The effect of ultrasound on the crystallization-precipitation process of transforming sodium amoxicillin into amoxicillin trihydrate

Crystallization is a process used in many industrial fields. However, in the pharmaceutical industry it plays a crucial role as the quality of the drug (finished product) in terms of therapeutic efficacy and stability is strongly related to the physical properties of the active ingredient, as well as other excipients determined and controlled through this process. This paper deals with the feasibility of the transformation of amoxicillin sodium salt into amoxicillin trihydrate. It will also be the opportunity of studying the physical properties of the crystals obtained (size and size distribution, shape, purity, morphology) as a result from the interaction of the crystallization-precipitation process with or without ultrasound and the selected antibiotic amoxicillin. The limiting factor for sonocrystallization is pH, the optimum pH being 4.5. Indeed, for a pH below 2, the crystallization yield does not exceed 10 %. Ultrasound has a synergistic effect on crystallization by improving the yield and the fineness of the powder, as well as on the kinetics of the reaction in relation to the nucleation time, even if it is not necessarily possible to attribute this phenomenon to solubility or de-supersaturation. For 30 min experiments, the best yield of 95 % was obtained using a low frequency (20 kHz), compared to the yield obtained without ultrasound (69 %) or with higher frequencies (581 kHz - 72 % and 864 kHz - 65 %). In addition to the ultrasound frequency used, temperature, pH and sonication time also have a profound effect on the product's crystal morphology and size. In the case of low frequency, particle size ranged from 0.4 to 60 μm, and from 0.7 to 250 μm under silent conditions.