Testing of the influence of media’s ph value on the solubility and partition coefficient of the acetylsalicylic acid

Abstract

Experimental data of the influence of media’s pH value on the solubility and partition coefficient of the acetylsalicylic acid (ASA) is presented in order to define the place in the human digestive system where ASA is absorbed after oral administration. The solubility of ASA in three media with different pH value (pH 1.2; purified water and pH 7.4) was determined using UV spectrophotometric measurement (based on Beer-Lambert’s law that says that absorbance represents the linear function of concentration). Data for partition coefficient calculating was obtained by using classical shake-flask method, whereby as polar phase previously mentioned media were used and as organic phase chloroform was used. The study showed that both solubility and partition coefficient are pH-dependent, whereby solubility increases with increasing pH value above the pKa value, while partition coefficient decreases in the same conditions. ASA is theoretically expected to be the best absorbed in a medium where it is mostly presented in the unionized form. In this case, it is a hydrochloric acid buffer (pH value 1.2) which is similar to one found in the stomach. However, data from previous experimental studies have shown that ASA is the best absorbed in the proximal small intestine, due to larger surface area and more permeable membranes.