Acid-base reactions are ubiquitous in solution chemistry, as well as in electrochemistry. However, macroscopic concepts derived in solutions, such as pKa and pH, differ significantly at electrified metal-aqueous interfaces due to specific solvation and applied voltage. Here, we measure the pKa values of an amino acid, glycine, at a gold/water interface under a varying applied voltage by means of spectroscopic titration. With the help of simulations, we propose a general model to understand potential-dependent shifts in pKa values in terms of local hydrophobicity and electric fields. These parameters can be tuned by adjusting the metal surface and applied voltage, respectively, offering promising, but still unexplored, paths to regulate reactivity. Our results change the focus with respect to common interpretations based on, for example, apparent local pH effects and open interesting perspectives for electrochemical reaction steering.