Standard Potential of the Ferric–Ferrous Couple
| Value | 771 mV (0.771 V) |
| Status | Measured: ± 1 mV (0.13 % relative) |
| Source | USGS PHREEQC 3 database minteq.v4.dat (MINTEQA2 version 4), the file the site's water pages use: Fe³⁺ + e⁻ = Fe²⁺, log K 13.032 at 25 °C, so E° = (2.302585 RT/F) × 13.032 = 0.771 V |
| Categories | Chemistry |
| Unit | Value |
|---|---|
| millivolt | 771 mV |
| volt | 0.771 V |
| kilovolt | 0.000771 kV |
| picovolt | 771,000,000,000 pV |
| nanovolt | 771,000,000 nV |
| microvolt | 771,000 μV |
| megavolt | 7.7100000e-07 MV |
| gigavolt | 7.7100000e-10 GV |
Learning zone
The ferric–ferrous couple is the redox equivalent of a pH indicator for natural systems. At +0.771 V it sits in the middle of the range that groundwater, sediments and soils actually occupy, so whether iron is present as soluble Fe²⁺ or insoluble Fe(OH)₃ tells you immediately whether an environment is oxidising or reducing. Anoxic well water carries clear Fe²⁺ that oxidises and turns orange within minutes of reaching the tap.
The couple is also fast and reversible, which makes it a favourite in analytical chemistry: the ceric–ferrous titration and the ferroin indicator both depend on it, and iron is a common mediator in electrochemical and biological electron transport chains. Note the potential shifts strongly with complexation: bind the iron with EDTA or cyanide and the ferricyanide/ferrocyanide couple drops to +0.358 V, a reminder that a standard potential describes a specific chemical species and not simply an element.
Cite this page
formula.expert. (2026, November 1). Standard Potential of the Ferric–Ferrous Couple. https://formula.expert/constants/standard-potential-iron-iii-ii