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Nernst Cell Potential Calculator

Enter E0, the number of electrons, and the reaction quotient Q.

Result

1.0704 V
0.0296 V
Spontaneous (E > 0)

How it is calculated

The tool applies the Nernst equation at a fixed temperature of 25 °C: E = E° − (0.025693 ÷ n) × ln(Q). Here E° is the standard cell potential in volts, n is the number of electrons transferred in the balanced reaction, and Q is the reaction quotient (products over reactants, each raised to its coefficient, with solids and pure liquids left out). The constant 0.025693 V is RT÷F at 25 °C, and ln is the natural logarithm.

It reports the cell potential E, the correction term (0.025693 ÷ n) × ln(Q) and a label: spontaneous if E is above 0, non-spontaneous otherwise. Values are shown to four decimals in volts. Q must be greater than 0 and n at least 1. When Q is 1 the correction is zero and E equals E°. If Q is below 1 the term is negative, so E rises above E°; if Q is above 1, E falls below it. The temperature cannot be changed, so it does not suit other conditions.

Worked example

Take a cell with E° = 1.10 V, n = 2 electrons and Q = 10. The correction term is (0.025693 ÷ 2) × ln(10) = 0.0128465 × 2.3026 = 0.0296 V. So E = 1.10 − 0.0296 = 1.0704 V. The value is positive, so the tool labels the reaction spontaneous as written. Because Q is above 1, meaning more products than at standard conditions, the cell voltage is slightly lower than its standard value of 1.10 V.

Try this example →

Common mistakes

  • Using log base 10 values for Q. The equation uses the natural logarithm, which the tool computes itself, so enter Q as a plain ratio, not log Q.
  • Writing Q upside down. Q is products over reactants, and swapping them flips the sign of the correction term.
  • Including solids or pure liquids in Q, such as the zinc and copper metals in a Daniell cell. They do not appear in the quotient.
  • Entering n for a single half-reaction step instead of the total electrons in the balanced overall reaction.
  • Using the result for temperatures other than 25 °C. The constant is fixed at that temperature.

About this calculator

The standard cell potential (E0) measured under standard conditions only tells part of the story — the actual voltage a battery or electrochemical cell produces shifts with the real concentrations of reactants and products involved, described by the Nernst equation: E = E0 − (RT/nF)·ln(Q), which simplifies at 25°C to E = E0 − (0.025693/n)·ln(Q), where n is the number of electrons transferred and Q is the reaction quotient.

This calculator computes the actual cell potential from E0, n and Q at that standard 25°C temperature, a core calculation in electrochemistry courses and battery/corrosion research — it's the same relationship behind why a battery's voltage sags as it discharges (as reactant concentrations fall, Q changes and E drops) and why pH electrodes and ion-selective sensors can measure concentration from a voltage reading.

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Comments (1)

  • Editorial team

    The correction is small for ordinary concentrations, so a big surprise usually means a wrong Q. With E0 = 1.1 V and n = 2, Q = 0.01 gives 1.1592 V and Q = 100 gives 1.0408 V. It matters most when E0 is small: E0 = 0.05 V with Q = 1000 comes out at -0.0387 V, flipping the label. The tool is fixed at 25 degrees Celsius. How do you set up Q for your cells?

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