Consider a wastewater with an alkaline pH of 11 containing copper(II) at a total
level of 5.0 mg/L and excess uncomplexed EDTA at a level of 200 mg/L (expressed
as the disodium salt, Na2H2C10H12O8N2•2H2O, formula weight 372). At this pH
uncomplexed EDTA is present as ionized Y4-. The questions to be asked are: Will
most of the copper be present as the EDTA complex? If so, what will be the equilibrium
concentration of the hydrated copper(II) ion, Cu2+? To answer the former
question it is first necessary to calculate the molar concentration of uncomplexed
excess EDTA, Y4-. Since disodium EDTA with a formula weight of 372 is present at
200 mg/L (ppm), the total molar concentration of EDTA as Y4- is 5.4 ´ 10-4 M. The
formation constant K1 of the copper-EDTA complex CuY2- is