# Modification of the Spectral Absorbance Difference Method for Determining the Dissociation Constants of Polyprotic Acids with Delocalized π‑Systems

## TL;DR

This paper modifies a method to determine dissociation constants of polyprotic acids with delocalized π-systems, improving accuracy for multi-step acid-base reactions.

## Contribution

A modified spectral absorbance difference method is introduced to enhance accuracy in determining higher dissociation constants of polyprotic acids.

## Key findings

- The absorbance spectra of tenacic and 4-nitrotenacic acids show strong pH dependence due to π-electron delocalization.
- The modified method successfully determined the second and third dissociation constants of the studied acids.
- The method can be applied to other compounds with pH-dependent π-electron delocalization.

## Abstract

Polyprotic organic acids with suitable molecular geometry
and acidic
properties are frequently employed in the development of novel nano-
and macroscale materials. These acids can bind to inorganic surfaces
in a bridging mode, effectively facilitating junctions at organic–inorganic
interfaces. This study was motivated by the potential of triprotic
2-hydroxyisophthalic (tenacic) acid and its derivatives to fulfill
this role. The absorbance spectra of the unsubstituted tenacic and
4-nitrotenacic acids were found to be strongly pH-dependent. This
behavior is attributed to varying degrees of electron delocalization
in the π-systems of the conjugate bases. The previously reported
spectral absorbance difference method was applied in this work without
modification to determine the first dissociation constants of both
acids. To determine the second dissociation constants for both acids
and the third for 4-nitrotenacic acid, we modified the method to improve
the accuracy of the measured K
b of their
conjugate bases. Utility of this method may be extended to other compounds
that exhibit pH-dependent variations in π-electron delocalization.