Thermodynamics of complexation of lead(II) and cobalt(II) with N-(carboxymethyl)aspartic acid in aqueous solutions

Abstract

754N (Carboxymethyl)aspartic acid (CMAA, H 3 Y) is classified as a mixed type complexone containing acid substituent moieties of different chemical nature at the amino nitrogen atom and may be regarded as an struc tural analogue of imidoacetic acid in which one car boxymethyl group is substituted by an succinyl residue. The structural formula of CMAA isIn [1,2] it is pointed to the fact that the first and second protons of CMAA are dissociated prior to its third proton, thus indicating the betaine molecular structure of this complexone in solution. Some evi dence for CMAA having the betaine structure not only in solution but also in a solid state is adduced in [3] on the basis of IR studies. In all CMAA complexes with 3d metals (Zn 2+ , Cu 2+ , Ni 2+ , Co 2+ , Mn 2+ ), the betaine structure is absent, the nitrogen atom is coor dinated to a metal ion, and CMAA is a tridentate ligand (the amino nitrogen atom and two carboxylate groups take part in coordination [3][4][5]).The thermodynamics of complexation of Pb 2+ and Co 2+ with CMAA is studied insufficiently. The most reliable stability constants of cobalt(II) and lead(II) CMAA complexes are given in [4,5], and the con stants of protolytic equilibria in CMAA solutions and the heats of acid-base interactions are found in [6] and [7], respectively. In these works, the results were obtained at several concentration ratios and ionic strengthes and processed with consideration for maxi mally possible processes, such as the dissociation of a complexone, the hydrolysis of a metal, and the com HOOCH 2 C NH 2 CH CH 2 COO − COOH + plexation and acid-base interaction of species of dif ferent natures.There are no literature data on the enthalpies and entropies of complexation of Pb 2+ and Co 2+ with CMAA, and this imposes some difficulties on thermo dynamic and other calculations.The objective of our work is to determine the heats of complexation of Pb 2+ and Co 2+ with CMAA via direct calorimetric measurements, to study the effect of the concentration of a supporting electrolyte, and to calculate the thermodynamic characteristics of com plexation processes. EXPERIMENTALThe CMAA sample used in this work was synthe sized at Tver State University [1]. This complexone containing a single highly basic nitrogen atom bound with mono and diacyl residues was synthesized via the reaction of addition of aminoacetic acid to unsat urated dicarboxy acid (maleic acid) [8].The results of elemental analysis of a separated product and its molecular weight are given below:Calcd., %: C, 38.55; H, 4.45; N, 5.62; M r , 249. Found, %: C, 38.61; H, 4.39; N, 5.56; M r , 267. These results indicate that the product corre sponded to the chemical formula C 8 H 11 NO 8 ⋅ H 2 O. The major component content determined by pH titration was 99.40%.Lead and cobalt nitrate solutions were prepared from recrystallized samples (chemically pure grade). Lead and cobalt in solution were determined com plexometrically. Potassium nitrate (chemically pure grade) recrystallized from distilled water was used as a support...