Determination of total phosphorus in soil. Alkaline fusion-molybdenum antimony spectrophotometric method (1)

1.

This method is suitable for the determination of total phosphorus content in various soils

Second, the principle of the method

The soil sample melts with sodium hydroxide under high temperature conditions , so that the phosphorus-containing minerals and organophosphorus compounds in the soil are all converted into soluble orthophosphate , and under acidic conditions, it reacts with molybdenum antimony color reagent to form phosphomolybdenum blue.

3.

Unless otherwise specified, analytically pure chemical reagents that meet national standards are used for analysis, and the experimental water is newly prepared deionized water or distilled water

(1) Concentrated sulfuric acid, p(H ₂ SO ₄ )=1.

(2) 65% concentrated nitric acid

(3) Sulfuric acid solution: c(H ₂ SO ₄ )=3mol/L

Carefully add 168.

(4) Sulfuric acid solution: c(H ₂ SO ₄ )=0.

In 800 mL of water, carefully add 28.

(5) Sulfuric acid solution (1+1)

(6) Nitric acid solution (1+1)

(7) Sodium hydroxide solution concentration = 2mol/L

Dissolve 20.

(8) Anhydrous ethanol

(9) Potassium dihydrogen phosphate , pure superior grade
.

Dry an appropriate amount of potassium dihydrogen phosphate (KH ₄ PO ₄ ) in an oven at 110° C.
for 2 hours, and cool in a desiccator for later use
.

(10) 100g/L ascorbic acid solution
.

10g of ascorbic acid is dissolved in a suitable amount of water, and water is added to a 100mL volumetric flask to the mark and mixed well.
Stored in a brown glass bottle, it can be stable for two weeks
.
If the color turns yellow, discard for reconstitution
.

(11) Molybdate mixed solution
.

①0.
13g/mL ammonium molybdate solution: 13g ammonium molybdate is dissolved in 100mL water;

②0.
003g/mL potassium antimony tartrate solution: 0.
35g potassium antimony tartrate dissolved in 100 mL of water;

③ Under constant stirring, slowly add the ammonium molybdate solution to 300 mL of sulfuric acid solution, and then add the prepared potassium antimony tartrate solution and mix well
.
The mixed solution is stored in a brown glass bottle and can be stable for two months
.

(12) Phosphate stock solution, 50ug/mL
.

Weigh 0.
2197g of potassium dihydrogen phosphate, dissolve it in an appropriate amount of water, and transfer it into a 1000mL volumetric flask
.
Add 5mL 0.
5mol/L sulfuric acid solution, dilute to the mark with water, and mix well
.
The solution is stable for six months when stored in a brown glass bottle
.

(13) Phosphate standard solution, 5pg/mL
.

Pipette 25.
00mL phosphate stock solution into a 250mL volumetric flask, dilute to the mark with water, and mix well
.
The solution is prepared immediately before use
.

(14) 0.
002g/mL 2,4-dinitrophenol indicator
.

Weigh 0.
2 g of 2,4-dinitrophenol and dissolve it in 100 mL of water
.

Four, instruments and equipment

In the experiment, the newly purchased glassware needs to be washed with detergent first, and then soaked in nitric acid solution (1+1) for 24 hours, and then washed with tap water and deionized water before use, and then air-dried for later use
.

(1) Soil sample crushing equipment (pulverizer, agate mortar)
.

(2) The upper soil sieve, the aperture is 1mm and 100 mesh (or 0.
149mm)
.

(3) Analytical balance, with an accuracy of 0.
0001g
.

(4) Nickel crucible and silver crucible, the capacity is 30mL
.

(5) Spectrophotometer
.

(6) 50mL colorimetric tube with stopper
.

(7) Electric furnace or electric heating plate with adjustable temperature
.

(8) Water bath
.

(9) Muffle furnace
.

(10) Centrifuge, 50mL centrifuge cup
.

(11) pH meter: various types of composite pH meters or ion activity meters, accurate to ±0.
02
.

(12) Commonly used instruments and equipment in general laboratories
.

5.
Sample preparation and pre-treatment

(1) Collection and preservation

Collect and preserve soil samples in accordance with the relevant regulations of HJ/T166
.

(2) Preparation of samples

The collected soil sample is air-dried to near dryness, then the debris is removed, crushed, thoroughly mixed, and passed through a 1mm aperture sieve.
Then the soil sample is spread on kraft paper into a thin layer and divided into small squares
.
Use a small spoon to extract an equal amount of soil sample (the total amount is greater than 20g) in each square, and further grind it in a soil sample crushing device or an agate mortar to make all of it pass through a 0.
149mm sieve, mix well and put it into the mill Reserve in the mouth bottle
.

Weigh 0.
2500g (accurate to 0.
0001g) of the air-dried soil sample passing through a 0.
149mm sieve on the bottom of the nickel crucible, wet the sample with a few drops of absolute ethanol, and then add 2g of solid sodium hydroxide and spread it on the surface of the sample
.
Put the crucible into a high-temperature electric furnace and continue to heat up.
When the temperature rises to about 400°C, keep it for 15 minutes; then continue to heat up to 640°C, keep it for 15 minutes, and take it out for cooling
.
Then add 10mL of water to the crucible and heat it to 80℃ in a water bath.
After the frit is dissolved, transfer the solution in the crucible into a 50mL centrifuge cup, and wash with 10mL of 3mol/L sulfuric acid solution and appropriate amount of water for several times.
In a crucible, pour the washing liquid into the centrifuge cup, and then centrifuge at 4000r/min for 3 minutes
.
Transfer all the supernatant to a 100mL volumetric flask, and dilute to the mark with water for measurement
.

Note: When processing a large number of samples, the crucible after adding sodium hydroxide should be temporarily placed in a large desiccator to prevent moisture absorption
.

(3) Preparation of blank samples

Without adding the soil sample, follow the same steps as the sample preparation to prepare a blank sample for testing
.

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