Calculating critical relative humidity from solubility according to Pitzer ion interaction model.

The solubility and the critical relative humidity (H(cr)) of 14 drugs and inorganic salts were determined, the relationship between the H(cr) and the solubility was explored theoretically, and the H(cr) was calculated in the light of Raoult's law and Pitzer ion interaction model from their solubility. The results indicate that the H(cr) values calculated by Raoult's law in high humidity (H(cr)>80%) and by Pitzer ion interaction model in low humidity (H(cr)<80%) are comparable to the measured ones.