Page 237 - Introduction to Colloid and Surface Chemistry

226 Colloid stability

coagulation concentration depends upon the criterion which is set for
judging whether, or not, the sol is coagulated, and this must remain
consistent during a series of investigations.
A common method for measuring critical coagulation concentrations
is to prepare a series of about six small test tubes containing equal
portions of the sol and to add to each, with stirring, the same volume
of electrolyte in concentrations, allowing for dilution, which span the
likely coagulation concentration. After standing for a few minutes, an
approximate coagulation concentration is noted and a new set of sols
is made up with a narrower range of electrolyte concentrations. After
standing for a given time (e.g. 2 h), the sols are reagitated (to break
the weaker interparticle bonds and bring small particles into contact
with larger ones, thus increasing the sharpness between stability and
coagulation), left for a further period (e.g. 30 min) and then
inspected for signs of coagulation. The critical coagulation concentra-
tion can be defined as the minimum electrolyte concentration which is
required to produce a visible change in the sol appearance.
An expression for the critical coagulation concentration (c.c.c.) of
an indifferent electrolyte can be derived by assuming that a potential
energy curve such as V(2) in Figure 8.2 can be taken to represent the
transition between stability and coagulation into the primary minimum.
For such a curve, the conditions V = 0 and dV7d// = 0 hold for the
same value of H. If V R and V A are expressed as in equations (8.7)
and (8.10), respectively,

Aa
UH
and

^-£k + ^A.- YlL-Q
f v r D
— i — KV — "
d// d// d// H
from which K.H = 1; therefore,
2 2 z
32ireak T y AOK
ex P[-l]—-~ =
e 2 12
giving

K
(coagulation) 2 2
Ae z