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 $\mathrm{kcal}{\mathrm{mol}}^{-1}$ and $\lambda$ is in $\text{nm}$.