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The solubility and the dissolution rate of the sodium salt of an acidic drug (REV 3164; 7-chloro-5-propyl-1H,4H-[1,2,4]triazolo[4,3-alpha]quinoxaline-1,4-dione) decreased by the effect of common ion present in aqueous media. The solubility of the sodium salt of REV 3164 in a buffered medium was much lower than that in an unbuffered medium. Also, the presence of NaCl decreased its solubility in water. The apparent solubility product (K'sp) of the salt, however, did not remain constant when the concentration of NaCl was changed. A decrease in K'sp value with the increase in NaCl concentration was observed; for example, the K'sp values at 0 and 1 M NaCl were 7.84 X 10(-4) and 3.94 X 10(-4) M2, respectively. Even when corrected for the effect of ionic strength, the solubility product decreased. This decrease in the solubility product in the presence of NaCl indicated a decrease in the degree of self-association (increase in activity coefficient) of the drug in aqueous media.

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