Electron Pair Donor and Acceptor Numbers

Dipolar protic ethanol, methanol, water, liquid ammonia 

2 In fact, tris(pyrrolidino)phosphane oxide has a donor number of 1.22. 

Quantitative determination of solvent polarity is difficult, and quantitative methods rely on physical properties such as dielectric constant, dipole moment and refractive index. It is not possible to determine the solvent polarity by measuring an individual solvent property, due to the complexity of solute-solvent interactions, and for this reason empirical scales of solvent polarity based on chemical 

The original solvent polarity scale, known as the E (30) scale, was defined simply as the energy of the longest wavelength adsorption band for the dye, measured in kcalmol-1. This scale has now been revised and normalized because of the introduction of SI units, and Ej is defined in Equation 1.4. 

From a practical viewpoint, Ej values are quickly and easily obtained, giving a very useful and convenient scale. However, a general polarity scale based on a single probe molecule has its limitations because a single compound cannot experience the diversity of interactions that the whole range of solvents can offer. The Kamlet-Taft parameters a, /3 and n tackle this problem by using a series of seven dyes to produce a scale for specific and nonspecific polarity of liquids [23], Whilst it undoubtedly gives a more detailed description of the solvents properties, 

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