Aug 1, 1976

Phenylbutazone ionization kinetics

Journal of Pharmaceutical Sciences
Valentino J Stella, J D Pipkin

Abstract

Phenylbutazone has been associated with bioavailability problems and has shown nonclassical behavior in phase-transport studies. This nonclassical behavior has been attributed, in part, to the fact that phenylbutazone, as a carbon acid, undergoes noninstantaneous ionization kinetics. Instantaneous reaction is an assumption made in many diffusion-limited transport models involving a simultaneous ionization reaction. The ionization kinetics of phenylbutazone were determined at an ionic strength of 0.1 and 25 degrees using a stopped-flow spectrophotometer. A log kobs versus pH profile for the approach to the ionization equilibrium was determined, and a mechanism consistent with the profile was postulated. The percent enol versus the diketo form of phenylbutazone acid as well as pKaenol and pKadiketo was kinetically calculated. The protonation reaction was highly catalyzed by general acids while the deprotonation reaction was highly catalyzed by general bases. The general acid, water, was a poor proton donor to the anionic form (the so-called mesomericanion) of phenylbutazone.

  • References2
  • Citations6

Mentioned in this Paper

Fenilbutazon
Phenylbutazone
Ampholytes
Spectrophotometry, Ultraviolet
Enol
PH Profile Measurement
Kobus kob
Sodium Chloride, (24)NaCl
Acids
Hydrogen-Ion Concentration

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