Standard Hydrogen Electrode
The reference half-cell, consisting of hydrogen gas at 1 bar bubbled over platinum in 1 M acid, assigned a potential of exactly zero.
Formula / equation
2H⁺ + 2e⁻ ⇌ H₂, E° = 0.00 V
Unit
V
In depth
Since absolute electrode potentials cannot be measured, one couple must be defined as the zero point, and hydrogen was chosen. The platinum is coated with platinum black to catalyse the electron transfer and provide a large surface area. It is accurate but inconvenient in routine use.
Examples in the real world
Measuring the copper electrode against it gives the tabulated +0.34 V for the Cu²⁺/Cu couple.