A thermodynamic study of step-wise complex formation in several aqueous Copper (II)-amino acid systems

• Main Author: Kothari, Vipin M.
• Format: Others
• Published: BYU ScholarsArchive 1961
• Subjects: Thermochemistry; Coordination compounds; Amino acids; Chemistry
• Online Access: https://scholarsarchive.byu.edu/etd/8293; https://scholarsarchive.byu.edu/cgi/viewcontent.cgi?article=9293&context=etd

pK_n values for step-wise dissociation of protons from the protonated ligands (glycine, α-amino isobutyric acid, sarcosine and theonine) were determined potentiometrically with a Beckman Model GS pH meter in aqueous solution at 10, 20, 30 and 40° C. Log K values for step-wise interaction of glycine, α-amino isobutyric acid, sarcosine and threonine with copper(II) ion were also determined potentiometrically with a pH meter at 10, 20, 30 and 40° C. The standard free energy of formation for the step-wise interaction of copper(II) with each of the above ligands was calculated from the values of log K. Enthalpy and entropy changes were calculated for each of the above reactions at 20° C. Measurements of pH were obtained at several ionic strengths for each metal io-ligand system at each temperature. It was assumed that the zwitterion has no charge. The hydrolysis of copper(II) ion and the liquid junction potential were assumed to be negligible. Log K_1 and log K_2 values for the copper(II) chelate of α-amino isobutyric acid are lower than those of glycine at 10 20° C. but higher at 30 and 40° C. This may be related to the positive inductive effect and the steric effect of methyl groups. The large decrease in the log K values of the copper(II) chelate of sarcosine is probably due to two effects, namely, steric and hydration. These two effects counteract the positive inductive effect introduced by the methyl group on the nitrogen and actually decrease its basic strength and thus decrease the bond strength between nitrogen and copper. THe decrease in the log K values of the copper (II) chelate of threonine is explained by the presence of the hydroxyl group which increases the acid strength of the NH_3^+ group and decreases its electron donating power. This is probably due to the hydrogen bonding between the hydrogen of the hydroxyl group and nitrogen of the amino group. Values of log K for the copper(II) chelate of each ligand decrease with the increase in the temperature, indicating that the entropy effect is greater at a lower temperature than at a higher temperature. Calculations of n, [A^-] and log K were carried out on an IBM 650 Computer.