Some standard content:
National Standard of the People's Republic of China
Determination of pH value in forest soil
Determination of pH value in forest soil This standard applies to the determination of pH value in forest soil. 1 Potentiometric method
1.1 Key points of the methodbzxZ.net
UDC 634.0.114
GB 7859 —87
The ratio of water or salt solution (1 mol/L potassium chloride for acidic soil, 0.01 mol/L calcium chloride for neutral and alkaline soil) to soil is 2.5:1, 5:1 for saline soil, and 10:1 for litter layer and peat layer. After adding water or salt solution, stir thoroughly and balance for 30 minutes, then insert pH glass electrode and calomel electrode into the leaching solution and measure with pH meter. The electromotive force value can also be measured with a millivoltmeter and then converted into pH value.
1.2 Reagents
1.2.1 pH 4.01 standard buffer: 10.21g potassium hydrogen phthalate (KHCH.O4 analytical grade) dried at 105℃, dissolved in water and diluted to 1L, that is, 0.05mol/L potassium hydrogen phthalate solution. 1.2.2 pH 6.87 standard buffer: 3.39g potassium dihydrogen phosphate (KH, PO4, analytical grade) and 3.53g anhydrous sodium hydrogen phosphate (NazHPO4, analytical grade) dried at 50℃, dissolved in water and diluted to 1L, that is, 0.025mol/L potassium dihydrogen phosphate and 0.025mol/L disodium hydrogen phosphate solution.
1.2.3pH9.18 standard buffer solution: 3.80g borax (Na,B.0,·10HO, analytical grade) is dissolved in cold water without carbon dioxide and the volume is adjusted to 1L, that is, 0.01mol/L borax solution. The pH of this solution is easy to change, so it should be stored carefully. 1.2.41mol/L potassium chloride solution: 74.6g potassium chloride (KCl, chemically pure) is dissolved in 400ml water, the pH of the solution should be between 5.5 and 6.0, and then diluted to 1L.
1.2.50.01mol/L calcium chloride solution: 147.02g calcium chloride (CaCl2·2H,O, chemically pure) is dissolved in 200ml water and the volume is adjusted to 1L, that is, 1.0mol/L calcium chloride solution. Pipette 10ml 1.0mol/L calcium chloride solution into a 500ml beaker and add 400ml water. Use a small amount of calcium hydroxide or hydrochloric acid to adjust the pH to about 6, and then dilute to 1L, which is 0.01mol/L calcium chloride solution. 1.3 Main instruments
Acidity meter, glass electrode, saturated calomel electrode, or pH composite electrode. 1.4 Determination steps
1.4.1 Preparation of test solution: Weigh 10g of air-dried soil sample that passes through a 2mm sieve into a 50ml tall beaker, add 25ml of carbon dioxide-free water or 1.0mol/L potassium chloride solution (for acidic soil determination) or 0.01mol/L calcium chloride solution (for neutral, calcareous or alkaline soil determination). Weigh 5g of litter layer or peat layer sample and add 50ml of water or salt solution. Stir vigorously with a glass rod for 1 to 2 minutes and let it stand for 30 minutes. At this time, avoid the influence of ammonia or volatile acids in the air. 1.4.2 Instrument calibration: Use a buffer solution with a pH value close to that of the soil extract to calibrate the instrument so that the pH value of the standard buffer solution is consistent with the pH value on the instrument scale.
1.4.3 Determination: Under the same conditions as above, insert the glass electrode and the calomel electrode into the soil suspension to measure the pH value. After each sample is measured, rinse the electrode with water and absorb the water with dry filter paper. 1.5 Result calculation
General pH meters can directly read the pH value without conversion. 1.6 Allowable deviation
Approved by the National Bureau of Standards on June 1, 1987
Implementation on January 1, 1988
GB7859-87
The allowable difference between the results of two parallel measurements of samples is 0.1pH; when the determination conditions and methods are strictly controlled indoors, the allowable difference of the precision pH meter can be reduced to 0.02pH.
