Potassium Ferrocyanide
Properties
| State | Solid (lemon-yellow crystals; trihydrate most common) |
| Color | Lemon yellow |
| Solubility | Soluble in water (289 g/L at 20°C); insoluble in ethanol |
| Melting Point | Decomposes at ~70°C (trihydrate loses water); anhydrous decomposes ~300°C |
| Boiling Point | Decomposes before boiling |
About Potassium Ferrocyanide
Potassium ferrocyanide, K4[Fe(CN)6], is the lemon-yellow Fe(II) sibling of red ferricyanide. The yellow color and diamagnetism both come from a low-spin d6 iron center sealed inside an octahedral cage of six cyanide ligands, with a formation constant around 10^35. That binding strength is why a salt containing six CN groups per formula unit ended up approved as food additive E536 — the European Food Safety Authority and FDA both treat it as safe because the cyanide simply does not dissociate at gastric pH. You'll see it on table salt ingredient lists in Europe at up to 20 mg/kg as an anti-caking agent, and winemakers use the bluefining (Blauschönung) process to drop excess copper and iron out of white wines as Prussian blue, which is then filtered out before bottling. In analytical chemistry, ferrocyanide is the classic spot-test reagent for Fe3+ — the resulting Fe4[Fe(CN)6]3 (Prussian blue) is so intense that low-ppm iron contamination in a clear solution turns the whole flask blue. In electrochemistry, the [Fe(CN)6]4-/3- couple at +0.358 V vs SHE is the standard one-electron reversible system everyone benchmarks new electrodes against.
Where you'll encounter it
If you've ever salted a steak with a European brand of table salt, sipped a German Riesling, or run a cyclic voltammogram in undergraduate physical chemistry, ferrocyanide has touched your day. In a winery lab, the bluefining titration uses a precisely calculated dose of K4[Fe(CN)6] based on prior ICP-OES analysis of the wine's iron and copper content — overshoot and you leave residual ferrocyanide; undershoot and the wine still hazes. In an electrochemistry lab, a freshly prepared 5 mM K4[Fe(CN)6] / 1 M KNO3 solution is what you scan to confirm a glassy carbon electrode is properly polished — anything wider than ~70 mV peak separation means the surface is fouled and needs another alumina pass.
Common Uses
- Anti-caking additive E536 in European table salt at up to 20 mg/kg
- Bluefining of white wine to remove copper and iron as Prussian blue precipitate
- Spot test reagent for Fe3+ in qualitative inorganic analysis
- Reference redox couple for cyclic voltammetry of new electrode surfaces
- Component of cyanotype solution B in alternative-process photography
- Case-hardening of steel in traditional Bower-Barff cyaniding heat treatment
- Mordant in cotton dyeing for Prussian blue and saxon blue shades
Safety Information
GHS: not classified as hazardous at normal handling concentrations. Oral LD50 in rats is around 6400 mg/kg, and permitted dietary intake as a food additive is capped orders of magnitude below that. The cyanide is locked inside the coordination sphere with a formation constant near 10^35, which is why E536 cleared food-additive review. Never heat the dry salt with concentrated H2SO4 or HCl — that decomposes the complex and releases HCN, whose exposure limit is a ceiling rather than a time-weighted average. Avoid prolonged intense UV on solutions, which slowly photodecomposes to ferricyanide. Dispose through cyanide-compatible waste streams even though free CN- isn't released under normal use.
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. We deliberately do not publish occupational exposure limits or other regulatory thresholds: those values are revised over time and differ between jurisdictions, so the only correct source is the current SDS and the regulations that apply where you work.