Exchange Properties of Allophanic Clays — Hqung, Uehara, and Sherman 
511 
presence seems to suppress the dissociation of 
the surface silanol groups. 
Comparison of Sodium Adsorption with 
Ammonium Adsorption 
The same procedure was applied to the 
Hanipoe surface soil, using IN NH 4 C1 and IN 
NH 4 OAc solutions. Concentrated ammonia and 
hydrochloric acid solutions were used to adjust 
the pH values of NH 4 C1 solutions; concen- 
trated ammonia solution and glacial acetic acid 
were used to adjust the pH values of MH 4 OAc 
solutions. The final total concentration of 
NH 4 + ions was not IN, but varied according 
to the desired pH values of the solutions. The 
results are shown in Figure 6. 
The soil adsorbed about the same amount of 
sodium and ammonium ions up to pH 7 with 
acetate solutions. The amount of sodium ions 
adsorbed beyond pH 7 increased rapidly with 
increase in pH, while the amount of ammonium 
Fig. 4. Exchange curves of allophanic soils. O > 
Hanipoe soil; ©, Waikii soil; 0, Mahoelua soil; 
and Waikaloa soil. 
ions adsorbed tended to level off beyond this 
point. Adsorption from NH 4 C1 and NH 4 OAc 
solution followed similar patterns, but adsorp- 
tion from the latter was significantly greater. 
The influence of anions on the amount of cation 
adsorbed by clays is well known, but as yet 
there seems to be no adequate explanation, 
limura (1961) showed that the amount of 
ammonium ions adsorbed increased with pH, 
following the same pattern of sodium adsorp- 
tion by allophanic clays obtained in this study. 
The discrepancy seems to be due to the differ- 
ence in the experimental procedures employed. 
His procedure was essentially that of Schofield 
(1949), under whose method the amount of 
ammonium ions adsorbed was determined with- 
out the removal of the excess salt present, while 
in the procedure used in this study excess salt 
was removed by alcohol and the ammonium 
ions adsorbed were apparently removed as the 
excess salt was removed from the system. This 
may be explained as follows: 
Since the hydronium ion activity is very low 
in a solution having a pH of 8 and above, the 
following equilibrium tends to shift to the 
right: 
NH 4 + + H,0 = NH 3 + H s O+ (11) 
When protons of the silanol groups are replaced 
by ammonium ions, the ammonium ions will be 
strongly polarized so that they will split more 
or less distinctly into ammonia molecules and 
protons, the protons being strongly attracted 
toward the oxygen atoms on silicon atoms. 
Considering the equilibria 
NH 3 + HoO = NH 4 + +OH- (12) 
and 
2H.O = H 3 0+ + OH- (13) 
with equilibrium constants of 1.8 X IQ -5 and 
10 -14 , respectively, the equilibrium constant 
for (11) will be 10~ 14 /1.8 X 10 -5 — 5.6 X 
10 -10 . The equilibrium constant for the equilib- 
rium 
NH 4 + + H 2 0 +(— Si)— OH = 
(—Si) — OH.NH 3 — H 3 0+ . . (14) 
is less than 10 -8 , as can be seen by comparing 
the results in Figure 6 with those in Figure 1. 
It may be concluded that the ammonium ions 
