5.5 : EDTA: Conditional Formation Constant

Each EDTA molecule has six binding sites: four carboxyl groups and two amino groups. The fully protonated form of EDTA is represented as H₆Y²⁺. However, it can exist in different forms, H₅Y⁺, H₄Y, H₃Y⁻, H₂Y²⁻, and HY³⁻, depending on the pH of the solution. In very basic solutions with pH > 10.17, the fully deprotonated form, Y⁴⁻, is the predominant species that readily complexes with metal ions in a 1:1 ratio.

For the equilibrium reaction of the metal with the Y⁴⁻ form of EDTA, the formation constant, K_f, is also known as the stability constant. Note that the K_f can be defined for any of the forms of EDTA in the solution. The value of K_f increases as the positive charge on metal ions increases. At a fixed pH, the different forms of the EDTA are in equilibrium so that the total molar concentration of all forms of EDTA equals the total molar concentration of the uncomplexed EDTA. The term c_ºEDTA is the total concentration of all forms of uncomplexed EDTA. At a fixed pH, the fraction of total EDTA present as Y⁴⁻ becomes constant, gives an expression where K_f′ is called the conditional formation constant or effective formation constant, which has a fixed value at a given pH. Therefore, it can calculate the equilibrium concentrations of free metal ions and the metal–EDTA complexes.