The Saha equation is useful for determining the ratio of particle densities for two different ionization levels. The most useful form of the Saha equation for this purpose is Z i N i = Z i + 1 Z e N i + 1 N e , where Z denotes the partition function. The Saha equation can be seen as a restatement of the equilibrium condition for the chemical potentials : μ μ i = μ μ i + 1 + μ μ e