert the electrode into the standard buffer solution whose pH value is not more than 2 pH units different from the tested sample, start the reading switch, adjust the positioner so that the reading is just the pH value of the standard solution, and repeat several times Allow the reading to stabilize. Take out the electrode and wash it, dry the water with a filter paper strip, and then insert it into a standard buffer solution. The allowable deviation between the two standard solutions is 0.1ph unit. If it exceeds, check whether there is a problem with the instrument electrode or the standard solution. The instrument can be used to measure samples only after it is calibrated correctly.
②Determination of ph of soil water immersion solution: Weigh 20g (minimum 0.1g) of air-dried sample that passed through a 2mm aperture sieve into a 50ml tall beaker, add 20ml of water that removes CO2, and stir with a stirrer for 1min to make the soil particles fully Disperse and measure after standing for 30min. Insert the electrode into the solution to be tested (note that the lower part of the glass electrode bulb is located under the soil-liquid interface, and the calomel electrode is inserted into the upper clear solution), shake the beaker gently to remove the water film on the electrode, and promote its rapid equilibrium. Lower the reading switch, and write down the pH value when the reading is stable. Release the reading switch, take out the electrode, wash it with water, dry the water with a filter paper strip, and then measure each sample. After measuring 5-6 samples, check the positioning with standard solution.
③ Determination of ph value of soil potassium chloride salt extract: When the pH value of soil water immersion is <7, the ph value of soil salt extract should be measured. The determination method is except that 1molLˉ1 potassium chloride solution is used instead of CO2-free water, and the water-to-soil ratio is 1:1.
1.6 Calculation of ph meter measurement results When the pH value is measured with a pH meter, the ph value can be read directly without calculation.
1.7 Measurement precision of ph meter Parallel results allow maximum difference: neutral and acidic soil ≤ 0.1ph unit, alkaline soil ≤ 0.2ph unit.
1.8 pH Meter Measurement Notes
① Glass electrodes that are not used for a long time need to be soaked in water for 24 hours to activate them before they can react normally. Those that are not used temporarily can be soaked in water. When they are not used for a long time, they should be stored dry. When the surface of the glass electrode is contaminated, it needs to be treated. The cavity of the calomel electrode should be filled with saturated potassium chloride solution, and there should be a little potassium chloride crystallization at room temperature, but the potassium chloride crystallization should not be too much, so as not to block the passage between the electrode and the solution to be measured. There must be no air bubbles between the inner electrode of the glass electrode and the bulb, and between the inner electrode of the calomel electrode and the ceramic core.
② The position of the electrode in the suspension affects the measurement results. It is required to insert the calomel electrode into the upper unclear liquid and avoid contact with the mud as much as possible to reduce the influence of the calomel electrode liquid junction potential.
③Shaking the beaker during the ph reading will result in a low reading, and you should wait a little while after shaking before reading.
④ Avoid the entry of acid and alkali steam during operation.
⑤ The standard solution can be stored for 1 to 2 months at room temperature, and the storage period can be extended in a refrigerator at 4°C. Do not pour the used standard solution back into the original solution for mixing. If turbidity and precipitation are found, it cannot be used again.
⑥The temperature affects the electrode potential and the ionization balance of water. The temperature of the temperature compensator, the standard buffer solution and the solution to be tested must be consistent. The pH value of the standard solution changes slightly with temperature.
⑦ When measuring batch samples, it is best to separate the samples with a large difference in pH value according to the soil type, etc., so as to avoid measurement errors caused by the slow influence of the electrode.
⑧ If the sample with low density is measured, the water-to-soil ratio can be changed moderately, but it must be indicated.
