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EC number: 237-410-6
CAS number: 13775-53-6
In a dissociation and acid/base equilibrium study Cryolite was observed to easily dissociate in water into various ions (Dykeman, 1985).
Although the available study from Dykeman
(1985a) is termed a "hydrolysis study", it is more appropriately a
dissociation and acid/base equilibrium study and the kinetics are unlike
the kinetics usually involved in a hydrolysis study. The results of the
test in parts per million free fluoride in each solution were as follows:
The traditional model for the dissociation of
cryolite at pH 7 is:
3NaF. AIF3↔ 3Na++
A 200 ppm solution of cryolite has a fluoride
equivalent of 108.6 ppm which might be present as freely dissociated
fluoride or complexed to some degree in some other ionic or nonionic
form. If it is a valid model, it would be expected that the "3F-" free
fluoride ion term on the right of the equation would represent 108.6 x
6/3 = 54.3 ppm fluoride ion if the reaction went completely to the
right. The above analytical results of 16.8 ppm, 40.0 ppm, and 47.0 ppm
free fluoride represent respectively 30.9%, 73.7%, and 86.6% of the
"theoretical 54.3 ppm "3F-" fluoride. This suggests that in the model of
Equation 1 the equilibrium shifts increasingly to the right as the pH is
increased but the dissociation is not complete.
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