When an electrode contacts an electrolyte, an interfacial electric field forms. This interfacial field can polarize the electrode’s surface and nearby molecules, but its effect can be countered by applying countering potential. We present an optical method for quantifying the value of this countering potential (“potential of zero charge”, pzc). Our approach uses phase-sensitive second harmonic generation to determine the potential where the interfacial electric field vanishes, using Pt-water interface as a model system. Our experiment reveals that the value of the pzc of the Pt/water interface and further shows that it is pH-independent from pH 1 to 13 on an absolute potential scale. We will discuss this result from the consideration of the solid-liquid interface, including the electrode materials and interfacial water structure. Our work provides a materials-agnostic tool for quantifying the interfacial electrical field at a solid-liquid interface and confirms the view that the interfacial electric field during hydrogen electrocatalysis is more intense in alkaline than acid.