Acid Dissociation Constants of EDTA

These values are somewhat abnormal. We know from statistical data that aliphatic carboxylic acids exhibit pKa values in the range 3.5-5.0, whereas for protonated aliphatic amines, they are located in the range 9.0-11.0. The abnormal third acidity value pKcn = 6.16 is in favor of a mixture of the following two isomeric compounds, which are both acids  

In one of the components of the mixture, the acidity is brought by a carboxylic rest, while in the other, it is one of the protonated amines that is acidic. This mixture is globally represented by the symbol H2Y . The composition of this mixture is constant regardless of the pH value (see the problem with the microconstants— Chap. 5). Moreover, pKa = 1.99 and even pKai =2.67 are values that are too low to belong to normal carboxylic acids, but this point cannot be explained by the occurrence of several protonation sites. They probably should be ascribed to structural factors, such as, for example, the withdrawing inductive effect of the protonated amino group located at the proximity of the carboxylic groups of interest. pK 4 = 10.26 seems to be a normal value for a protonated amine. The structure of the HY form should probably be written as 

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From the acid dissociation constants of EDTA (1,2) it can be calculated that between pH 7,4 and 6,6 an EDTA solution is represented to an extent greater than 99,8<7oby the forms HY and HjY . If the EDTA is in sufficient excess with respect to the cation studied, the pH of the solution, on complexing, will remain between the above limits and the experimental reaction will be described unambigously by the equation ... 

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