[Kinetics of ampicillin breakdown in the solid state]

The degradation kinetics of solid sodium ampicillin, ampicillin trihydrate and anhydrous ampicillin was studied in relation to the disintegration level of the powder before and after its screening by the particle size. It was shown that the effect of the powder disintegration level on the degradation rate constant proceeded by the spherical model and the reaction of the antibiotic degradation proceeded according to the equation of the first order reaction. The degradation rate constants of the total fractions and the fractions with the particle size of 60, 80 and 100 micron at elevated and room temperatures and their half-lives were evaluated. Different mechanisms of degradation of three ampicillin forms were shown on the basis of the dependence of the degradation rate constant on the temperature and the data of thin-layer chromatography. Higher stability levels of ampicillin powder with the particle size of 100 micron and anhydrous ampicillin in comparison with sodium ampicillin and ampicillin trihydrate were observed. This allows recommending anhydrous ampicillin for preparation of oral dosage forms.