CATION EXCHANGE CAPACITY
SA10
Last Revised:
June 2008
1. PURPOSE
This exercise uses a simple and fast method to determine cation exchange capacity (CEC). The
extracted Mg & Ca are titrated, the result corrected for Na & K and a typical %saturation used.
2. REAGENTS & EQUIPMENT
2.1 pH meter
2.2 Air-dried soil (passed through 2 mm sieve)
2.3 1M ammonium ethanoate
2.4 Standardised 0.01 M EDTA
2.5 pH buffer
2.6 EBT indicator
3. PROCEDURE
3A. pH determination
3.1 In duplicate, weigh about 10 g of soil into a small beaker.
3.2 Add about 20 mL of distilled water to each beaker and mix well with a clean stirring rod.
3.3 Let stand about 15 minutes. Stir several times during this time.
3.4 Calibrate the pH meter with appropriate standards.
3.5 Carefully insert the pH meter electrodes into each sample and determine the pH.
3B. Ca & Mg by EDTA titration
3.6 In duplicate, weigh accurately about 0.5g of soil into a 250 mL conical flask.
3.7 Add 100 mL of 1M ammonium ethanoate solution.
3.8 Shake vigorously once a minute for 5 minutes.
3.9 Filter the sample using #2 (or equivalent) filter paper.
3.10 Add 10 mL of buffer solution and sufficient EBT indicator to the filtrate.
3.11 Titrate the filtrate with EDTA until the colour has changed from red through purple to blue. The
end point is reached when the last reddish tinge has disappeared and pure blue remains.
4. REPORT
Calculations
1.
Calculate the meq (milliequivalents) of Ca & Mg using the formula below.
meq Ca Mg =
2.
titre (mL ) x molarity EDTA x 200
sample mass ( g)
Multiply this value by 1.05 to determine meq total.
Soil Analysis Practical Manual
3.
Using the table below, determine the %base saturation (assuming kaolinitic soils).
pH
4.5
5.0
5.5
6.0
6.5
7.0
7.5
8.0
4.
%saturation
0
11
22
33
44
55
68
89
Using the equation below, calculate CEC.
CEC =
5.
SA10
100 x meq total
%saturation
Average your two values.
Questions
If you know the soil texture for thus soil, compare your CEC value to the typical values in Table
1.
5.1.
2.
Explain why the meq Ca/Mg equation has a factor of 200 (the 100 is simply a percentage, so
where does the 2 come from?).
Explain the correction factor of 1.05 for conversion to total meq.
3.
4.
Describe how this method could be made more accurate. Why is it used?
Page 2 of 3
Soil Analysis Practical Manual
SA10
Page 3 of 3