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 combination of spectrophotometric and potentiometric methods at 25 °C in wide range of ionic strength. The ionic strength of solutions was kept constant by sodium perchlorate as background electrolyte at 0.10 to 3.0 mol dm-3. Potentiometer was calibrated according to Gran method at each of ionic strength. The pKa values were calculated using STAR program by multivariate curve fitting of absorbance data. The dependence of deprotonation 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 interaction parameters were extracted associated with the thermodynamic protonation constant.