This article throws light upon the three groups of ion-exchange equations. The groups are: 1. Empirical Formula 2. Kinetic or Statistical Formula 3. Mass Action.
1. Empirical Formula:
The Freundlich equation is one of the first used in soil studies and it states that,
m = ACB
where, m = amount adsorbed per unit weight of soil;
C = ion concentration (ions to be allowed for adsorption) in soil solution
A & B are constants which vary from soil to soil.
2. Kinetic or Statistical Formula
The Langmuir equation is used to characterize ion adsorption particularly phosphorus in soils. The most common form of the Langmuir equation is,
m = ABC/1 + BC
where, m = amount of ion adsorbed per unit weight of soil
c = ion concentration in soil solution to be considered
A = adsorption maximum
B = constant related to bonding energy.
3. Mass Action:
In homo-valent (here monovalent-monovalent) exchange,
RK + Na+DRNa + K+,
The equilibrium equation may be written as
[Na+]i (K+)o/[K+]i (Na)o = Kk.Na
For monovalent and divalent ions
RCa + 2K+D2RK+ Ca2+
At equilibrium, [K+]i2 (Ca2+)o/[Ca2+]i (K+)o = KCa.k
Brackets denote concentration; parentheses, activity; i, exchanger phase or inner solution; O, outer solution; K, the equilibrium constant which varies with the mole fractions of these two ions. Because of the difficulties encountered in the determination of the activities of the ions present in the exchanger phase or inner solution, concentration has been used here.
All exchange equations are the same in the case of exchange of equal valence ions. This, however, is not the case when the ions are of unequal valencies like monovalent and divalent ions.
Na x/Ca xÃ— âˆšCa/Na+= K
where, x = is the exchange complex
K = is the constant.
This formula is used to characterize alkali soils and it also predicts the alkali hazard of irrigation water. The Gapon constant K gives a measure of the relative tightness of binding of cations to a clay.