Document Type


Date of Award


Degree Name

Master of Science in Chemical Engineering - (M.S.)


Chemical Engineering

First Advisor

George C. Keeffe

Second Advisor

C. L. Mantell

Third Advisor

Michael Frederick


The kinetics of an apparent solubility decrease of commercial sodium fluoride encountered in a piece of equipment designed to produce saturated sodium fluoride solution was Investigated, in order to understand why a consistently saturated solution was not possible to achieve with plain tap water as solvent.

The main impurity, about 1 percent sodium silicofluoride, coats the sodium fluoride crystals, resulting in an equilibrium shift during dissolution of the salt in water; and under certain conditions results in the formation of a protective gel. Therefore, in order to produce a constant strength sodium fluoride solution, this coated impurity must be decomposed under conditions which are unfavorable to the formation of the silica gel.

The investigation was concerned specifically with the kinetics of the hydrolysis of sodium silicofluoride in water and in saturated sodium fluoride solutions.

Two different mechanisms for the hydrolytic decomposition of the silicofluoride ion in water have been proposed by previous authors. Ryss and Slutskaya1 and Kubelka and Pristonpil2 postulate the reaction:

2 H2O + SiF-6 ---> 4H+ 6F- + SiO2

A two step mechanism is offered by Hudleston and Bassett3 and also by Rees sad Hudleston, a slow reaction,

SiF6- ---> SiF4 + 2F-

followed by a rapid reaction,

SiF4 + 3 H2O ---> 4HF + H2SiO3

This author has shown from experimental considerations of the primary and secondary salt effects, the effect of dielectric constant of the media on the reaction rate constant, end from theoretical calculations of the entropy factor that the following mechanism is more probable:

Na2SiF6 <--H2O--> 2 MA+ + SiF6- ionization step

SiF6* + xH2O < -- in presence of alkali such as NaOH -- > SiF6* . xH2O (Formation of an activated complex)

SiF6* . x H2O + (2 + y) H2O <--H2O--> SiO2 y H2O + HF + 2F- + x H2O the rate determining step

followed by the rapid neutralization of the HF formed by the alkali (sodium hydroxide) that has been added

2 HF + 2(Na+ + OH-)---> -2(Na+ + F-) + 2 H2O

4(Na+ + OH-) + Na2 SiF6 -- H2O --> SiO2 + 6(Na+ + F-) + 2H2O

In this mechanism x and y are unknown due to the difficulty of determining their exact values.

A minimum alkalinity, pH of 10.2, is apparently necessary in order that the silica produced from the Nat SiF6 present is not activated to form a gel. Equilibrium shift cannot explain entirely the required high alkaline conditions necessary in the effluent for complete dissolution of the commercial sodium fluoride in a continuous dissolution bed.

1. Ryss, I. G. and Slutskaya, M. M., J. Phys. Chem. (USSR). 14 (1940) p. 701.

2. Kulbelka, V. P. and Pristoupil, V., Z. Anorg. Allgem. Chem. 197, 393 (1931).

3. Hudleston, J. and Bassett, H., J. Chem. Soc. (1921) p. 403.

4. Rees, A. a, and Hudleston, J. L., J. Chem. Soc. (1921) p. 1334.



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