title: Kinetics of thermal degradation of betamethasone valerate and betamethasone dipropionate in different media.
abstract: The effect of pH, media, phosphate concentration and ionic strength on the kinetics of thermal degradation of betamethasone valerate and betamethasone dipropionate has been investigated. A validated HPLC method has been used to determine the parent compounds and their major thermal degradation products identified in the reaction. Betamethasone-17-valerate gave rise to two major products, namely, betamethasone-21-valerate and betamethasone alcohol, and betamethasone dipropionate degraded into three major products, namely, betamethasone-17-propionate, betamethasone-21-propionate and betamethasone alcohol, in different media. Betamethasone valerate showed maximum stability at pH 4-5 while betamethasone dipropionate was maximally stable at pH 3.5-4.5. The degradation of betamethasone valerate and betamethasone dipropionate was found to follow first-order kinetics and the apparent first-order rate constants (k(obs)) for thermal degradation in different media range from 0.399-9.07×10(-3) h(-1) and 0.239-1.87×10(-3) h(-1), respectively. The values of the rate constants decrease with increasing solvent polarity, phosphate concentration and ionic strength. The second-order rate constants (k΄) for the phosphate ion inhibited reactions lie in the range of 3.02-1.30×10(-6) M(-1) s(-1).
authors: Faculty of Pharmacy, Hamdard University, Shahra-e-Madinat Al-Hikmah, Muhammad Bin Qasim Avenue, Karachi-74600, Pakistan.
authors: S U R
authors: SU
authors: Khattak
authors: D
authors: D
authors: Sheikh
authors: I
authors: I
authors: Ahmad
authors: K
authors: K
authors: Usmanghani
keywords: Betamethasone valerate; betamethasone dipropionate; kinetics; thermal degradation