A thermodynamic study of complex formation in aqueous solution.|nI.|pThe copper (II)-alanine and copper (II)-phenylalanine systems.|nII.|pThe iron (III)-acetylacetone system


Values of thermodynamic equilibrium constants were obtained by potentiometric measurements with a Beckman Model GS pH meter in aqueous solutions for the following reactions: 1. The step-wise dissociation or protons from CH_3CHNH_3COOH^+ and C_6H_5CH_2CHNH_3COOH^+ at 0, 10, 20, 30, and 40°C. 2. The step-wise formation of bis-alamino copper(II) and bis-phenylalanino copper(II) at o, 10, 20, 30, and 40°C. 3. The step-wise formation of tris-acetylacetonato iron(III) at 30°C. Enthalpy and entropy changes were calculated for the reactions in 1 and 2, above, from the temperature dependence of the equilibrium constants. Measurements were made and data obtained for the reactions in 3 at several different ionic strengths, metal ion concentrations, and pH values. It was found that the addition of a phenyl group to alanine lowers the pK of dissociation of both protons from the acid species of the amino acid. This is attributed to the attraction which the phenyl group has for electrons. The same effect caused the log K of the first step-wise chelate of copper(II) to be lower with phenylalanine than with alanine. The second step-wise chelate of copper(II) with phenylalanine, however, has a higher log K value than the equivalent alanine chelate. This shows that the phenyl group influences the electronegativity of the copper(II) ion in the first chelate. The calculated formation constants of tris-acetylacetonato iron(III) were independent of metal ion concentration and ionic strength but increased as the pH of the measurements decreased. This was attributed to hydrolysis of the iron(III) ion. It was shown that at least qualitative measurements of equilibrium constants maybe made at pH values as low as 1.5 with a Beckman Model GS pH meter. There was no measurable increase in the proton concentration when Fe^+++ was added to solutions containing glycine or alanine. It was concluded from this that chelation does not occur to any great extent between Fe^+++ and these amino acids, although it has recently been reported in the literature that strong chelates are formed between Fe^+++ and simple amino acids. It is possible that complexes, but not chelates, are formed in the reactions of F^+++ with simple amino acids. The reaction of dilute perchloric acid with certain metal ions produces a small amount of Cl^-, indicating that an oxidation-reduction reaction occurs between the perchlorate ion and the metal ion. It was previously thought that dilute perchloric acid is not an oxidizing agent. Most of the calculations necessary in processing the data obtained were programmed for the IBM 650 computer.



College and Department

Chemistry and Biochemistry



Date Submitted


Document Type





Organometallic compounds, Thermochemistry



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