Determination of pKa of benzoic acid- and p-aminobenzoic acid-modified platinum surfaces by electrochemical and contact angle measurements

Acidity constant values of benzoic acid (BA)-modified platinum electrode (Pt-BA) and p-aminobenzoic acid (pABA)-modified platinum electrode (Pt-NHBA) surfaces were determined using cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and contact angle measurements (CAM). Diazonium tetrafluoroborate salt reduction and pABA oxidation reactions were used to prepare (Pt-BA) and (Pt-NHBA) surfaces, respectively. Both surfaces exhibited pH dependence with [Fe(CN)(6)](3-/4-) redox probe solutions at different pH; this allowed us to estimate the surface pK (a) values. Acidity constants for Pt-BA surface were found to be pK (a) (3.09 +/- 0.25), (4.89 +/- 0.11), and (3.91 +/- 0.54) by CV, EIS, and CAM techniques, respectively, while the values for Pt-NHBA surface were pK (a) (3.16 +/- 0.45), (4.24 +/- 0.40), and (5.64 +/- 0.12). The Pt-BA surface pK (a) values were lower in CV and CAM measurements relative to the bulk solution of BA, while a higher value was observed in EIS for Pt-BA surface. The pK (a) values determined for Pt-NHBA surface via both CV and EIS were lower than the bulk value; however, the result obtained from CAM was one unit higher than pK (a) of bulk pABA.