Microionization constants of sulphonamides.

The macroionization constants and thermodynamic parameters of twelve commercial sulphonamides were determined by potentiometric titration at 15 degrees , 25 degrees and 35 degrees and by spectrophotometry at 25 degrees , in aqueous solution of ionic strength 0.1 (NaClO(4)). The microionization constants and the tautomeric constants (concentration ratios of zwitterion to uncharged or neutral species) were obtained according to the Edsall method combined with complementary tristimulus colorimetry. By utilizing the microionization constants, the species distribution as a function of pH were calculated for the three kinds of sulphonamides.