THERMOMETRIC TITRATIONS OF POLYPROTIC ACIDS

The authors formulate n equivalent equations for the thermometric titration curve of a polyprotic acid HnA, the pK values of which are comparable, neutralized by a strong base. The main term of the pth equation corresponds to a line segment with slope proportional to the enthalpy of neutralization ΔHP of the pth acidity. Terms which are numerically small justify the roundoff sections connecting these segments to the base lines and to each other. These equations have been experimentally checked with a diacid, glutaric acid, with a triacid, citric acid. In the case of a diacid, the deviation between the experimental curve and the theoretical lines, at the point corresponding to semi-neutralization, is evaluated as a function of the acidity constants and enthalpies of neutralization. A rapid graphic method is used to estimate the ratio of the two acidity constants. © 1979, American Chemical Society. All rights reserved.