Kinetic studies of proton-exchange reactions between imidazole groups of small molecules and hydrogen phosphate ions in aqueous solution using ultrasonic relaxation

Abstract

Ultrasonic absorption in aqueous solutions of imidazole and three substituted derivatives, N-methyl-, 2-methyl-, and 4-methylimidazole, has been studied in the presence of potassium phosphate as a function of pH and salt concentration in the frequency range 0.46-5.7 MHz at 25°C. For all of these compounds, the ultrasonic absorption curves as a function of pH showed a sharp maximum in the vicinity of pH 7, due to a disturbance of the proton-exchange equilibrium between the imidazolium ion and hydrogen phosphate ion by the ultrasonic waves. An expression for the ultrasonic absorption due to this proton transfer has been derived from relaxation theory, using a reaction scheme of Eigen. A quantitative interpretation is given for both the pH and the phosphate concentration dependence of the ultrasonic absorption in the neutral pH range. The rate constant k R and the volume change ΔV for the reaction of dihydrogen phosphate ion with the unprotonated imidazole group have been determined by a curve-fitting procedure in which k R and ΔV were the only adjustable parameters