Potassium Ferricyanide
Properties
| State | Solid (bright red crystals) |
| Color | Bright red |
| Solubility | Very soluble in water (464 g/L at 20°C); slightly soluble in ethanol |
| Melting Point | Decomposes at ~300°C |
| Boiling Point | Decomposes before boiling |
About Potassium Ferricyanide
Potassium ferricyanide, K3[Fe(CN)6], is the bright red counterpart to lemon-yellow ferrocyanide, and the color difference is the giveaway that the iron sits in the +3 oxidation state inside an octahedral cage of six cyanide ligands. The complex has a formation constant near 10^44, which is why this salt sits comfortably in your reagent cabinet despite the CN groups — they simply do not come off under benchtop conditions. The most cited use is the Turnbull's blue test: drop a Fe2+ solution into ferricyanide and you get the same intensely blue Fe4[Fe(CN)6]3 lattice that Diesbach stumbled into around 1706. In photography, ferricyanide plus sodium thiosulfate is Farmer's reducer, the classic darkroom bleach for pulling highlights back on overexposed silver prints. The cyanotype process — those blue architectural prints that named the word 'blueprint' — pairs ferricyanide with ferric ammonium citrate. In electrochemistry, the [Fe(CN)6]3-/4- couple is the workhorse system every electrochemist runs to verify that a new electrode is behaving (E° = +0.358 V vs SHE, well-defined one-electron transfer).
Where you'll encounter it
If you've ever taught a sophomore lab the Prussian blue test, run cyclic voltammetry to characterize a glassy carbon electrode, or developed a cyanotype on cotton in the sun, you've worked with ferricyanide. In a working electrochemistry lab, a 5 mM ferricyanide / 0.1 M KCl solution is the standard 'is my potentiostat plugged in correctly' check — you should see a textbook reversible CV with a peak separation near 60 mV. In undergraduate qualitative analysis labs, instructors often hand out unknown iron solutions and let students figure out the oxidation state by comparing what happens with ferricyanide versus ferrocyanide: both produce Prussian blue, but only Fe2+ goes blue with the red salt and only Fe3+ goes blue with the yellow salt.
Common Uses
- Turnbull's blue qualitative test for Fe2+ in undergraduate inorganic labs
- Cyanotype photographic printing on cotton, paper, and engineering blueprints
- Farmer's reducer in darkroom silver-print bleaching of overexposed highlights
- Reference redox couple for cyclic voltammetry electrode characterization
- Electron acceptor in PSII photosynthesis assays and Hill reaction studies
- Etching agent in semiconductor and printed circuit board fabrication
- Ferroxyl indicator solution for detecting iron corrosion under coatings
Safety Information
GHS classification: Eye irritation Cat 2 (H319), skin irritation Cat 2 (H315). The complex itself is low-toxicity (oral LD50 in rats around 2970 mg/kg) because the cyanide ligands stay bound. The hazard scenario you actually need to avoid is heating the salt with concentrated mineral acid or exposing solutions to intense UV — both can break the Fe-CN bonds and liberate HCN gas (OSHA PEL 10 ppm ceiling, IDLH 50 ppm). Never mix ferricyanide waste with acid-bearing waste streams. Store away from sunlight to prevent slow photodecomposition to Prussian blue, and dispose through cyanide-compatible waste channels even though free CN- isn't present in the bulk material.
This safety summary is for educational reference only and may not be complete. It is not a substitute for Safety Data Sheets (SDS), medical advice, or professional chemical safety guidance. Always consult appropriate SDS and qualified professionals before handling chemicals.