Cation Exchange Capacity
Soil 206 – Soil Ecosystem Lab
Objectives:
After completing this laboratory the student should be able to:
1. List and explain three factors influencing the magnitude of cation exchange capacity in soil.
2. Perform cation exchange calculations involving cmolC/kg and atomic mass data.
3. Know and understand the procedures to determine CEC.
Cation Exchange Capacity
The use of soil for biomass production, waste disposal and wastewater treatment, and chemical,
industrial and pharmaceutical uses, requires controlling and manipulating the chemistry of soil.
Fundamental to understanding soil chemistry is the study of soil colloids which are principle in the
process of ion exchange and also principle in the plethora of uses of this natural resource. The
abundance or the absence of ion exchange, how this can be predicted and determined and how this
affects the potential use of soil is the purpose of this lab.
The clay, hydrous oxides, and humus (the end product of soil organic matter degradation) fractions of
the soil are primarily made up of colloidal particles. Colloidal particles are extremely small (0.20.001m in diameter), exhibit a large surface area per unit weight and, in soils, usually exhibit a negative
charge. Colloidal charge may be constant, as in the case of isomorphous substitution, or pHdependent, as in the case of residual charges associated with the broken edges of clay crystals and the
carboxylic and phenolic groups of humus and the hydroxyl groups of oxides. This pH dependent cation
exchange capacity (CEC) increases with increasing soil pH. An important result of the net negative
charge is the attraction soil colloids exhibit for positively charged ions (cations). Adsorption is the term
used to describe the electrical attraction of ions to a charged surface. Refer to chapter 4 in Gardiner and
Miller for a complete discussion of colloidal charge.
Cation Exchange
Adsorbed soil cations are held very weakly by soil colloids. The weak nature of this attraction allows an
adsorbed cation to be easily replaced by another cation. Cation exchange is the process where cations
in solution exchange places with adsorbed cations on an exchange complex. The exchange complex
refers to all soil solids exhibiting charge (e.g. clay, hydrous oxides, and humus).
An exchange reaction between calcium in the soil solution and adsorbed hydrogen ions can be
represented as follows:
4H+-X + Ca2+
2Ca2+-X + 4H+
where “X” represents a negatively charged exchange complex. This reaction is oversimplified for the
sake of illustrating the basics of cation exchange. The soil system contains a great abundance and
variety of cations on the exchange complexes and in the soil solution. The ability to retain and exchange
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cations is one of the most important chemical properties of a soil. Through cation exchange, soil colloids
greatly increase the retention of plant nutrients such as calcium, magnesium, potassium, and ammonium
or other positively charged ions as may be desired in other industries. These adsorbed cations are
partially protected from movement, yet in the case of plant nutrients, they are readily available for plant
consumption. To a certain extent, the long-term fertility of a soil is dependent upon its CEC. The CEC is
also responsible for the soils ability to prevent leaching of pollutants, promote filtration of contaminated
water, and allow selective extraction of aqueous species.
The cation exchange capacity (CEC) of a soil is defined as the total amount of exchangeable cations
adsorbed by a given weight of soil. Cation exchange capacity is expressed in terms of moles of positive
charge per unit weight. For the convenience of being able to express CEC in whole numbers, the
accepted unit of CEC is centimoles of positive charge per kilogram of soil (cmolC/kg). Cation exchange
capacities range from about 2 to 100 cmolC/kg in mineral soils and as high as 200 cmolC/kg in organic
soils.
The total negative charge associated with a given colloid can be partitioned into constant charge (due
to isomorphous substitution) and variable charge (pH dependent). Constant charge indicates that the
surface of the colloidal particle has a permanent charge and is not affected by changes in pH. This is the
result of a cation substitution during the formation of the colloid and is incorporated into the crystal
structure of the colloid.
Variable charge, also called pH-dependent charge, on the other hand is not a permanent charge and is
due to deprotonation (loss of H) of hydroxyl (-OH) groups on the surface of organic and inorganic
colloids. As hydroxyl groups’ deprotonate, a negative charge is created. In this manner, the total negative
charge and CEC of most soils increases with pH. The illustration below represents this reaction.
Illustration 1: pH and Surface Hydroxyl Groups
Si-----OH
Protonated surface
hydroxyl group
Increase pH
Si------ODeprotonated surface
hydroxyl group
The magnitude of negative charge, as expressed in cmolC/kg, is summarized in table below and is
related to the type and amount of colloid present. For example, a soil with a high content of humus and
2:1 type clay with high net negative charge (such as vermiculite or smectite) will exhibit a high CEC. A
soil high in 1:1 type clay (such as kaolinite), which does not have appreciable permanent net negative
charge, is low in humus and at neutral pH will have low CEC. The table below summarizes the trend in
CEC for various soil fractions at a pH of 7.
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Table 1: CEC Estimates for Various Soil Fractions, pH 7
Negative Charge
Constant Charge
Colloid
(cmolc/kg)
(%)
Humus
200
10
Vermiculite (2:1)
150
95
Smectite (2:1)
100
95
Illite (2:1)
30
80
Kaolinite (1:1)
8
5
Al and Fe oxides
4
0
Variable Charge
(%)
90
5
5
20
95
100
Most colloids in soils act as a “giant anion” in cation exchange reactions. However, positively charged
colloids do exist, particularly in acidic soils high in hydrous oxides of Fe, Al, and Mn. In this case, anion
exchange reactions may occur.
