Last updated: 2026-06-16

The standard cell potential, written $E^\circ_{cell}$, is the voltage an electrochemical cell produces when every species is in its standard state — 1 M for dissolved ions, 1 bar for gases, at a stated temperature, conventionally 25 °C. It measures the thermodynamic driving force behind the cell's overall oxidation–reduction reaction. Anyone studying galvanic cells, batteries, or corrosion uses it to predict whether a redox reaction will run on its own and how much electrical work it can deliver.

How the two half-cells combine

Every cell is built from two half-reactions. Reduction tables list each half-reaction as a reduction with its standard reduction potential. In a working cell one electrode is forced to run in reverse (oxidation), but the potentials are still combined as tabulated:

$E^\circ_{cell} = E^\circ_{cathode} - E^\circ_{anode}$

The cathode is the electrode where reduction happens and the anode is where oxidation happens. Because both numbers come from the table as reduction potentials, you subtract the anode value directly rather than flipping its sign by hand.

Worked example

Take the classic Daniell cell, copper paired with zinc. From a reduction table, $E^\circ(\mathrm{Cu^{2+}/Cu}) = +0.34\ \mathrm{V}$ and $E^\circ(\mathrm{Zn^{2+}/Zn}) = -0.76\ \mathrm{V}$. Copper has the higher reduction potential, so it is the cathode:

The positive result confirms the cell is galvanic: zinc spontaneously gives up electrons to copper ions, producing about 1.10 V.

Why the sign matters

A positive $E^\circ_{cell}$ corresponds to a spontaneous reaction because of the link $\Delta G^\circ = -nFE^\circ_{cell}$ — a positive potential gives a negative free energy. If you assign the electrodes so that the result comes out negative, you are describing the reverse reaction, which only proceeds when an external voltage drives it, as in electrolysis. To go from a positive potential to the corresponding free energy, use the Gibbs Free Energy from Cell Potential Calculator, and to find how far the reaction proceeds, the Equilibrium Constant from Cell Potential Calculator.

Standard potentials assume standard-state concentrations. When concentrations differ, the actual potential shifts according to the Nernst Equation Calculator, which reduces to the standard value when all activities equal one.