Pt.2-Effect of Temperature and Composition on Oxidation-Reduction Equilibria....
JSGT 1951 V35 T448-T461
There is no basic difference between oxidation–reduction processes in glasses and slags on the one hand and those in aqueous solutions on the other. Examples are given. In the case of the dissociation of Fe2O3, it is postulated that the Fe3+ ion is stable only in a system in which not only its excess electrical charge is neutralised but its potential field is also completely screened by anions. Such a condition of stability is obtained by six fluorine ions (FeF63- complexes), four highly polarisable oxygen ions in close proximity, or six less polarisable O2- ions at a greater distance. Since the polarisability of an oxygen ion increases with the decreasing positive fields of its neighbours, the O2- ions in magnesium ferrite are more polarisable than those in haematite. Hence, magnesium ferrite is stable at a much higher temperature than ferric oxide. The variations in colour of different glasses containing chromium are to be ascribed to variations in the Cr3+:Cr6+ ratio, and this, so far as glass composition is concerned, depends on the polarisation effect on the O2- ions produced by the cations present. P2O5 having a high potential field, reduces the polarisability, whereas K2O and BaO, with a low potential field, have the opposite effect and stimulate chromate formation. Numerous other cases are considered. The meaning of the terms acidity and basicity applied to glasses and slags is discussed in relation to the screening requirements of the ions present. All silicate glasses are “acid,” and the polymerisation of silicates is regarded as indicating a demand by the Si4+ions for better screening. For a constant O:Si ratio, the basicity of a glass can be increased by replacing some of the network-modifying cations by others with weaker potential fields (Na+ by K+, or Ca2+ by Ba2+ ions).
The Significance of the Co-ordination Requirements of the Cations in the Constitution of Glass. II. The Effect of Temperature and Composition on Oxidation-Reduction Equilibria in Glasses
W. A. Weyl
