Effect of Ionic Strength on Deprotonation of Salicylic Acid and its Derivative

The protonation constant values of salicylic and 5-nitrosalicylic acid werestudied by a combinationof spectrophotometric and potentiometric methods at 25 °C in wide range of ionic strength. The ionicstrength of solutions was kept constant by sodium perchlorate as background electrolyte at 0.10 to 3.0 moldm-3. Potentiometer was calibrated according to Gran method at each of ionic strength. The pKa valueswere calculated using STAR program by multivariate curve fitting of absorbance data. The dependence ofdeprotonation constants on ionic strength were explained by means of Specific ion Interaction Theory (SIT).Activity coefficients of species were calculated by a modified SIT approach. The specific ion interactionparameters were extracted associated with the thermodynamic protonation constant.