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Soil potassium analysis
The total potassium content of the soil is generally 1 to 2%, and the higher ones can reach 3% of the upper energy, and the lower ones can be lower than 0.2%. The structure potassium accounted for about 90~98%, slow-acting potassium accounted for 2~8%, and quick-acting potassium 1~2%. The decomposition of soil potassium samples can be broadly divided into alkali melting and acid dissolution, and the earlier adopted NH4Cl-CaCO3 alkali fusion method, due to the high purity of the flux used, the large amount of sample, volatile loss, and procedures compared Cumbersome, it is rarely used at present. Na2CO3 alkali fusion method and HF-HClO4 acid fusion method are more convenient, complete decomposition, high accuracy but expensive platinum crucibles are required, and require good ventilation equipment. In recent years, NaOH has been gradually replaced by NaOH melting method, using NaOH. The melting method is not only easy to operate, but also completely decomposed, and it can be replaced by silver crucible (or nickel crucible), which is suitable for general laboratory use. The determination of potassium in the test liquid is now mostly flame-based, which is both quick and easy, accurate and sensitive. 1. NaOH melt-flame photometric method: (1) Method points: The sample was melted with NaOH at high temperature in the crucible, the melt was dissolved with water, and the sample was measured for potassium by flame photometry. The potassium concentration of the reading solution can be determined by the potassium concentration of the multiple potassium standard solution and the working curve of the galvanometer reading, and then the potassium content of the sample can be calculated. (2) Main instruments: flame photometer, silver crucible (30 ml), high temperature electric furnace, volumetric flask (50~100). (3) Reagents: A. Sodium hydroxide (analytically pure, granular). B. Anhydrous (analytically pure). C. 1:1 hydrochloric acid (chemically pure). D. 4.5 mol/l sulfuric acid solution. E. Potassium standard solution. 0.1907 g of potassium chloride (KCl analytical pure at 105°C for 2 hours) is dissolved in water and the volume is 1 liter, which is 100 ppm K standard solution, stored in a plastic bottle, and a series of 0, 5, 10, 20, 40 is prepared. , 60 ppm contains K standard solution which is similar to the ion composition in the test solution. (4) Measurement procedure: A. Preparation of test solution. Weigh out 0.2 g (through a 0.25 mm screen) 0.2 g (accurate to 0.0001 g) on ​​the bottom of the crucible (don't stick to the wall), moisten the sample with a few drops of anhydrous alcohol, and add 2 g of solid NaOH. . Tile the surface of the sample and temporarily store it in a desiccator to prevent dehydration. The crucible was placed in a high-temperature furnace and raised from room temperature to 300-400. C, keep warm for 15 minutes, rise to 750. C, heat for 15 to 30 minutes, remove and cool. Add 10 ml of water to 80 in an electric oven. About C, the frit dissolved and then micro-boiling for 5 minutes. The solution in the crucible was transferred to a 50 ml volumetric flask. The crucible was washed several times with hot water and 2 ml of 4.5 mol/l H2SO4 and poured into a volumetric flask. The total volume to about 40 ml, and finally to the volumetric flask plus 5 drops of 1:1 HCl solution and 5 ml of 4.5 mol / liter H2SO4 solution shaking, cooled to room temperature, with water to volume, shaken, left to clarify or filter paper filter. This test solution can be used for determination of total potassium. B. Determination: Pipette 5 to 10 ml of the test solution in a 50 ml volumetric flask (potassium concentration is preferably controlled at 20 to 30 ppm), dilute to volume with water, measure directly with a flame photometric method, record the readings, and then record on the working curve. Check the potassium concentration in the reading solution. (5) Calculation of results (not recorded) 2. Acid-dissolve-flame photometric method: (1) Method points: Dissolve the potassium in the soil with hydrogen acid-perchloric acid, and measure the potassium with a flame photometric method. (2) Main instrument: platinum crucible (30 ml) or plastic teflon crucible (high temperature resistant, 30 ml), flame photometer, volumetric flask (50 ml), plastic measuring cylinder (10 ml). (3) Reagents: A. Concentrated hydrofluoric acid (HF, 48%, analytically pure). B. Concentrated perchloric acid (HClO4, 60%, analytical grade). C. 3 mol/l hydrochloric acid. D. K standard solution. Pipette 100ppmK standard solution to prepare 0, 5, 10, 20, 40, 60ppm K standard solution containing HCl. (4) Measurement step*: A. Preparation of test solution. Weigh 0.3 g (accurate to 0.0001 g) of 0.25 mm mesh soil into 30 ml platinum crucible, moisten the sample with a few drops of water, add 5 ml of 60% perchloric acid, and add 5 ml of hydrofluoric acid solution. Shake it carefully and mix it evenly. The crucible was placed on an electric furnace and heated at a low temperature. The HF and the sample were fully applied. And to prevent its rapid evaporation or splash. When Changbai White Perchlorate is used, remove the moromi and cool it slightly, add 5 ml of HF, continue heating and boil, and evaporate to near dryness. Remove the hydrazine and add 3 ml of HClO4, continue to dry to drive off the remaining HF, and slowly heat up until the white smoke ran out, basically removing the excess HClO4. Add 4 ml of 3 mol/l HCl and heat at low temperature to dissolve the residue. Then wash it into a 100 ml volumetric flask and bring it to volume and shake for total potassium determination. B. Determine the same method 1. (5) The calculation result is the same as the calculation of Law 1. 1 The entire digestion process should be carried out in a fume hood. 2 Soil samples should first be added with HClO4 followed by HF. On the contrary, splashes may be caused. 3 The standard for digestion is that when the second subsidy plus HF is in HC1O4 white fume, the contents of the plutonium should be crystal clear. If there is a precipitate, it must be treated once again with HF-HClO3. 4 There should be no fluoride ion in the test solution, otherwise it will affect the result. Determination of slow-acting potassium in soil (1 mol/litre nitric acid boiling extract-flame spectrophotometry) 1. Method points: Boil slow release of potassium in the soil by boiling 1 HN/L HNO3 solution. This method not only has a short extraction time, but also has a small amount of cultivating reagents. The coefficient of variation for multiple determinations is also small, and it is often used as an indicator of soil potassium supply replacement. 1 mol/L HNO3 boiled leaching potassium minus the available potassium is the slow-potassium content. 2. The main instruments: flame photometer, electric furnace, pressure regulator, high beaker, volumetric flask. 3. Reagents: (1) Extractant (1 mol/l HNO3 solution) (2) 0.1 mol/l HNO3 solution) (3) Potassium standard solution: Prepare 0, 5, 10, 20, 40, 60 ppm with 100 ppm K standard solution. HNO3 K standard solution. 4. Assay procedure: Weigh out 5.0 g of air dried soil through a 1 mm aperture sieve in a 200 ml high beaker, add 50 ml of 1 mol/l HNO3 solution, cover with a watch glass, boil for 10 minutes on an electric oven, and filter while hot. The volumetric flask was washed 4 to 5 times with 0.1 mol/l HNO3 (or hot water) and cooled to a constant volume. This solution can be directly measured with potassium photometric flame, record galvanometer readings, and then check the potassium concentration of the reading solution from the working curve.