Prussian Blue Synthesis

Create the first synthetic pigment
Beginner🕐15 min▲Moderate hazardprecipitationredox

Summary

Mixing two pale iron solutions instantly precipitates1 an intense blue pigment — the first synthetic pigment of known composition. You’ll be able to explain how iron in two oxidation states (Fe²⁺ and Fe³⁺) creates the deep blue by “intervalence charge transfer,” and why the tightly-bound cyanide in the reagents is safe.

History

Prussian blue was the first synthetic pigment with a known chemical composition. It was accidentally discovered around 1706 by Berlin paint maker Johann Jacob Diesbach, who was trying to make a red pigment but used contaminated potash. The result was an intense blue that immediately became prized by artists.

The pigment got its name from its use by the Prussian military for dyeing uniforms. It became the blue in blueprints (cyanotypes), invented in 1842 by John Herschel2. The word “cyan” itself derives from the Greek word for blue used to describe Prussian blue.

The chemistry involves iron in two different oxidation states - Fe²⁺ and Fe³⁺ - bridged by cyanide groups. This mixed-valence compound absorbs red and yellow light, transmitting only the intense blue. Today, Prussian blue is used medically to treat thallium and radioactive cesium poisoning by binding these toxic metals in the gut.

Hazards & preparation

Warning

PPE: safety glasses and gloves.

  • Ferric chloride is corrosive and stains badly — protect skin, clothing, and surfaces.
  • Never mix the ferrocyanide or ferricyanide with strong acids. The cyanide is safely locked to iron, but strong acid can release toxic hydrogen cyanide gas. Keep acids away from this experiment.

Disposal: the pigment and iron solutions must not go down the drain — filter off the solid Prussian blue, bin it, and hold the iron filtrate for hazmat collection or dilute heavily. See the Safety page.

Materials

  • Ferric chloride - 5g dissolved in 50mL water (Method 1)
  • Potassium ferrocyanide - 5g dissolved in 50mL water (Method 1)
  • Ferrous sulfate - 5g in 50mL water (Method 2)
  • Potassium ferricyanide - 5g dissolved in 50mL water (Method 2)
  • Beaker - 150mL
  • Stirring rod
  • Filter paper (optional, to collect pigment)

Procedure

Method 1 (with ferrocyanide):

  1. Dissolve 5g ferric chloride in 50mL water (yellow-brown solution).
  2. Dissolve 5g potassium ferrocyanide in 50mL water (yellow solution).
  3. Pour the ferrocyanide solution into the ferric chloride while stirring.
  4. An intense dark blue Prussian blue precipitate forms instantly.

Method 2 (with ferricyanide):

  1. Dissolve 5g ferrous sulfate in 50mL water (pale green solution).
  2. Dissolve 5g potassium ferricyanide in 50mL water (yellow solution).
  3. Pour the ferricyanide into the ferrous sulfate while stirring.
  4. An intense dark blue precipitate (Turnbull’s blue, identical to Prussian blue) forms instantly.

What you should see

Two pale, transparent yellow/green solutions combine into an instant, opaque, almost black-blue cloud — startlingly intense from such faint starting colours. Left to stand, it settles into a deep blue sludge that filters to a vivid pigment.

Symptom Likely cause Fix
Colour weak/greenish Wrong ratio or dilute reagents Use stronger solutions; ensure a slight excess of the iron salt
No precipitate One reagent didn’t dissolve Make sure both solutions are fully dissolved before mixing
Pigment won’t settle Very fine colloid Let it stand longer, or filter through fine paper

The reactions

\[\ce{4 Fe^{3+} + 3 [Fe(CN)6]^{4-} -> Fe4[Fe(CN)6]3}\] (Prussian blue)

iron(III) ions + hexacyanoferrate(II) ions → insoluble Prussian blue pigment

The Science

Prussian blue (iron(III) hexacyanoferrate(II)) is an intense pigment used since the 1700s. The deep blue color comes from intervalence charge transfer between Fe²⁺ and Fe³⁺ ions in the crystal structure. Light energy promotes an electron from Fe²⁺ to Fe³⁺, absorbing red/orange wavelengths and transmitting blue.

Questions to Explore

  1. What is intervalence charge transfer? The colour comes from electrons jumping between Fe²⁺ and Fe³⁺ centres bridged by cyanide. How can an electron transfer across the bridge, and why does it absorb red light?

    Hint / answer

    The cyanide bridge conducts the electron between the two irons, letting light push an electron from an Fe²⁺ to a neighbouring Fe³⁺. That jump takes exactly the energy of red/orange light, so those colours are absorbed and the intense blue is what’s left.

  2. Why is the colour so intense? Simple iron solutions are pale (weak d–d transitions); Prussian blue is deeply coloured. Why are charge-transfer transitions so strong?

    Hint / answer

    d–d transitions are quantum-mechanically “forbidden,” so they absorb light weakly — pale colours. Charge-transfer moves a whole electron between atoms and is fully allowed, absorbing light strongly, so even tiny amounts of pigment look deeply coloured.

  3. Why is potassium ferrocyanide not toxic despite containing cyanide? How does binding cyanide to iron change its reactivity, and what happens with strong acid?

    Hint / answer

    The six cyanides are locked tightly around the iron in a stable complex, so they’re not free to poison you the way loose cyanide would. Strong acid, however, can pry them off and release toxic hydrogen cyanide gas — which is why you must keep acids away from these reagents.

  4. How does Prussian blue treat thallium poisoning? Given its lattice of iron ions bridged by cyanide, what lets it trap thallium so selectively?

    Hint / answer

    Its open cage structure has cavities exactly the right size and charge to snugly hold large ions like thallium and cesium. Swallowed, it grabs those toxic ions in the gut and carries them out in the stool instead of letting them be absorbed.

  5. What does “first synthetic pigment” mean? Natural blues (lapis, azurite) were rare and costly. How was Prussian blue different, and why did cheap blue matter?

    Hint / answer

    It wasn’t a mined mineral or an imitation of one — it was a brand-new compound made deliberately in a workshop, the first blue of known, reproducible chemistry. Being cheap and plentiful, it put a strong blue in the hands of ordinary artists for the first time.

Going further

  • Collect the pigment. Filter, rinse, and dry the precipitate to get usable Prussian blue paint, and try it on paper.
  • Compare the routes. Make Prussian blue (Method 1) and Turnbull’s blue (Method 2) side by side — they’re the same compound from different starting states.
  • Next in the Redox track: watch iron chemistry “develop” a permanent black in Iron Gall Ink.

Footnotes

  1. Precipitate — An insoluble solid that forms and separates out when two solutions are mixed.↩︎

  2. John Herschel — English polymath (1792–1871) who invented the cyanotype photographic process in 1842.↩︎