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

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J. A. Duffy

Department of Chemistry, University of Aberdeen, Aberdeen, Scotland, UK

J. Chem. Educ., 1996, 73 (12), p 1138

DOI: 10.1021/ed073p1138

Publication Date (Web): December 1, 1996

Section:

History, Education, and Documentation

Abstract

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.