Prussian blue forms instantly when you mix an iron(III) solution with a ferrocyanide solution. The two chemicals react in a 1:1 ratio to produce a deep blue precipitate that you then filter, wash, and dry into a usable pigment. The process is straightforward enough for a home chemistry setup, though it does require some basic safety precautions and careful washing to get a clean, vibrant result.
What Prussian Blue Actually Is
Prussian blue is an iron-based pigment with a crystal lattice structure containing two different forms of iron. One iron atom sits at the center of a cage made of carbon and nitrogen, while a second iron atom links those cages together. This arrangement absorbs red and yellow light intensely, giving the compound its characteristic deep blue color.
There are two forms. “Soluble” Prussian blue has the formula KFe[Fe(CN)₆], meaning potassium ions sit inside the crystal lattice. “Insoluble” Prussian blue, Fe₄[Fe(CN)₆]₃, lacks those potassium ions and has a slightly different structure. The soluble form isn’t truly soluble in water; it forms a colloidal dispersion rather than settling out. Both are the same intense blue. The method below produces the soluble (potassium-containing) form, which is the classic pigment.
What You Need
- Ferric chloride (iron(III) chloride): Available from chemistry suppliers or electronics suppliers, where it’s sold as a circuit board etchant.
- Potassium ferrocyanide: Also called yellow prussiate of potash. Available from chemical suppliers and some photography supply shops.
- Distilled water: Tap water contains minerals that can contaminate the pigment.
- Filter paper and a funnel: Coffee filters work in a pinch, but proper lab filter paper gives a cleaner result. A Büchner funnel with vacuum filtration speeds things up considerably.
- Glass or plastic containers: Ferric chloride corrodes metal, so avoid metal vessels.
- Safety glasses, chemical-resistant gloves, and good ventilation: Potassium ferrocyanide can cause irritation, nausea, and headaches with prolonged exposure. Work in a well-ventilated area or under a fume hood. If you generate dust or mist, wear respiratory protection.
The Synthesis Process
Dissolve a small amount of ferric chloride in distilled water in one container, and dissolve an equal molar amount of potassium ferrocyanide in distilled water in a second container. For a small batch, about 5 grams of each dissolved separately in 50–100 mL of water works well. Both dissolve easily at room temperature.
Slowly pour the ferrocyanide solution into the ferric chloride solution while stirring. A dark blue precipitate forms immediately. This is Prussian blue. The reaction is simple: one iron(III) ion reacts with one ferrocyanide ion, and a potassium ion gets incorporated into the crystal structure. Continue stirring for several minutes to ensure the reaction goes to completion. Adding the ferrocyanide slowly and with constant stirring helps produce finer, more uniform particles, which matters if you want a smooth pigment.
You can also go the other direction: mixing an iron(II) solution (ferrous sulfate, for example) with potassium ferricyanide (red prussiate of potash) produces what was historically called Turnbull’s blue. Modern analysis has confirmed this is chemically identical to Prussian blue.
Filtering and Washing
Once the reaction is complete, you need to separate the blue solid from the liquid and wash away leftover salts. Set up your filter paper in a funnel over a collection vessel. Pour the mixture through the filter, using a spatula or a small stream of distilled water to transfer every bit of blue precipitate from your container onto the filter.
Washing is the step that separates a muddy, dull pigment from a brilliant one. Pour distilled water over the collected precipitate several times, allowing it to drain through each time. This removes dissolved salts (mostly potassium chloride) that would otherwise dry as a white residue mixed into your pigment. Three to five washes typically do the job. You’ll notice the wash water starts clear rather than tinted, which tells you the excess reactants are gone.
After washing, let the precipitate dry. You can leave the filter paper with its blue cake on paper towels in a dry, dust-free area. Drying can take a day or two at room temperature. If you’re in a hurry, a low-temperature oven (around 60°C) speeds things up without damaging the pigment. Once fully dry, scrape the powder off the filter paper and grind it with a mortar and pestle to break up any clumps. The result is a fine, intensely blue powder ready for use as a pigment.
Getting a Better Pigment
The quality of Prussian blue as an artist’s pigment depends heavily on particle size and purity. A few adjustments to the basic process make a noticeable difference.
Using dilute solutions (around 1–2% concentration) and adding them together very slowly produces smaller, more uniform particles. This gives a smoother pigment with better tinting strength. Concentrated solutions dumped together quickly tend to form large, clumpy aggregates that are harder to grind fine.
Temperature also matters. Carrying out the reaction at room temperature rather than with heated solutions generally yields a finer precipitate. Some pigment makers add a small amount of hydrochloric acid to the ferric chloride solution before mixing, which can improve the crystallinity of the final product and make the blue slightly more vivid.
The number of wash cycles directly affects color purity. Historical pigment manufacturers put the raw precipitate through extensive rounds of filtering and washing before considering it a marketable product. If your dried pigment looks dull or has a greenish tinge, insufficient washing is the most likely cause.
Safety Considerations
The word “cyanide” in potassium ferrocyanide understandably raises alarm, but the cyanide groups in this compound are tightly bound to iron and do not release free cyanide under normal conditions. Potassium ferrocyanide has an oral toxicity (LD50) of about 3,600 mg/kg in rats, placing it in a relatively low toxicity category. It’s actually approved as a food additive in some countries for use as an anti-caking agent in table salt.
That said, heating potassium ferrocyanide to decomposition (above about 300°C) can release hydrogen cyanide gas, carbon monoxide, and nitrogen oxides. Never heat the dry powder with a flame or in a high-temperature oven. At the temperatures involved in making and drying the pigment, this is not a concern.
Ferric chloride is corrosive and will stain skin, clothing, and most surfaces a rusty orange-brown. It can cause skin and eye irritation on contact. Wear gloves and eye protection, and clean up spills promptly. Keep it away from metal tools and surfaces.
Prussian blue itself is considered nontoxic. The FDA approved it in 2003 under the brand name Radiogardase as a treatment for thallium and radioactive cesium poisoning. The pigment’s crystal structure has open spaces that trap these toxic metal ions, preventing the body from reabsorbing them in the gut. The approved dose for adults is 3 grams taken three times daily, which gives you a sense of how safe the finished compound is. This medical application only works with pharmaceutical-grade insoluble Prussian blue, not homemade pigment.
Using the Finished Pigment
Prussian blue is compatible with most painting media. For watercolor, mix the dry powder with gum arabic solution. For oil paint, grind it into linseed oil on a glass slab with a muller. It has extremely high tinting strength, so a little goes a long way. Start with small amounts when mixing, as it can easily overpower other colors.
The pigment is lightfast in oil but can fade in strong sunlight when used in watercolor washes, especially in very thin applications. It’s stable in acidic conditions but can decompose in strongly alkaline environments, turning brown. Avoid mixing it with highly alkaline pigments or using it on lime-based surfaces like fresh plaster.
Store the dry powder in a sealed container away from strong light. Prussian blue doesn’t degrade with age under normal storage conditions, so a well-made batch will last indefinitely.