Method of measuring soil pH value with PH meter (acidity meter)
Soil samples were extracted with water or neutral salt solution (for example, 1mol/L potassium chloride solution can be used for acidic soil; 0.01mol/L calcium chloride solution can be used for neutral and alkaline soil). The ratio of water to soil is generally 2.5:1; the ratio of water to soil in saline-alkali soil is 5:1; peat is 10:1. After fully stirring, equilibrate for 30 minutes, and measure it with an acidity meter or a pH meter.
Reagent preparation:
1. pH4.01 standard buffer solution: 10.21g potassium hydrogen phthalate (KHC8H4O4, analyzed
Pure, dried at 105°C) dissolved in 1000m1 distilled water.
2. pH6.87 standard buffer solution: dissolve 3.39g potassium dihydrogen phosphate (KH2PO4, analytically pure, dried at 45°C) and 3.53g anhydrous disodium hydrogen phosphate (Na2HP4, analytically pure, dried at 45°C) In 1000m1 distilled water.
3. pH9.18 standard buffer solution: Weigh 3.80g of borax (Na2B4O7·10H2O) and dissolve it in 1000ml of boiled and cooled distilled water. Bottled and sealed for preservation.
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;
4. 1mol/L potassium chloride solution: Dissolve 74.6g potassium chloride (KCl, analytically pure) in 1000m1 distilled water. The pH of the solution is 5.5-6.0
5. 0.01mol/L calcium chloride solution: Dissolve 147.02g calcium chloride (CaCl2 2H2O, analytically pure) in 1000ml distilled water, namely 1mol/L calcium chloride solution. Take 10ml of 1mol/L calcium chloride solution in a 500ml beaker, add 500ml of distilled water, add dropwise calcium hydroxide or hydrochloric acid solution to adjust pH = 6, then use distilled water to make up to 1000ml, namely 0.01mol/L calcium chloride solution .
[instrument]
pH meter or acidity meter; pH composite electrode (or pH glass electrode and calomel standard electrode). [Sample processing]
Dry the soil sample and pass it through a 2mm sieve, weigh 10.0g of the sample into a 50ml beaker, add 25ml
Carbon dioxide-free distilled water or 1mol/L potassium chloride solution (acidic soil), or 0.01mol/L calcium chloride solution (neutral and alkaline soil), vigorously stirred with a glass rod for 1-2min, let stand for 30min, and Prepare for determination. At this time, attention should be paid to the influence of laboratory ammonia and volatile acid mist.
[determination]
Turn on the pH meter (or acidity meter) according to the instrument manual, and select a pH standard close to the pH value of the soil extract.
The quasi-buffer solution (pH4.01 buffer solution for acidic, pH6.87 buffer solution for neutral, pH9.18 buffer solution for alkaline) is used as the standard, and the pH value indicated by the calibration instrument is consistent with the standard value. Insert the composite electrode (or pH glass electrode and calomel standard electrode) of the pH meter into the soil extract, turn the beaker gently, and read the pH value. After each sample was measured, the electrodes were rinsed with distilled water and blotted dry with filter paper.
[calculate]
The pH value read by the pH meter. [Remark]
1. The analysis accuracy is 0.1pH value, but the precise pH meter can be accurate to 0.02pH unit, and use temperature compensation. 2. Soil samples should not be ground too finely, preferably with a 2mm sieve. If the ground sample cannot be measured immediately, it should be
Bottled and sealed to prevent ammonia and volatile acids in the air. Gently swirling the beaker after the electrode is inserted into the soil extract is necessary to speed up the equilibration time, which is especially important for poorly buffered or high pH soils. If a standard calomel electrode is used as a reference electrode, it must be inserted into the upper solution to reduce errors.
