508 
PACIFIC SCIENCE, Vol. XX, October 1966 
respectively, in equal amounts, a curve with 
two inflection points (such as is shown in 
Fig. I A) is obtained. This fact provides a 
possibility of characterizing different functional 
groups that may be present on the clay surface, 
if the constants Ks/Ka differ by a factor of 
10 4 or greater. 
It is important to note that in a given ex- 
change material the factors governing the de- 
gree of saturation by a specific cation are the 
ratio of the cation and the relative affinity of 
the exchanger toward the cations, as expressed 
by the ratio Ks/Ka. If the acid functional 
groups are very weakly dissociable, it is im- 
possible to saturate the exchanger by repeated 
treatment of the exchanger with the electrolyte 
solution unless the metal ion concentration of 
the solution is extremely high and the hydrogen 
ion concentration is extremely low. 
EXPERIMENTAL PROCEDURE 
A 1-g sample of soil or 0.2 g of soil colloid 
was placed in the carbon funnel and plugged 
with balls of filter pulp to serve as the filtering 
medium. A series of samples thus prepared were 
leached with IN NaOAc solutions previously 
adjusted with NaOH or HO Ac to give various 
pH values covering the range 4.5-10. Incre- 
ments of the leachates were collected from time 
to time for pH measurements. Leaching was 
continued until the pH of the leachate became 
identical with or close to the value of the 
original solution. The samples were then 
leached with 150 ml of methyl alcohol while 
maintaining a 2—5 mm head over the sample. 
After the alcohol washing, the samples were 
leached with IN NH 4 OAc in the same manner, 
and the leachates were collected in 50 ml 
volumetric flasks. The sodium concentration was 
determined by flame photometry, using a Beck- 
man DU spectrophotometer. 
Poor buffering of the sodium acetate solution 
at higher pH range made it necessary to use 
unusually large amounts of leaching solution 
before the pH of the leachate approached that 
of the solution. Similar buffering problems were 
encountered in the acid range by Schofield 
(1949) and limura (1961) when ammonium 
chloride was used. 
345 6783 10 
log(M+) - log (H 4 ") 
Fig. 1 . Theoretical exchange curves. Curve A is 
the exchange curve of a hypothetical exchanger con- 
taining two kinds of acid functional groups in equal 
amounts, one with Ks/Ka = 10 4 , the other with 
Ks/Ka = 108 . 
In the later part of this experiment, sodium 
carbonate solutions adjusted to the same pH 
value as that of the sodium acetate solutions 
were used prior to the leaching by sodium ace- 
tate solutions. Displacement of sodium carbon- 
ate solutions with sodium acetate solutions was 
necessary, since sodium carbonate is quite in- 
soluble in alcohol. 
MATERIALS 
Silica gel. This sample, manufactured for 
chromatographic work, was obtained from the 
Baker Chemical Company. Particle size ranged 
from 0.18-0.07 mm. The appearance of a 
broad band near 3.7 A on the X-ray diffraction 
pattern indicated that this material was essen- 
tially noncrystalline. 
Halloysite. Sample mined in Eureka, Utah, 
and obtained through Ward’s Natural Science 
Establishment, Inc. 