Note: ①Precautions for using glass electrodes: a. Before using, the electrode should be immersed in 0.1mol/L hydrochloric acid solution or water for more than 12 hours to activate it. b. When using, the electrode should be gently shaken first to allow the solution inside to flow into the bulb part to prevent the existence of bubbles. The bulb part of the electrode is very easy to be damaged. It must be used carefully and cautiously. It is best to use a small sleeve to protect it. d. The electrode can be stored in water when not in use. If it is not used for a long time, it can be placed in a paper box. e. The surface of the glass electrode cannot be stained with oil. Avoid using concentrated sulfuric acid or chromic acid cleaning solution to clean the surface of the glass electrode. It cannot be parked in a strong alkali and fluoride-containing medium or in a colloidal system such as clay for too long to avoid damaging the electrode or causing the electrode to react slowly. ② Precautions for using saturated calomel electrode: a. The electrode should be supplemented with saturated potassium chloride solution and potassium chloride solid from the side port of the electrode at any time. When not in use, it can be stored in a saturated potassium chloride solution or the front end can be tightly covered with a rubber sleeve and placed dry. b. When using, the small rubber plug at the side port of the electrode should be removed to allow the potassium chloride solution to maintain a certain flow rate. c. Do not immerse in the solution to be tested for a long time to prevent the outflowing potassium chloride from contaminating the solution to be tested. d. Do not directly contact solutions that can corrode mercury and calomel, such as S! 1 solution with a high concentration. At this time, a double liquid junction salt bridge should be used instead, and potassium fluoride solution should be poured into the outer tube. Agar salt bridge can also be used. Preparation of agar salt bridge: weigh 3g of superior agar and 10g of potassium chloride (KC1, analytical grade), put them in a 150ml beaker, add 100ml of water, heat and dissolve in a water bath, and then use a dropper to pour the dissolved agar solution into a U-shaped tube with a diameter of about 4mm. There should be no bubbles in the middle and both ends should be covered, and then soak it in 1mol/L potassium chloride solution. ③ Precautions during measurement: a. Do not grind the soil too finely, it is better to pass through a 2mm aperture sieve. When the sample is not measured immediately, it is best to store it in a specimen bottle with a ground mouth to avoid the influence of ammonia and other volatile gases in the atmosphere. b. The equilibration time after adding water or 1.0 mol/L potassium chloride solution has an impact on the measured soil pH value, and it varies with the soil type. For fast equilibration, 1 min is enough to reach equilibrium, while for slow equilibration, it can take up to 1 h. Generally speaking, 30 min of equilibration is appropriate. c. After the pH glass electrode is inserted into the soil suspension, it should be shaken slightly to remove the water film on the glass surface and accelerate the equilibrium, which is especially important for soils with weak buffering and high pH. d. The saturated calomel electrode is best inserted into the upper clear solution to reduce the error caused by the soil suspension affecting the liquid junction potential. 2 Colorimetric method
2.1 Key points of the method
According to the color produced by the mixed indicator and the standard buffer series of pH 4.0-8.0, various colored inorganic reagent solutions are mixed in different proportions to form a simulated pH permanent color scale storage solution, and its pH value is marked (each interval is 0.2 pH unit), and then visually compared with the test solution to determine the pH value of the test solution. 2.2 Reagents
2.2.1 pH mixed indicator: 0.1g methyl red is placed in an agate mortar, and a small amount of anhydrous alcohol is added to grind to make 100ml alcohol solution, and 7.4ml 0.050mol/L sodium hydroxide solution is added, and diluted to 500ml with water. Another 0.1g bromothymol blue is weighed and dissolved in 52ml anhydrous alcohol, and 3.2ml 0.050mol/L sodium hydroxide solution is added, and diluted to 250ml with water. Mix 1 part of methyl red solution and 2 parts of bromothymol blue solution to make a pH mixed indicator. This solution should be placed in a dark dropper bottle. 2.2.2 Cobalt chloride solution: 59.5g cobalt chloride (CoC12·6H,O, analytical grade) is dissolved in 1L 1% (V/V) hydrochloric acid solution. 2.2.3 Ferric fluoride solution: 45.05g ferric chloride (FeC13·6H0, analytical grade) is dissolved in 1L 1% hydrochloric acid solution. 2.2.4 Copper chloride solution: 400g copper chloride (CuCl, ·2H,O, analytical grade) is dissolved in 1L 1% hydrochloric acid solution. 2.2.5 Copper sulfate solution: 200g copper sulfate (CuSO4·5H2O, analytical grade) is dissolved in 1L 1% sulfuric acid solution. Mix the above prepared solutions according to the proportions in Table 1, and add a certain amount of water to prepare a pH permanent color scale. Each level of solution is purchased and stored in a flat-bottomed test tube (required specifications-consistent), marked with pH value, and sealed with a stopper for storage. 172
2.3 Determination steps
GB7859-87
Preparation of pH permanent color scale solution
Take 10ml of water extract (water: soil is 2.5:1) and place it in a flat-bottomed test tube, add 12 drops of pH mixed indicator, shake well and immediately compare visually with the permanent color scale to determine its pH value. 2.4 Allowable deviation
The allowable difference between two parallel determination results is 0.5pH. Additional Notes:
This standard was proposed by the Ministry of Forestry of the People's Republic of China and is under the jurisdiction of the Forestry Research Institute of the Chinese Academy of Forestry. This standard was drafted by the Forestry Soil Research Institute of the Chinese Academy of Sciences and the Forest Soil Research Office of the Forestry Research Institute of the Chinese Academy of Forestry.
Tip: This standard content only shows part of the intercepted content of the complete standard. If you need the complete standard, please go to the top to download the complete standard document for free.