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Environmental fate & pathways

Hydrolysis

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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).

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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.