Potassium Ferricyanide
Formula: K₃[Fe(CN)₆] — Potassium hexacyanoferrate(III), red prussiate of potash
Appearance: Deep red-orange crystalline solid
Hazard: Irritant · Do not mix with acids
Properties
Bright red crystalline solid, very soluble in water producing a yellow solution. The Fe³⁺ center is held within a stable octahedral cyanide cage ([Fe(CN)₆]³⁻), making it far less toxic than simple cyanide salts. Strong oxidizer that reacts with ferrous (Fe²⁺) ions to immediately precipitate Prussian blue (iron(III) ferrocyanide, Fe₄[Fe(CN)₆]₃). Related compound: potassium ferrocyanide (the Fe²⁺ analogue, yellow prusiate of potash), which does the same reaction in reverse. The two together are the heart of the Prussian blue chemistry used in blueprints, dyes, and pigments.
Historical Context
Prussian blue was the first modern synthetic pigment, discovered in Berlin around 1704 by the paint-maker Johann Jacob Diesbach, apparently by accident while attempting to make a red lake pigment using potash contaminated with animal blood. Its distinctive deep blue — intense enough that a little goes a very long way — transformed painting. Watteau, Hokusai, and Picasso (in his Blue Period) all used Prussian blue extensively.
The cyanotype photographic process was invented by Sir John Herschel in 1842. Paper coated with a mixture of potassium ferricyanide and iron(III) ammonium citrate turns deep blue wherever light strikes — the ferricyanide oxidizes the ferrous ions produced by photoreduction, and Prussian blue precipitates in place. Blueprints, the architectural drawing reproductions that gave their name to a whole concept, used exactly this chemistry for over a century.
Experiments
Prussian Blue Synthesis: Mix a few mL of dilute ferric chloride or ferric ammonium sulfate solution with dilute potassium ferricyanide solution — a deep blue precipitate of Prussian blue forms immediately. The color is intense; even very dilute solutions produce vivid results. Compare with the reaction using potassium ferrocyanide (which gives Turnbull’s blue — actually the same compound formed a different way).
Cyanotype Prints: Dissolve potassium ferricyanide and iron(III) ammonium citrate in water separately (equal volumes), mix, and coat onto paper or fabric in subdued light. Place objects or a photographic negative on the surface and expose to sunlight for 10–15 minutes. Rinse in water — where light hit, vivid Prussian blue develops; unexposed areas wash away, leaving white forms. The definitive photogram process.
Experiments using this chemical:
- Prussian Blue Synthesis — Instant deep-blue pigment from iron salts
Safety
Moderate hazard — stable under normal use; hazardous in acid.
The cyanide ligands are tightly bound and not bioavailable under neutral or alkaline conditions. However, do not mix with strong acids — acid decomposes the complex and releases toxic hydrogen cyanide gas. Never heat strongly. Keep away from acids; dispose of solutions by dilution and normal drain disposal (the complex is stable and the quantities involved in lab use are minimal). Wear gloves — the dye stains skin persistently.
Incompatible with: Strong acids (releases HCN gas — serious hazard); strong reducing agents; reactive metals; concentrated strong bases at high temperature