Optical Basicity: A Practical Acid-Base Theory for Oxides and Oxyanions

The optical basicity concept relies on the Lewis approach to acids and bases and was developed for dealing with chemical problems in non-aqueous, non-protonic media such as silicates, phosphates and borates which are important in glass making and (as slags) for refining steel. Basic oxides such as Na2O or CaO are ionic while SO3, P4O10 or SiO2 are covalent, and it is the magnitude of negative charge borne by the oxygen atoms or ions which governs the degree of acidity or basicity. The oxygen atoms of sulfates, phosphates or network systems such as silicates bear charges which are between those of their parent oxides. In principle, the negative charge can be estimated using the optical (ultraviolet) spectra of certain probe ions and is represented by the optical basicity value, Lambda. Optical basicity values, available for 16 oxides, increase from the acidic SO3 to the very basic Cs2O in a way which conforms with electronegativity and polarizability. The optical basicity concept also extends to fluorides and sulfides.