Kinetics of proton-transfer reactions of polyacrylic and polymethacrylic acids with an indicator

The kinetics of proton transfer between polyacrylic acid and the pH indicator phenol red and between polymethacrylic acid and phenol red have been studied by the temperature-jump relaxation technique in the pH range 6-8.5. In every case only a single and well defined relaxation time was observed. The rate constant for direct proton transfer from the polyacid to the indicator was found to be (2 ± 1) × 10⁷ M^-1 sec^-1 (acid concentration expressed in terms of monomeric units) for both systems, essentially independent of pH in the pH range studied. No evidence was found from these kinetic studies and additional ones at pH 5-6 for conformational transitions of the polyacids. Possible effects of the macromolecular nature of the polyacids on the proton-transfer kinetics are discussed.