Ammonium acetate method for measuring cation exchange capacity

Soil cation exchange capacity (CEC) refers to the total amount of various cations that soil colloids can absorb. Its value is expressed in terms of the amount of substances containing various cations per kilogram of soil, and the unit is cmol (+)/kg. Ammonium acetate method is a more commonly used method for measuring soil cation exchange capacity, which is described in detail below.

1. Experimental principle
Repeatedly treat the soil with 1mol/L ammonium acetate solution of pH 7.0 to make the soil saturated with ammonium ions. The excess ammonium acetate is washed off with 95% ethanol, then magnesium oxide is added, and distilled by fixed nitrogen distillation method. The distilled ammonia is absorbed with boric acid solution, titrated with standard acid solution, and the soil cation exchange capacity is calculated according to the amount of ammonium ions.

II. Experimental instruments and reagents
1. ** Instruments **: balance (accurate to 0.0001g), oscillator, centrifuge, Kjeldahl nitrometer, distillation device, conical bottle, pipette, volumetric bottle, etc.
2. ** Reagent **:
- ** 1mol/L ammonium acetate solution **: Weigh 77.08g ammonium acetate (analytically pure), add water to dissolve, dilute to nearly 1L, adjust to pH 7.0 with dilute acetic acid or dilute ammonia water, then bandwidth evaluation to 1L.
- ** 95% ethanol **: analytically pure.
- ** Magnesium oxide **: Burn magnesium oxide in a high temperature furnace at 500 - 600 ° C for 2h, cool and store in an airtight container for later use.
- ** 2% boric acid solution **: Weigh 20g of boric acid (analytically pure), dissolve in 1L of water, add 5mL of methyl red-bromocresol green mixed indicator per liter of boric acid solution, and adjust to purplish red (pH about 4.5) with dilute acid or dilute base.
- ** 0.01mol/L hydrochloric acid standard solution **: Accurately absorb 100mL of 0.1mol/L hydrochloric acid standard solution in a 1L volumetric flask, bandwidth evaluation with water to scale, shake well.

III. Experimental steps
1. ** Weigh sample **: Weigh air-dried soil sample 1.0000 - 2.0000g (accurate to 0.0001g) through a 2mm sieve and put it into a 100mL centrifuge tube.
2. ** Ammonium ion exchange **: Add 1mol/L ammonium acetate solution 20mL, plug the centrifuge tube tightly with a rubber plug, oscillate on an oscillator for 30min, then centrifuge at 3000r/min for 5min, and transfer the supernatant to a 100mL volumetric flask. Repeat the above operation, and then treat the soil with 1mol/L ammonium acetate solution 20mL twice. Transfer the supernatant after each centrifugation to the same 100mL volumetric flask. Finally, use 1mol/L ammonium acetate solution bandwidth evaluation to scale, and shake well. This is the liquid to be tested.
3. ** Wash off excess ammonium acetate **: Add 95% ethanol 20mL to the centrifuge tube containing the soil sample, shake for 1-2min, centrifuge for 5min, and discard the supernatant. Repeat this operation 3-4 times until the supernatant after the last centrifugation is free of ammonium ions (check with Nye's reagent, take 1-2 drops of supernatant on a white porcelain plate, add 1-2 drops of Nye's reagent, if no yellow appears, indicating no ammonium ions).
4. ** Distillation **: Transfer all the washed soil samples to the Kjeldine nitrogen bottle without damage, add 1g of magnesium oxide and 100mL of water, connect the distillation device, and insert the lower end of the condenser tube into the conical bottle containing 25mL of 2% boric acid solution. Turn on the power supply, carry out distillation, and stop distillation when the distilled liquid reaches about 100mL.
5. ** Titration **: Titration the absorption solution with 0.01mol/L hydrochloric acid standard solution. The solution changes from blue-green to purple-red as the end point. Record the volume of the hydrochloric acid standard solution consumed.

IV. Results Calculation
Soil cation exchange capacity (cmol (+)/kg) = c × V × 1000/m

where:
c - concentration of hydrochloric acid standard solution (mol/L);
V - volume of hydrochloric acid standard solution consumed by titration (mL);
m - mass of air-dried soil sample (g).

V. Precautions
1. The oscillation time should be strictly controlled to ensure that the exchange reaction is fully carried out.
2. When washing off excess ammonium acetate with ethanol, make sure that it is washed out, otherwise it will affect the measurement results.
3. During the distillation process, pay attention to check the airtightness of the device to prevent ammonia from escaping.
4. The judgment of the titration end point should be accurate to avoid excessive titration.

The ammonium acetate method can more accurately determine the soil cation exchange capacity, providing important basic data for soil fertility evaluation, soil improvement, etc.