On the Titration of a Weak Acid with a Weak Base: Application to the Potentiometric Determination of the Degree of Acidity of Vinegar

Although the number of direct instrumental methods of chemical analysis is continuously increased, titrations are still important in routine practice in many laboratories. The aim of this contribution is to carry out the study of weak acid/ weak base titration curves, while also emphasizing titration error. A diagram for the titration error in function of the difference between the end and equivalence point (pHeq) is drawn in order to facilitate the graphical calculation of titration error. The approach to the construction of redox titration curves, based on the use of a hyperbolic sine expression, is applied here to weak acid/ weak base titrations. While the approach in its simplified form is limited by the water ionization, herein we derive a complete expression applicable in all situations when dealing with weak and medium strength acids titrated with weak and medium strength bases. The acetic acid/ammonia system is selected first as example in order to derive the equations, the reaction is studied in detail and applied to the determination of acetic acid (acidity degree) in vinegar.