The first generation of the Edwards equation was
where k and k are the rate constants for a nucleophile and a standard (HO). H is a measure of the basicity of the nucleophile relative to protons, as defined by the equation:
where the pK is that of the conjugate acid of the nucleophile and the constant 1.74 is the correction for the pK of HO.
E is the term Edwards introduced to account for the polarizability of the nucleophile. It is related to the oxidation potential (E) of the reaction  (oxidative dimerization of the nucleophile) by the equation:
where 2.60 is the correction for the oxidative dimerization of water, obtained from a least-squares correlation of data in Edwards’ first paper on the subject. α and β are then parameters unique to specific nucleophiles that relate the sensitivity of the substrate to the basicity and polarizability factors.
However, because some β's appeared to be negative as defined by the first generation of the Edwards equation, which theoretically should not occur, Edwards adjusted his equation. The term E was determined to have some dependence on the basicity relative to protons (H) due to some factors that affect basicity also influencing the electrochemical properties of the nucleophile. To account for this, E was redefined in terms of basicity and polarizability (given as molar refractivity, R):
:     where   
The values of a and b, obtained by the method of least squares, are 3.60 and 0.0624 respectively. With this new definition of E, the Edwards equation  can be rearranged:
where A= αa and B = β + αb. However, because the second generation of the equation was also the final one, the equation is sometimes written as , especially since it was republished in that form in a later paper of Edwards’,  leading to confusion over which parameters are being defined.