Determination of the physical state of salicylic acid in hydrogel formulations by X-ray diffractometry

Hydrogel topical formulations containing salicylic acid (SA) are available for the treatment of warts. An X-ray diffractometric method was developed to determine the physical state of the SA in one of these products. Formulations were prepared with high SA concentrations (26-45% w/w) so that a fraction of the incorporated SA was undissolved. A fixed weight of each formulation was loaded into an X-ray sample holder and the integrated intensities of two peaks of SA were determined. The intensity of each of these peaks was linearly related to the weight percent of SA in the formulation. The intercept on the x-axis was the solubility of SA in the matrix at room temperature and was found to be about 20% w/w. The use of the two X-ray peaks provided two independent solubility values and these were in good agreement. Based on scanning electron microscopy, the solubility of SA was determined to be close to 19% w/w at room temperature.