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: \[Z = \frac{\pu{2.859E4}}{\lambda}\] where \(Z\) is in \(\pu{kcal mol-1}\) and \(\lambda \) is in \(\pu{nm}\).