The donor number of a solvent can be measured via calorimetry, although it is frequently measured with nuclear magnetic resonance (NMR) spectroscopy using assumptions on complexation. A critical review of the donor number concept has pointed out the serious limitations of this affinity scale. Furthermore, it has been shown that to define the order of Lewis base strength (or Lewis acid strength) at least two properties must be considered. For Pearson qualitative HSAB theory, the two properties are hardness and strength, while for Drago's quantitative ECW model, the two properties are electrostatic and covalent.
^Françoise Arnaud-neu; Rita Delgado; Sílvia Chaves (2003). "Critical evaluation of stability constants and thermodynamic functions of metal complexes of crown ethers". Pure Appl. Chem.75 (1): 71-102. doi:10.1351/pac200375010071.