Standard states of oxide components

The curves shown in Fig. 11 are of some interest because the cation effect is the opposite of that which might be predicted from studies of the effects of cation polarizing power on the polymerization constants (Hess, 1971). Hess has pointed out that values of K tend to increase as the polarizing power of the cation (Z/r2) increases. Referring to the polymerization reactions  

The origin of this dilemma appears to lie in the standard states assumed by both the Toop - Samis and Masson models for the basic oxide component. These mixing models assume 100 dissociation of the basic oxide constituent of the binary. Thus in the pure liquid end-member oxide melt the ion fraction of free oxide Xq2- is assiuned to be 1.0. In oxide melts containing strongly polarizing cations this is unlikely to be correct since the dissociation reactions e.g. 

The polymer models assume ideal mixing and so the ion fractions are related to fugacities (via Raoult s Law) 

However oxide activities are by definition ratios of the fugacities of 02 in the melt to those in the pvire liquid oxide standard state  

The oxide activities shown in Fig. 11 for the Mg and Ca systems refer to a common (hypothetical) standard state value of Xq2- =1.0 for both systems so that the activities can be compared. Note that if either oxide fails to dissociate completely then Xq2-in the pure liquid oxide will be less than 1.0. The departure of X 2- from unity is likely to increase with cation polarizing power (cf. SiO ). 

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