Chemistry Reference
In-Depth Information
As is the case in pure water, the solubility constant in HF solutions depends on the form
K
of
For hydrated silica formed by precipitation from an aqueous solution,
has a
709
value of
may further react with fluoride ions:
The reaction constants indicate that
is
the dominant dissolved fluorosilicate
species in HF-containing solutions.
Fluorine may exist in different forms depending on the composition of the solu-
tion. As a weak acid, hydrofluoric acid ionizes only partially to fluoride The aqueous
solutions HF may contain an appreciable quantity of fluorine in the form of un-ionized
HF and partially ionized bifluoride
824,894
The concentrations of these species in
aqueous solutions are defined by the following equilibrium conditions:
75
Different values of the dissociation constants may be found in the litera-
ture.
57,75,123,142,238,709
In aqueous solutions of concentration up to at least 1 M fluoride, the
significant fluorine species are
tends to polymerize.
100,238
Figure 2.7 shows the concentration of each dissolved species
as a function of HF concentration.
142
The concentration of commercially available con-
centrated HF solution, is 49 wt%, equivalent to 24.5M.
451
57,75
At relatively high concentrations, HF
is commonly used to prepare buffered solutions for etching of silicon and
its oxides. For
HF solutions, the equilibrium is further complicated by
the reaction
