Titrations in non-aqueous media. Part XII. Potentiometric titrations of symmetrical aliphatic dicarboxylic acids and some of their binary mixtures in different media

Abstract

Solutions of oxalic, malonic, succinic, glutaric, adipic, pimelic, suberic and azelaic acids in pyridine, propan-2-ol and mixtures of both solvents with benzene were titrated potentiometrically with tetrabutyl-ammonium hydroxide in propan-2-ol. The titration curves of the first three acids showed two distinct end-points whereas those of the last three acids showed only one distinct end-point in both pyridine and propan-2-ol and in all mixtures of these two solvents with benzene. The titration curves of glutaric and adipic acids in pyridine (dielectric constant, ε= 13.6) exhibited two and one end-points, respectively. The titration curve of glutaric acid in pyridine-benzene (1 + 2)(ε= 6) gave only one end-point. On the other hand, the titration curve of adipic acid showed two end-points in propan-2-ol (ε= 19) and one end-point in propan-2-ol-benzene (1 + 2)(ε= 8.4). An increase in the number of methylene groups or a decrease in the dielectric constant of the medium decreased the number of end-points in the titrations of these dicarboxylic acids. The number of end-points was also found to be dependent on the nature of the solvent. For example, the titration curve of glutaric acid exhibited two end-points in propan-2-ol-benzene (1 + 4)(dielectric constant, ε= 6) whereas the titration curve of the same acid showed only one end-point in pyridine-benzene (1 + 2), which has an identical dielectric constant. Although the inductive effects of the carboxyl groups in dicarboxylic acids can influence both groups up to seven bonds along, such an effect may not necessarily be responsible for a second end-point. The acidity of these acids decreases with the increase in the number of methylene groups up to pimelic acid in both pyridine and propan-2-ol, although pimelic acid did not give two end-points in either of these solvents.

The potentiometric titrations of the dicarboxylic acids have shown that in order to observe two end-points, it is not necessary to have a 200 mV difference between the two half-neutralisation potentials of the acidic groups. In some instances, a 130 mV difference is sufficient for two end-points to be observed.

Some of the binary mixtures of the dicarboxylic acids could be titrated to determine the percentage of each acid in these mixtures. For such titrations, pyridine seemed to be a better solvent than propan-2-ol.