Titrimetric applications of multiparametric curve-fitting : Part 1. Potentiometric titrations of weak bases with strong acids at extreme dilutions

A multiparametric curve-fitting procedure is described for locating the equivalence point of a potentiometric titration and is applied to data obtained in titrations of acetate ion with hydrochloric acid over a wide range of concentrations (down to 6.5 · 10^-5M acetate). It does not depend on the existence of a point of inflection on the titration curve, and therefore yields useful results in titrations of this very weak base at concentrations well below that at which the point of maximal slope disappears. Three parameters are involved: the concentration of the base being titrated, the concentration dissociation constant K_a of its conjugate acid in the medium employed, and the apparent activity coefficient y_H⁺ of hydrogen ion in that medium, and all three of these must be evaluated when a new supporting electrolyte is employed. In 3.0 M potassium chloride at 24°, the apparent activity coefficient of hydrogen ion is 1.746₀, and this value permits data obtained in titrations of other or unknown bases in this medium to be interpreted by two-parameter fits. The value of K_a for acetic acid in this medium is 1.313₄ · 10^-5M; by means of these two numerical values routine titrations of acetate in this medium can be interpreted by one-parameter fits. It is possible to locate the equivalence point with an accuracy and a precision that cannot be approached by other techniques and even to obtain useful and reliable results under conditions so unfavorable that other techniques fail completely.