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Nephelauxetic effects (measured from uv spectra) in the outer s and p orbitals of the probe ions T1+, Pb2+, and Bi3+ are used for setting up scales of Lewis basicity for oxide systems. Values of Lewis basicity, expressed as "optical basicities," are assigned to a variety of oxide media including oxyanion glasses and oxyacids, thereby making it possible to compare the basicity of media for which comparisons based upon the Brqnsted-Lowry or Lux-Flood concepts are not feasible. A critical evaluation of optical basicity is made in comparison with the conventional Lux-Flood approach to basicity in oxyanion media. The relevance of electronegativity to optical basicity is also considered. Scales for molten halides are also proposed, and from the optical basicities for the LiCl-KCl (41 % KC1) melt and the NaCl-AlCl3 (67 % A1C13) melt, it is apparent that chloride is affected in a similar way to oxide by highly polarizing cations, but to a lesser extent.that it has not been applied quantitatively is that, until(2) G. N.

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