Rotating-disk dissolution kinetics of nitrofurantoin anhydrate and monohydrate at various temperatures.

The dissolution behavior of nitrofurantoin anhydrate and monohydrate in JP XI, second fluid (pH 6.8) was investigated at various temperatures using a dispersed-amount method and a rotating-disk method. The initial dissolution process of the monohydrate obtained by the rotating-disk method followed the Noyes-Whitney-Nernst equation, but that of the anhydrate did not. The initial dissolution process of the anhydrate was analyzed by a dissolution kinetics equation involving the phase transformation process from anhydrate to monohydrate. The maximal concentration, the dissolution rate constant, and the rate constant of the phase transition process were estimated. The thermodynamic parameters for the dissolution processes of the anhydrate and monohydrate were obtained from van't Hoff plots and Arrhenius plots, respectively. The results of the intrinsic solubility and dissolution parameters of anhydrate and monohydrate suggest the possibility that the difference in the dissolution rates of the anhydrate and monohydrate affect the bioavailability of nitrofurantoin preparation. Information on the dissolution behavior of nitrofurantoin pseudopolymorphs is therefore useful for designing high-quality preparations.