An index of the ionizing power of a solvent based on the frequency of the longest wavelength electronic absorption maximum of 1-ethyl-4-methoxycarbonylpyridinium iodide in the solvent. The Z-value is defined by: \[Z = \frac{2.859\,\times\,10^{4}}{\lambda}\] where Z is in kcal mol-1 and λ is in nm.