Soil organic matter determination potassium dichromate volumetric method (1)

Soil organic matter refers to the organic compounds containing carbon in the soil, and its sources are very wide

Now take the sixth part of soil testing: determination of soil organic matter (NY/T1121.

1.

It is suitable for soil with organic matter content below 15%

Second, the principle of the method

Under heating conditions, use excess potassium dichromate -sulfuric acid solution to oxidize soil organic carbon, and the excess potassium dichromate is titrated with ferrous sulfate standard solution.

Three, the main equipment

(1) Electric furnace (1000W)

(2) Hard test tube (medium 25mm×200mm)

(3) Oil bath

Use an aluminum pot with a height of 15-20cm, with glycerin (or intelligent control digital display oil bath)

(4) Wire cage

The size and shape are matched with the oil bath, there are several small compartments inside, and one test tube can be inserted into each compartment

(5) Automatic zero adjustment titrator or 10mL single-standard pipette, 50mL burette

(6) Thermometer (0～300℃)

Four, reagents

(1) 0.

Weigh 40.

(2) 0.

Weigh 28.
0g of ferrous sulfate or 40.
0g of ferrous ammonium sulfate and dissolve in 600-800mL of water, add 20mL of concentrated sulfuric acid and stir evenly.
After a while, filter with filter paper into a 1L volumetric flask, then wash the filter paper with water and add water to 1L
.
This solution is easy to be oxidized by air and the mass concentration decreases.
The accurate mass concentration should be calibrated each time it is used
.

Calibration of 0.
1 mol/L ferrous sulfate solution: draw 20.
00 mL of 0.
1000 mol/L potassium dichromate standard solution into a 150 mL Erlenmeyer flask, add 3 to 5 mL of concentrated sulfuric acid and 3 drops of o-phenanthroline indicator.
Iron solution titration, the accurate mass concentration of ferrous sulfate solution can be calculated according to the consumption of ferrous sulfate solution
.

(3) Potassium dichromate standard solution

Accurately weigh 4.
904g of potassium dichromate (superior grade) dried at 130℃ for 2～3h, first dissolve it with a small amount of water, then transfer it to a 1000mL volumetric flask without damage, add water to make the volume, the mass concentration of this standard solution is c(1/ 6K ₂ Cr ₂ O ₇ )=0.
1000mol/L
.

(4) Phenanthroline indicator

Weigh phenanthroline dissolved 1.
49g of FeSO containing 0.
70 g of _.
4 · 7H ₂ (NH2 or O 1.
00 g of _.
4 ) ₂ SO _.
4 · of FeSO _.
4 · 6H ₂ 100 mL of an aqueous solution of O
.
This indicator is perishable and should be kept tightly closed in a brown bottle
.

In this method, attention is paid to the preparation of potassium dichromate- sulfuric acid solution
.
The success of the reagent preparation has a key influence on the accuracy of the experimental results
.

Five, experimental steps

(1) Sample determination

Accurately weigh 0.
05～0.
5g of air-dried sample through 0.
25mm aperture sieve (accurate to 0.
0001g, the weighing amount depends on the range of organic matter content), put it into a hard test tube, and then accurately add 10.
00mL0 from the automatic zero-adjusting burette.
4mol/L potassium dichromate - sulfuric acid solution, shake well
.
Insert the test tubes into the wire cage one by one, and then sink the wire cage into the oil bath that has been heated to 185～190℃ on the electric furnace, so that the liquid level in the tube is lower than the oil level, and the temperature of the oil bath is required to drop to 170～180℃, start timing when the solution in the test tube boils, lift the wire cage from the oil bath after 5±0.
5min, cool for a while, and wipe off the oil outside the test tube
.
Transfer the digestion liquid and soil residue in the test tube into a 250mL Erlenmeyer flask without damage, rinse the test tube with water, and incorporate the washing liquid into the Erlenmeyer flask, so that the total volume of the solution in the Erlenmeyer flask is controlled at 50-60 mL
.
Add 3 drops of o-phenanthroline indicator and titrate the remaining K ₂ Cr ₂ O ₇ with ferrous sulfate standard solution .
The color change process of the solution is orange-blue-green-brown-red
.
If the number of milliliters of ferrous sulfate solution used in the titration is less than 1/3 of the number of milliliters of ferrous sulfate solution consumed in the following blank test, the soil weight should be reduced
.

(2) Blank experiment

For each batch of analysis, two blank tests must be done at the same time, that is, about 0.
2g of burned soil is taken instead of the soil sample, and the other steps are the same as the soil sample determination
.

Burning the upper soil: Take 200g of the upper soil and pass through a 0.
25mm sieve, put it into several porcelain evaporating dishes, burn it in a muffle furnace at 700-800℃ for 1 to 2 hours, completely burn out the organic matter, and set aside
.

Related links: Determination of water-soluble fluoride and total fluoride in soil by ion selective electrode method (3)