Abstract: In electrochemistry, φ can be calculated via φ^⊖ and Nernst equation. Current physical chemistry textbooks only provide substances’ φ^⊖ under acidic conditions and corresponding practices. Hence, the calculation of φ works well under acidic conditions. However, it becomes complicated under alkaline conditions for some substances cannot exist in acidic conditions. These might lead to incorrect results and confused conclusions. To address this issue, the present study calculates φ^⊖ (φ_B^⊖) from ∆_rG_m^⊖ based on ∆_fG_m^⊖ of involved substances in redox half reaction. Then, φ at different pH is calculated via Nernst equation, resulting in a φ−pH diagram. Moreover, thermodynamic isotope effects on φ^⊖ and φ are included. This may serve as a reference for teaching and research purposes.

Key words: Redox reactions of water, Electrode potential calculation, φ–pH Diagram, Thermodynamic isotope effect