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EC number: 237-410-6 | CAS number: 13775-53-6
Hydrolysis
Administrative data
Link to relevant study record(s)
Description of key information
In a dissociation and acid/base equilibrium study Cryolite was observed to easily dissociate in water into various ions (Dykeman, 1985).
Key value for chemical safety assessment
Additional information
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:
pH |
ppm F- |
5 |
16.8 |
7 |
40.0 |
9 |
47.0 |
The traditional model for the dissociation of cryolite at pH 7 is:
3NaF. AIF3↔ 3Na++ 3F-+ 3AlF3
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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