standard hydrogen electrode
/ STAN-derd HY-druh-jen ih-LEK-trohd /
You cannot weigh how high something is without choosing a sea level to measure from. Electrode potentials have the same problem: you can only ever measure the difference between two electrodes, never one alone. So chemists agreed on a single reference point and called its potential exactly zero. That reference is the standard hydrogen electrode.
It is a specific, reproducible setup: a piece of inert platinum, coated to be spongy, bathed in acid where hydrogen ions are at exactly 1 molar (technically, unit activity), with hydrogen gas at 1 bar bubbling over it, all at 25 °C. The half-reaction at its surface — hydrogen ions turning into hydrogen gas and back — is defined to have a potential of 0.00 volts, by convention. Every other electrode potential in the tables is quoted relative to it.
This electrode is the agreed sea level of electrochemistry. Because everyone measures against the same zero, the standard potentials of zinc, copper, gold, and hundreds of others can be tabulated, compared, and added up to predict cell voltages. The real hydrogen electrode is finicky to build, so labs usually use a sturdier reference (like silver/silver chloride) and convert back to the hydrogen scale.
To find zinc's standard potential, you build a cell with a zinc electrode on one side and a standard hydrogen electrode on the other. The voltmeter reads about 0.76 V, and since hydrogen is defined as zero, zinc's potential is −0.76 V.
The hydrogen electrode is the zero against which every other potential is measured.
Its zero is a convention, not a measured fact — nobody has ever measured an absolute single-electrode potential. The whole table of values would simply shift if a different reference were chosen.