Titrations Based on Acid–Base Reactions

Titrations Based on Acid–Base Reactions

The earliest acid–base titrations involved the determination of the acidity or alka- linity of solutions, and the purity of carbonates and alkaline earth oxides. Before 1800, acid–base titrations were conducted using H₂SO₄, HCl, and HNO₃ as acidic titrants, and K₂CO₃ and Na₂CO₃ as basic titrants. End points were determined using visual indicators such as litmus, which is red in acidic solutions and blue in basic solutions, or by observing the cessation of CO₂ effervescence when neutraliz- ing CO₃2–. The accuracy of an acid–base titration was limited by the usefulness of the indicator and by the lack of a strong base titrant for the analysis of weak acids.

The utility of acid–base titrimetry improved when NaOH was first introduced as a strong base titrant in 1846. In addition, progress in synthesizing organic dyes led to the development of many new indicators. Phenolphthalein was first synthe- sized by Bayer in 1871 and used as a visual indicator for acid–base titrations in 1877. Other indicators, such as methyl orange, soon followed. Despite the increas- ing availability of indicators, the absence of a theory of acid–base reactivity made se- lecting a proper indicator difficult.

Developments in equilibrium theory in the late nineteenth century led to sig- nificant improvements in the theoretical understanding of acid–base chemistry and, in turn, of acid–base titrimetry. 

Sørenson’s establishment of the pH scale in 1909 provided a rigorous means for comparing visual indicators. The determination of acid–base dissociation constants made the calculation of theoretical titration curves possible, as outlined by Bjerrum in 1914. For the first time a rational method ex- isted for selecting visual indicators, establishing acid–base titrimetry as a useful al- ternative to gravimetry.