What is the formula for standard cell potential?

E°cell = E°cathode − E°anode, where both values are standard reduction potentials (measured in volts at 25 °C and 1 M concentration).

How do I know which half-reaction is the cathode?

The cathode undergoes reduction (gains electrons) and has the higher (more positive) reduction potential. The anode undergoes oxidation and has the lower reduction potential.

What does a positive E°cell mean?

A positive E°cell indicates the reaction is spontaneous under standard conditions (ΔG° < 0). A negative value means the reaction is non-spontaneous and requires external energy.

Are these values at standard conditions?

Yes. All potentials in the reference table are standard reduction potentials (E°) measured at 25 °C, 1 atm pressure, and 1 M ion concentrations. Use the Nernst equation for non-standard conditions.

Can I enter custom potentials not in the table?

Yes. Select "Custom" from either dropdown and type any reduction potential (in volts) directly into the input field.

Why is the anode potential subtracted, not added?

The anode half-reaction is reversed from its standard reduction form (it runs as oxidation), which negates its sign. Subtracting E°anode is equivalent to reversing it and adding.

Half Cell Potential Calculator

Name: Half Cell Potential Calculator
Availability: InStock
Author: Nham Vu

Enter or pick cathode and anode half-reactions to instantly compute E°cell = E°cathode - E°anode.

Cathode (Reduction)

Reduction Potential E° (V)

Anode (Oxidation)

Reduction Potential E° (V)

Result

Select half-reactions and click Calculate.

Standard Reduction Potentials (25 °C)

Half-Reaction	E° (V)

Summary

Enter or pick cathode and anode half-reactions to instantly compute E°cell = E°cathode - E°anode.

How it works

Select a cathode (reduction) half-reaction from the reference table or enter a custom reduction potential.
Select an anode (oxidation) half-reaction from the reference table or enter a custom reduction potential.
The calculator applies E°cell = E°cathode − E°anode automatically.
The full cell reaction is displayed by combining the two half-reactions.
A spontaneity indicator shows whether the cell reaction is favorable (E°cell > 0).

Use cases

Verify galvanic cell voltage in general chemistry coursework.
Compare reduction potentials when designing electrochemical experiments.
Quickly check whether a proposed cell reaction is thermodynamically spontaneous.
Look up standard reduction potentials for common half-reactions.
Prepare for AP Chemistry or university electrochemistry exams.
Validate manual calculations for lab reports.

Frequently Asked Questions

Last updated: 2026-06-18 · Reviewed by Nham Vu