Prussian Blue Synthesis

Create the first synthetic pigment

Difficulty: Easy | Time: 15 minutes | Visual Impact: Very High

Historical Context

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 Herschel. 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.

Materials

  • Ferric chloride - 5g dissolved in 50mL water
  • Potassium ferrocyanide - 5g dissolved in 50mL water (or use Method 2)
  • Alternative: ferrous sulfate - 5g in 50mL water
  • Sodium hydroxide - a few drops of 10% solution (Method 2 only)
  • 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 ferrocyanide solution into ferric chloride while stirring
  4. Intense dark blue precipitate (Prussian blue) forms instantly!

Method 2 (from iron salts):

  1. Mix 50mL ferric chloride solution with 50mL ferrous sulfate solution
  2. Add sodium hydroxide solution drop by drop (5-10 drops)
  3. Blue-green precipitate forms, turning deep blue on standing

Reaction

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

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.

Safety

Ferric chloride solutions are corrosive and stain badly. Wear gloves. Potassium ferrocyanide is not toxic (unlike simple cyanide compounds) - the cyanide is tightly bound to iron.

Resources