Exchange Properties of Allophanic Clays — Houng, Uehara, and Sherman 
513 
It may be considered that the pH-dependent 
charges produce strong polarizing effect on 
hydronium ions, so that the protons are de- 
tached from the water molecules and bonded 
to the oxygen atoms of the aluminosilicate ex- 
changer, forming hydroxyl groups. Permanent 
charges are not polarizing, so that hydronium 
ions neutralizing them remain as such in the 
system. Therefore, the hydronium ions on the 
permanent charge sites can be readily replaced 
by other cations, but the protons on the silanol 
groups are not so easily replaced. In this con- 
nection, it may be considered that the charges 
arising on the surface of the exchanger will 
have a stronger polarizing effect on the hydron- 
ium ions, while the charges arising inside the 
exchanger will have a weaker polarizing effect. 
The charges due to the broken bonds and sur- 
face hydroxyls will be pH-dependent, and the 
charges due to isomorphous replacement can 
be pH-dependent or permanent, depending on 
the distance from the surface of the exchanger 
to the site where the isomorphous replacement 
takes place. Here, permanent charges can be 
defined as the charges which do not give strong 
polarizing effect on neutralizing ions, and the 
pH-dependent charges are those which give 
strong polarizing effect on the neutralizing ions. 
It may be concluded that the charges of the 
allophanic clays of the soils studied are due 
mostly to the surface silanol groups and tetra- 
hedrally-coordinated aluminum atoms. The 
former charges show weak acidic character, the 
latter strong acid character. Contribution of the 
hydroxyl groups of aluminum hydroxide to the 
total exchange capacity seems to be insignificant. 
It is quite unlikely that the silanol groups 
of the soil colloids will contribute to cation 
exchange of a soil under natural conditions, 
except in saline and alkaline soils. It is con- 
sidered more desirable for most practical pur- 
poses to express cation exchange determinations 
in terms of permanent charges. 
SUMMARY 
1. Cation exchange in soils is considered to 
be an equilibrium reaction between the day 
acid and the day salt, assuming that the 
hydrogen-saturated and metal ion-saturated days 
behave as if they were weak acid and salt, 
respectively. 
2. Comparisons of the exchange curves ob- 
tained on clay minerals, allophanic days, soils, 
and on synthetic silica, aluminosilicate, and 
aluminum hydroxide gels indicated that the 
so-called pH-dependent charges of the days 
were most likely due to the dissociation of sur- 
face hydroxyl groups associated with silicon 
atoms (silanol groups), and that the so-called 
permanent charges were due to isomorphous 
replacement. In synthetic gels and the allophanic 
clays, the permanent charges were most prob- 
ably due to tetrahedrally coordinated aluminum 
atoms in the aluminosilica network. 
3. The lower exchange capacity obtained 
with NH 4 + ions at higher pH values is ex- 
plained on the basis that competition of protons 
between the pH-dependent charges and the 
ammonia molecules resulted in dissociation of 
NH 4 + ions to protons and ammonia molecules, 
Fig. 6. Exchange curves of Hanipoe soil using 
different saturation solutions. 
