Standard Potential of the Ferric–Ferrous Couple
| Value | 0.771 V |
| Status | Measured: ± 0.001 V (0.0013 relative) |
| Source | CRC Handbook |
| Categories | Chemistryelectrochemistry |
| microvolt | 771,000 μV |
| millivolt | 771 mV |
| volt | 0.771 V |
| kilovolt | 0.000771 kV |
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.