Abstract
It is suggested, when working with drug—urea—water systems, that more than passing consideration be given to the effect or effects produced by addition of a fourth component, as, for example, when an acid is used to adjust pH. Data are cited which indicate that these effects are measurable and often predictable. Solubility data for methyl salicylate and methyl benzoate are given as functions of varying acid, salt, and urea concentrations. These data represent the equilibrium solubilities of the esters in the various systems at 30°. The solubilities were obtained by sampling and subsequent determination of the ester concentration using UV spectrophotometry. A mathematical model was derived which permits a quantitative evaluation of salt effects in urea solutions. The theoretical calculations, based on this model, were found to be in good agreement with the experimental values observed for neutral and pH 1 solutions of methyl salicylate and methyl benzoate in urea. Extension of these findings to an earlier investigation indicates that significant error in interpretation of solubility in urea may result if these salt effects are disregarded.
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