A failed experiment by a Berlin pigment merchant unexpectedly gave birth to the first modern synthetic pigment of mankind - Prussian blue. This deep and rich blue quickly prevailed in the art world, endowing Hokusai's waves and Monet's water lilies with an immortal hue.
However, the true legend of Prussian blue began with its chemical nature. Its unique crystal structure not only endows it with a charming blue color but also conceals the ability to save lives. It possesses the ability to precisely capture lethal toxins such as thallium and cesium like a molecular sponge, becoming the "blue antidote" against heavy metal poisoning and nuclear pollution.
From the halls of art to the frontlines of medicine, from environmental restoration to the energy revolution, this compound, born by a miraculous accident, has continuously pushed the boundaries of human imagination. This article will uncover the chemical code of Prussian blue, exploring how a blue precipitate, in three hundred years, has evolved from a pigment merchant's mistake into a scientific legend that has changed the world.
In the decisive and epic moment in Berlin in 1704, when the pigment manufacturer Johann Jacob Dissbach mixed animal blood, plant ash, and ferric sulfate and heated it, he expected a deep red precipitate - a replica of the highly valuable carmine pigment that was comparable to gold at that time. However, the chemical reaction gave him a very unexpected result. What he saw before his eyes was a deep, rich, and unprecedented blue precipitate. This "failed experiment" was actually the first time in human history that a delicate and exquisite coordination chemical reaction was accomplished unintentionally. Nitrogen-containing organic substances decompose under alkaline high temperature to produce cyanide ions, which combine with iron ions in the solution and eventually self-assemble to form a structurally regular iron cyanide compound. Dissbach did not know that he had just synthesized the world's first modern synthetic pigment - Prussian blue, and this accidental discovery was about to initiate a blue revolution for around three centuries.
The chemical reaction pathway of the original process:
Step 1: Precursor generation (under high-temperature heating conditions)
Animal blood/antler (nitrogen-containing organic matter) + Burnt plant ash (K₂CO₃) → KCN (cyanide) + Other decomposition products
Step 2: Formation of ferrocyanide
KCN + FeSO₄ (sulfate iron) → K₄[Fe(CN)₆] (ferrocyanide potassium)
Step 3: Oxidation and Precipitation (Key Step)
Some Fe²⁺ is oxidized by oxygen in the air: 4Fe²⁺ + O₂ + 4H⁺ → 4Fe³⁺ + 2H₂O
Final precipitation reaction: 4Fe³⁺ + 3[Fe(CN)₆]⁴⁻ → Fe₄[Fe(CN)₆]₃↓ (Prussian blue precipitate)
Explaining and analysing the series of reactions from a modern perspective
1. Reaction vessel: A simple heating pot
2. Temperature conditions: High-temperature heating (approximately 300-400°C)
3. Atmosphere: Partially oxygen-deficient environment
4. Self-assembly process:
The generated cyanide ions (CN⁻) act as linear bridging chelating ligands
Fe²⁺ and Fe³⁺ serve as the metal centers
A face-centered cubic crystal lattice spontaneously forms in the solution
Key chemical points:
1. Formation of mixed oxidation states:
Iron in the reaction system exists simultaneously in +2 and +3 oxidation states
And this is the fundamental reason why Prussian blue presents its characteristic blue color
2. Coordination self-assembly:
Each Fe²⁺ is coordinated with 6 CN⁻, forming an octahedral coordination configuration
Each Fe³⁺ is also coordinated with 6 N atoms
CN⁻ acts as a rigid bridging group: Fe²⁺—C≡N—Fe³⁺
3. Structural template effect:
Potassium ions from the plant ashes play a template role in crystal formation
The pores in the final structure can accommodate water molecules and ions
Although Disbach had no understanding of modern chemical principles at all, his formula happened to meet all the necessary conditions, which was ingenious. Every condition is surprisingly idealized.
Nitrogen source: Animal blood provides organic nitrogen
Alkaline source: Burnt plant ash creates an alkaline environment
Iron source: Ferrous sulfate provides the metal center
Reducing atmosphere: Decomposition of organic matter creates partial anoxic conditions
Oxidizing agent: Oxygen in the air causes some Fe²⁺ to be oxidized to Fe³⁺
Why does Prussian blue appear blue? Scientists have always been dedicated to researching this profound topic of color. The mystery lies in the palace of the three-dimensional network infinite crystal structure, in the interaction between Fe²⁺and Fe³⁺. The principle of its coloration is a classic metal-metal charge transfer.
You can imagine it this way: Fe²⁺acts like a supplier of excess electrons, while the adjacent Fe³⁺behaves like an acceptor eager for electrons. The cyanide ions between them not only provide structural support but also build an energy channel. When a beam of white light shines in, specific energies, namely the photons of red and orange light, are absorbed and used to drive an electron to jump from Fe²⁺ through the cyanide ion bridge to Fe³⁺.
This process occurs instantaneously, and then the electron jumps back to its original position, repeating this cycle. The macroscopic result is that the orange-red light is selectively absorbed, while the remaining light, mainly its complementary color, blue, is reflected or transmitted and enters our eyes. