Soil Science

Cation Exchange

Introduction

The negative charges that soil colloids usually possess must be balanced by an equal quantity of positive charges.

The positive charges are supplied by cations, which are held by electrostatic attraction in a layer close to the colloid surface.

These cations are known as exchangeable cations because they are readily displaced by other cations from the soil solution.

Start

This section is designed to show you, step by step, the chemical changes that occur during the laboratory procedure for measuring exchangeable cations and cation exchange capacity (CEC).

The CEC is the amount of negative charge that the soil possesses per unit mass and is expressed as centimoles of charge per kilogram of soil (cmolc kg^-1).

Leach with aqueous ethanol

Aqueous ethanol is passed through the soil to leach out ions dissolved in the soil solution. This step is omitted in soils where the concentration of ions in the soil solution is already low.

The aqueous ethanol does not affect the exchangeable cations, which must remain in place to balance the negative colloid charges.

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Add NH4C1 solution

A high concentration of NH4^+ ions is next passed through the soil and displaces the original exchangeable cations by mass action.

The displaced cations are leached from the soil and collected as filtrate.

By chemically analyzing the filtrate, the amount of each exchangeable cation per kilogram of soil is determined.

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Leach with aqueous ethanol

Aqueous ethanol leaching is again used to remove all of the soil solution ions. Note that each negative colloid charge is now balanced by just one NH4^+ ion.

If we measure the amount of adsorbed NH4^+, we will know the amount of negative colloid charge per kilogram of soil, which is the CEC.

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Add K+ & Ca2+ as nitrate

The exchangeable NH4^+ ions are displaced by adding an excess of a solution containing K^+ and Ca^2+ as potassium and calcium nitrates. The solution containing the displaced NH4^+ ions is filtered off and analyzed to determine the CEC. For example, if 0.1 mol NH4^+ was displaced from 1 kg of soil, then the CEC would be 0.1 mole kg^-1, or 10 cmolc kg^-1.

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