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 = 2.859 × 10 4 λ
where Z is in kcal mol −1 and λ is in nm.
See also: Dimroth–Reichardt E T parameter, Grunwald–Winstein equation
PAC, 1994, 66, 1077 (Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)) on page 1176