Acid-base equilibria in ethylene glycol--III: selection of titration conditions in ethylene glycol medium, protolysis constants of alkaloids in ethylene glycol and its mixtures.

Theoretical titration curves are used for the selection of appropriate conditions for the acid-base volumetric determination of weak bases in ethylene glycol medium. The theoretical curves for titration of some alkaloids are deduced graphically on the basis of the logarithmic concentration diagram. The acid-base constants used for the construction of the theoretical titration curves were determined by potentiometric titration in a cell without liquid junction, equipped with a glass and a silver-silver chloride electrode. It is shown that the alkaloids investigated can be determined accurately by visual or potentiometric titration. The same approach for the selection of titration conditions seems to be applicable to other non-aqueous amphiprotic solvents.

Acid-base equilibria in ethylene glycol--II: autoprotolysis constants and acid-base properties of ethylene glycol and its mixtures.

The acid-base behaviour of ethylene glycol and its mixtures with water (1, 5 and 10%), methanol, ethanol, isopropanol, nitromethane (each 10%) and ethyl methyl ketone-chloroform (5 + 5%) have been investigated by potentiometric titration, in a cell without liquid junction, equipped with a glass and a silver-silver chloride electrode. The autoprotolysis as well the protolysis constants of phthalic acid were determined for each mixture. The added solvents improve the properties of ethylene glycol, decreasing the viscosity without changing the acid-base behaviour of the ethylene glycol itself, which is favourable for the titration of weak bases. Water increases the basic, and nitromethane the acidic, properties of the mixture. Small quantities of water (ca. 1%) do not impair the titration conditions.

Potentiometric determination of autoprotolysis constants of solvents.

A new method is proposed for determination of autoprotolysis constants by titration of a weak acid with strong base or strong acid with weak base, and applied to water and ethylene glycol (EG). The constants found were pK(w) = 13.90-13.97, and pK(EG) = 15.4 +/- 0.2.

Acid-base equilibria in ethylene glycol--I: definition of pH and determination of pk-values of acid-base indicators.

A pH-scale in ethylene glycol is defined, based on potentiometric measurements for a cell without liquid junction: glass electrode || 0.1M (HCl + KCl), ethylene glycol | AgCl:Ag; and a potentiometric-spectrophotometric method for the determination of pK-value of acid-base indicators in ethylene glycol on the basis of this pH scale is proposed, in which the pH scale is established and the pK-value determined in a single titration at constant ionic strength. The following pK-values of indicators are reported: Cresol Red-3.00, Thymol Blue-3.39, Bromophenol Blue-6.49, Bromocresol Green-7.38, Bromocresol Purple- 9.00 and Cresol Red (2nd change)-11.07. A linear relationship was found between the pK-values of four sulphonphthalein indicators in ethylene glycol and in water according to the following equation (pK(HI)(c))(EG) = 2.3 + 1.1 (pK(HI)(c))(W). An attempt is made to explain this dependence, based on the idea of solute-solvent interaction of species of the same charge type.