Habit modifications of nitrofurantion crystallized from formic acid mixtures

The crystal size and the length to width ratio of Nitrofurantoin, an anti-bacterial urinary tract drug, can be controlled using an appropriate mixture of solvents and suitable crystallization conditions. Some solvents will form undesirable complexes with the drug (INF) while with others no crystal structure modification or complexation was detected (HCO₂H). The length (y) to width (x) ratio of the Nitrofurantoin varies from 2.5 to 1.5 when crystallized from pure formic acid or in a mixture with water or ethanol. The y/x values correspond to the solvent interactions and super saturation (S). The crystal growth regularity is ascribed to the solvent power and thus when more regular crystals will precipitate bioavailability and solubilization of the drug will increase. Best results were obtained when mixture of formic acid-ethanol solution was used as crystallization media yielding large tabular crystals.