An Experimental Investigation of the Interaction of Aspirin with Urea in Water

A study of the interaction of aspirin with urea in water was initiated in an effort to obtain evidence of complex formation between these compounds and to observe possible changes in aspirin stability resulting from complexation. A solubility method was used to detect complexation and consisted of observing changes in the solubility of aspirin in the presence of varying urea concentrations at pH 2.0 and at pH 3.5. The rate of degradation of aspirin in the presence of various urea concentrations was observed at five pH values. All data was obtained at 30.0°C. The results indicated that urea markedly affected the solubility of aspirin. The solubilization was attributed to complex formation and apparent equilibrium constants for one to one and two to one species were obtained from the solubility curves. An analysis - of the degradation data indicated that the aspirin-urea interaction caused faster aspirin hydrolysis at pH values lower than 2.6 and inhibited aspirin hydrolysis at pH values from 2.6 to 3.5. This effect could be attributed to changes in the aspirin species, i.e., different charges at different hydrogen ion concentrations, as well as changes in the nature of the aspirin-urea complex. The rate of degradation of aspirin in the complex at pH 2.0 was calculated and was reasonably constant over a wide concentration of urea