Chemistry Reference
In-Depth Information
considered to occur at a near-saturation condition. According to Rimstidt and Barnes,
899
the rate of reaction of silica in water can be expressed as
r=k
(1 -
S
), where
k
is the
product of the rate constant and activities of the reacting species, and
S
is a quantity
measuring the degree of saturation. The term (1 -
S
) is a measure of departure from
equilibrium and when
S =
1 the system is at equilibrium and the net rate of reaction is
zero. Table 4.4 lists the factors that may influence the rate of silica-water interaction
as summarized by Rimstidt and Barnes.
899
The dissolution rate equation for silica in nonfluoride solutions may also be
expressed according to the surface complexation model described by Eq. (4.3) and Fig.
4.32 considering the contributions of different surface complexes:
and
898
The rate equation for the dissolution of silica in NaCl solu-
tions at pH 2-13 at 25 °C can be described as
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According to Fig. 4.32, is the predominant species and its concentration deter-
mines the dissolution rate at low pH. The low dissolution rates of silica in acidic solu-
tions (Fig. 4.32) indicate that this species is most resistant to hydrolysis. The more
reactive species and become important at higher pH at which the dis-
solution rate is faster as shown in Fig. 4.13.
In acidic fluoride solutions, many etch rate equations, empirically or mechanis-
tically derived, have been proposed.
57,153,239,451,491
The most quoted rate equation is that
of Judge,
57
who found that the etch rate of thermal oxide in HF and
solutions can
be expressed as a function of HF and
concentrations:
Curve fitting indicated that the rate of attack by is about four to five times that of HF
as shown in Fig. 4.36. A more general equation, including the effect of temperature, is
where
and
