The linear free energy relation: \[\log _{10}(\frac{k_{\text{s}}}{k_{0}}) = m\ Y\] expressing the dependence of the rate of solvolysis of a substrate on ionizing power of the solvent. The rate constant k0 applies to the reference solvent (ethanol–water, 80:20, v/v) and ks to the solvent s, both at 25°C. The parameter m is characteristic of the substrate and is assigned the value unity for tert-butyl chloride. The value Y is intended to be a quantitative measure of the ionizing power of the solvents. The equation was later extended to the form: \[\log _{10}(\frac{k_{\text{s}}}{k_{0}}) = m\ Y+l\ N\] where N is the nucleophilicity of the solvent and l its susceptibility parameter. The equation has also been applied to reactions other than solvolysis.