Dissociation constants of some substituted cinnamic acids in protic solvents: measurements by hyper-Rayleigh scattering and potentiometric techniques

Abstract

Recently, we have demonstrated [J. Phys. Chem. 99 (1995) 17891] that dissociation constants of weak organic acids in protic solvents can be measured by the recently revived hyper-Rayleigh scattering technique. In this paper, a series of para- and meta-substituted cinnamic acids has been considered whose dissociation constants in protic solvents such as methanol, ethanol, and ethylene glycol, are not known. Using the hyper-Rayleigh scattering technique, we have measured the ionization constants of the cinnamic acids in these solvents and compared our results with those obtained by the more conventional potentiometric method. A third method, namely the spectrophotometric method, was also employed for determination of the equilibrium constants of these acids but was proven to be unreliable. The advantages and drawbacks of these various methods for determination of K_a are also discussed.