Standard Hydrogen Electrode Potential

No one can measure the absolute potential of a single half-cell; only the difference between two of them shows up on a voltmeter. Electrochemistry therefore chose a reference and declared it zero: platinum in 1 M H⁺ with hydrogen gas bubbling at 100 kPa, 2 H⁺ + 2 e⁻ → H₂, defined as exactly 0.0000 V at all temperatures. Every other standard potential in every table is a difference measured against it.

The choice is arbitrary in the way sea level is arbitrary: useful, universal, and not a claim about anything physical. In practice almost nobody builds a hydrogen electrode: it needs a clean platinum black surface, a regulated hydrogen supply, and it poisons easily. Working laboratories use a silver–silver chloride electrode at +0.197 V or a saturated calomel electrode at +0.241 V and add the offset back. The pH electrode in your bench meter is exactly this arrangement, with the reference and the glass sensing membrane packaged in one body.