Prussian blue is a deep blue pigment made from iron and cyanide compounds, with the molecular formula C₁₈Fe₇N₁₈. It was the first modern synthetic pigment, accidentally created around 1706, and has since found uses far beyond art. Today it serves as an FDA-approved medication for radiation poisoning and a promising material in next-generation battery technology.
How It Was Accidentally Discovered
Prussian blue was born from a mistake in a Berlin laboratory around 1706. A Swiss pigment maker named Johann Jacob Diesbach was trying to produce Florentine lake, a red pigment made from cochineal (a dye extracted from insects). The process required potash, a potassium-based salt, but Diesbach had run out. He borrowed some from Johann Konrad Dippel, an alchemist who shared the lab and had been using potash in his own experiments with animal oils.
Dippel’s potash was contaminated with a compound called hexacyanoferrate, a byproduct of his work. When Diesbach mixed this tainted potash into his solution, which already contained iron sulfate, the chemical reaction produced a vivid blue precipitate instead of red. The result was Prussian blue, originally called “Caeruleum Berolinense” (Berlin blue). Within a few decades, the pigment spread across Europe and became a staple in painting, textile dyeing, and ink production.
What It’s Made Of
Prussian blue is an iron-based compound where iron atoms exist in two different states: some carry a +2 charge and others carry a +3 charge. These iron atoms are linked together by cyanide groups (carbon bonded to nitrogen), forming a rigid, cage-like crystal structure. This structure is what gives the pigment its intense blue color and remarkable stability.
A common concern is whether the cyanide in Prussian blue makes it dangerous. It doesn’t. The cyanide groups are locked tightly to the iron atoms in a bond so strong that they don’t release free cyanide into the body. Toxicological studies in both animals and humans have found no adverse health effects from Prussian blue compounds, which is why it’s considered safe enough for medical use.
An FDA-Approved Treatment for Poisoning
In 2003, the FDA approved Prussian blue under the brand name Radiogardase as a treatment for internal contamination with radioactive cesium-137 or thallium (both radioactive and non-radioactive forms). These are substances that can enter the body after a nuclear accident, a dirty bomb, or deliberate poisoning.
The way it works is elegantly simple. Cesium and thallium, once ingested or inhaled, get absorbed into the bloodstream and eventually pass into the intestines through bile. Normally, the intestines reabsorb these substances back into the body in a continuous loop, which is why they linger for so long and cause damage. Prussian blue’s cage-like crystal structure acts as a trap: it binds cesium and thallium ions inside the gut and holds onto them, preventing reabsorption. The trapped contaminants then pass out of the body in bowel movements.
The standard adult dose is 3 grams taken three times daily (9 grams total per day), while children ages 2 to 12 take 1 gram three times daily. One visible side effect: because Prussian blue is an intensely colored pigment, it turns stool blue for the duration of treatment.
Use in Art and Industry
For centuries, Prussian blue’s primary identity was as a pigment. Before its discovery, blue pigments were rare and expensive. Ultramarine, derived from lapis lazuli, cost more than gold. Prussian blue offered a vivid, stable blue at a fraction of the price, and it quickly became one of the most widely used pigments in European art. It appears in paintings by Hokusai, Gainsborough, and Picasso. It’s also the blue in blueprints, which get their name from the Prussian blue-based cyanotype printing process developed in the 1840s.
As a pigment, Prussian blue is valued for its high tinting strength, meaning a small amount produces a deep color. It’s lightfast, resistant to acids, and works well in oil paints, watercolors, and inks. It remains in use today in fine art supplies, though synthetic alternatives have replaced it in some industrial applications.
Prussian Blue in Battery Technology
The same cage-like crystal structure that traps cesium in the gut turns out to be excellent at storing and releasing charged ions in batteries. Prussian blue analogues, compounds that share its basic framework but swap in different metals, have emerged as leading candidates for cathode materials in sodium-ion and potassium-ion batteries. These battery types are being developed as cheaper, more abundant alternatives to lithium-ion technology.
The appeal is practical: Prussian blue analogues are cheap to synthesize, structurally robust, and offer solid electrochemical performance. Depending on which metals are used in the framework, these materials can achieve theoretical energy capacities of 150 to 170 milliamp-hours per gram. In laboratory tests, some versions have demonstrated stability over 10,000 charge-discharge cycles and can operate across a temperature range of -10°C to 50°C. Researchers have also developed scalable manufacturing methods, with one team producing kilogram quantities of a potassium-manganese variant in just 10 minutes using a water-free process.
The technology is still moving from lab scale to commercial production, but Prussian blue’s low cost and the abundance of sodium (compared to lithium) make it a strong contender for grid-scale energy storage and affordable electric vehicles.