On the Exchange Properties of Allophanic Clays 1 
K. H. Houng, G. Uehara, and G. D. Sherman 2 * 
The origin of the exchange sites on the 
clay minerals has been considered to be due to 
isomorphous replacement of Mg for A1 in the 
octahedral layer and/or A1 for Si in the tetra- 
hedral layer and to the broken bonds at the 
edges of the crystals. The charges originated by 
the former mechanism are supposed to be pH- 
independent and are called permanent charges, 
while the charges due to the broken bonds are 
considered pH-dependent (Coleman and Meh- 
lich, 1957). A more detailed classification of 
charges on the clay surface has been proposed 
by Mehlich (I960). These negative charges 
can be compared with acid groups of cation- 
exchange resins ; the permanent charges re- 
semble the strong acid groups, and the pH- 
dependent charges resemble the weak acid 
groups. The pH-dependent charges show 
stronger affinity for the hydrogen ions, so that 
only when the metal ion activity of the external 
solution becomes much greater than that of 
hydrogen ion activity can they be saturated with 
the metal ions. It is evident that the activity 
ratio of metal ions and hydrogen ions is the 
main factor that determines the degree of 
saturation with respect to the metal ions. 
THEORETICAL 
If an ion exchange reaction takes place ac- 
cording to the following equation: 
HX + M+ = MX + H+ (1) 
where X is the ion exchanger, an equilibrium 
equation is obtained: 
1 Published with the approval of the Director of 
the Hawaii Agricultural Experiment Station, Univer- 
sity of Hawaii, Honolulu, as Technical Paper 687. 
Manuscript received November 30, 1964. 
2 East- West Center grantee from Taiwan, China 
(now Associate Professor of Soils, Department of 
Agricultural Chemistry, National Taiwan University, 
Taipei), and Assistant Soil Scientist and Senior Soil 
Scientist, University of Hawaii, respectively. 
(MX) (H+) 
(HX) (M+) 
• (A 
If (MX) and (HX) are expressed in terms of 
mole fractions of the cations occupying the ex- 
change sites, it is reduced to Vanselow’s ex- 
change equation (1932), and K can be cal- 
culated from the measurable quantities. 
If the exchange reaction is expressed as the 
sum of the following two reactions: 
HX = H++X- (3) 
and 
X- + M+ = MX (4) 
with Ka and Ks as their equilibrium constants: 
Ka=(H+)(X-)/(HX) (5) 
Ks= (M+)(X-)/(MX) (6) 
then Equation (2) can be expressed as: 
K= Ka/Ks = (MX) (H+ ) / 
(HX)(M+) (7) 
In equation (7), K is given as the ratio of two 
dissociation constants Ka and Ks. Assuming 
the exchange capacity is A, an expression of the 
degree of saturation of the exchanger with re- 
spect to M ions can be derived as follows: 
(MX) + (MX)= A (8) 
Ka (MX) (H+) 
Ks [A— (MX) ] (M+) 1 ; 
and, in rearranging the above equations, we 
have : 
(MX) 1 
A Ks(H+) i 
Ka(M+) 
( 10 ) 
By arbitrarily setting values for Ks/Ka and 
(H+)/(M+) of the equilibrium solution, a 
family of curves is obtained, as shown in Figure 
1. If the exchanger contains two different acid 
functional groups with Ks/Ka of 10 4 * * and 10 8 , 
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