Multiproticity of Weak Acids: Inflection Point vs. Equivalence Point

Abstract

Most of the laboratory experiments of undergraduate chemistry curriculum are based on pH metry for qualitative and quantitative analysis. Establishment of any acid’s profile in terms of it being mono, di, or triprotic is a common trend in lab exercises. In these tasks, experiments are designed to classify multiprotic acids in terms of their stepwise proton releasing capability. Strong acid-base pH titration curves have such equivalence and inflection points that could be identified with ease. However, some multiprotic weak acids have such equivalence or inflection points which are difficult to identify from the titration curves, developed during these lab experiments. Hence students arrive at inaccurate conclusions. This difficulty appears when nothing is apparent from the experimental curve that the acid in question is mono, di, or triprotic. The purpose of this study is to describe the method for prediction of the strength of such weak multiprotic acids, which are difficult to be classified in terms of their multiprotic nature. An acid base titration may be employed to evaluate acid’s equivalent weight provided one is able to fix its multiprotic behavior. The experimental approach that is presented here, highlights the suitability of equivalent weight based calculation to establish the multiprotic nature of any weak acid whose inflection point is difficult to detect in titration curves due to very close values of their successive dissociation constants.