Abstract
The concept of surface pKa is introduced and developed for self-assembled monolayers (SAMs). Students use a titration method to determine the pKa of 3-mercaptopropionic acid (MPA) in solution. At the same time they determine the surface pKaof a SAM of MPA on a gold electrode by running a series of cyclic voltammograms of ferricyanide in buffers of differing pH. At different pH the extent of ionization of the SAM changes. Therefore, as ferricyanide is anionic, the charge exclusion of the ferricyanide, as ascertained from cyclic voltammetry peak currents, allows the inference of the charge of the MPA SAM. A plot of the peak current versus pH yields surface pKa values consistent with the literature. The dual aims of elucidating pKa and gaining experience in cyclic voltammetry are complementary. This practical exercise highlights the differences between surface layers and solution components with applications across a range of disciplines.
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