Cation Exchange Estimates
The CEC of a soil can be estimated from the percentage of each colloid present and the magnitude of
negative charge associated with each colloid. The CEC is the sum of the net negative charge
contributed by each colloidal fraction. The following example illustrates the calculations involved in
estimating CEC from soil colloid composition.
Example 1: Soil CEC Estimate
Estimate the CEC if a soil contains 1.6% humus, 18% vermiculite, and 3% Al and Fe oxides. Use
the values from the table to determine the negative charge associated with each colloid.
CEC Estimate from Colloid Composition
Composition
Colloid
(%)
Humus
1.6
Vermiculite
18.0
Oxides
3
Potential CEC
(cmolC/kg)
200
150
4
Contribution
(cmolC/kg)
3.3
27
0.12
Total CEC = 30.42 cmolC/kg Soil
Cation Exchange Calculations
Cations are adsorbed on a chemical equivalency (equivalent charge) basis. For example, exactly one
mole of charge (molC) of H+ is required to replace one molC of Ca+2, Al+3, or any other cation. Keep in
mind that a molC, the unit of equivalent charge, is not the same as a mole, the weight of exactly 6.02 x
1023 atoms (Avogadro’s number) of a given element. Although 1 molC of H+ equals 1 molC of Ca+2, the
weight of each element required to supply one equivalent charge is different. The following example
compares mole weights to mole charges.
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Example 2: Avogadro’s Number, Mole Charge and Mole Weight Comparrison
For H+:
For Ca+2:
For H+:
For Ca+2:
1 mole
1 mole
1 mole
1/2 mole
=
=
=
=
6.02 x 1023 atoms
6.02 x 1023 atoms
6.02 x 1023 (+) charges
6.02 x 1023 (+) charges
For H+:
1 molC  1 mol H+  1 g H+
1 molC
1 mol H+
=
1 g H+
For Ca+2:
1 molC  1 mol Ca+2  40 g Ca+2
2 molC
1 mol Ca+2
=
20 g Ca+2
In terms of cation exchange reactions, 1 molC is chemically equivalent to 1 mol H+ or 1g of H+ and ½ mol
Ca+2 or 20 g Ca+2. The following example illustrates the calculations involved in converting equivalent
charge to the mass (or weight) of a given element or molecule.
Example 3: Equivalent Charge Conversion to Atomic (or molecular) Mass.
a. Calculate the grams of Mg+2 equivalent to 4 cmolC.
4 cmolC  1 molC
 1 mol Mg+2  24 g Mg+2
100 cmolC
2 molC
1 mol Mg+2
=
0.48 g Mg+2
b. Calculate the grams of KCl equivalent to 4 cmolC.
Note: Use the same procedure as above, substituting the equivalent charge and molecular
weight of KCl. Remember that KCl dissociates into K+ and Cl- producing one mol of positive
charge.
4 cmolC  1 molC
 1 mol KCl  74.5 g KCl
100 cmolC
1 molC
1 mol KCl
=
2.98 g KCl
c. Calculate the pounds of KCl required to replace 4 cmolC of exchangeable Mg+2 in one acre
foot of soil. Assume an acre foot of soil weighs 4x106 lbs.
4 cmolC  1 molC
 1 mol KCl  74.5 g KCl  1 lb KCl
kg soil
100 cmolC 1 molC
1 mol KCl
454 g
=
6.6x10-3 lb KCl
kg soil
6.6x10-3 lb KCl  1 kg soil  4x106 lb soil
1 kg soil
2.2 lb soil
1 ac-ft
=
11,934 lb KCl
ac-ft
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Measurement of Cation Exchange Capacity (CEC)
There are three parts to this (or any) CEC measurement procedure:
1. Saturate all the exchange sites with a known cation
2. Replace all of the cations on the exchange sites and
3. Quantify (measure) the displaced cations
***** PUT ON GOGGLES AND GLOVES *****
1) Weigh out 5 g of soil into a 50 ml centrifuge tube.
Saturate the exchange sites with H+
2) Add 30 ml 1 N HCl to tube and tighten lid securely. Shake gently for ~5 min. Balance tubes by adding
DI water if necessary (weights should be within one gram). Centrifuge samples for 5 min on level 25.
Discard supernatant.
Wash all H+ not on exchange sites from the soil
3) Add 30 ml DI water. Dislodge any soil that is caked to the bottom of the tube. Shake each tube for 2
min after the soil is well dispersed. Centrifuge as before. Discard supernatant. Repeat this step one
more time.
Displace H+ on exchange sites with Ca2+
4) Add 30 mL of 1M CaOAc. Dislodge any soil that is caked to the bottom of the tube. Shake each tube
for 2 min after the soil is well dispersed. Centrifuge. SAVE WASHINGS in an Erlenmeyer flask.
Repeat this step one more time. Combine the supernatants and add 5 drops phenolphthalein
solution if washings are relatively clear and if the solutions are not clear add 10 drops
phenolphthalein.
Neutralize H+ in solution
5) Titrate into Erlenmeyer flask with 0.1 N NaOH drop-wise, stirring solution until endpoint is reached.
We have used H+ as the saturating cation and Ca+2 as the displacing cation.
Data:
Soil Name
Weight
(g)
NaOH
(ml)
NaOH
(N)
CEC
(cmolC)
Calculations:
CEC = [H+] displaced by Ca/ Weight of Soil
ml NaOH
g Soil
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mol NaOH  1 L
 1 molC 100 cmolC 1000 g =
L
1000 ml 1 mol NaOH 1 molC
1 kg
cmolc
kg soil
5