Thermodynamic changes associated with the formation of aminopolycarboxylate chelates

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Abstract
By emf. measurements of cells of the type Pt‐H2, H+, A− (buffer), KCl, Ag‐AgCl, and extrapolation of the data to infinite dilution, thermodynamic dissociation constants and metal chelate stability constants have been obtained for ethylenediaminetetraacetic and nitrilotriacetic acid. From measurements of this type made over a series of temperatures, the enthalpy and entropy changes associated with complex formation were calculated. In all cases the enthalpy changes were found to be relatively small, while the entropy contributions to the stability of the chelate compounds were found to be considerably greater. The results are interpreted on the basis of the structures of the compounds, and of present theories of chelate ring formation.