Polarographic and Spectrophotometric Studies on Trimethylpyruvic Acid

Abstract

Abstract Polarographic and spectrophotometric behavior of trimethylpyruvic acid has been investigated. The limiting current of trimethylpyruvic acid shows a minimum value in pH about 0.5. The absorption spectra show the absorption maximum at about 310 mμ in aqueous solutions, indicating the decrease of the optical density in strongly acid pH. These facts have been accounted for by the hydration of carbonyl group of undissociated acid. pKa for apparent dissociation constant of trimethylpyruvic acid has been spectrophoto-metrically obtained to be 1.52 at 25°C. Hydration constant of the undissociated acid (Kh=[AHaq]/[AH]) has been kinetically obtained to be 0.46 at 25°C. The rate constant of dehydration of hydrated trimethylpyruvate in buffer solution of pH about 5 was 2.8×10−2 sec−1 at 25°C. From the temperature dependence of the hydration constant, the heat of hydration of unhydrated acid has been found to be −8.2kcal. A quantitative interpretation of the pH-dependence of the limiting current has been attempted by deriving the equation, involving the hydration and dissociation factors. The agreement of the theoretical curve with the experimental is good, except for the pH range below 0.5, where acid catalyzed dehydration of hydrated acid should be taken into consideration. Rate constants of the dissociation and recombination of trimethylpyruvic acid have been found to be kd=6.0×105 sec−1 and kr=1.4×107 1. mol−1 sec−1, respectively.