Silica Polymorphs, Dissolution, and Precipitation

The reaction (which is elementary) describing the dissolution and precipitation of silica polymorphs is 

The rate equals the change in dissolved silica concentration with time, or d(H4Si04)/dt. Multiplying both sides of the differential rate equation by the activity coefficient y H4SI04 (see Chap. 4), leads to 

Thus the dissolution reaction is zero-order and the precipitation reaction first-order. 

The precipitation rate constant for quartz and a and p cristobalite up to about 300°C, and of amorphous silica to about 200°C, with the reaction written as in Eq. (2.91) is given by 

Because the solubilities of the four polymorphs are known as a function of temperature (see Chap. 7, Section 7.5) the forward rate constants, A+ (M/s) can be computed from the relationship -for each elementary reaction. Thus the versus 7(K) expressions for quartz and amorphous silica written as in Eq. (2.91), are 

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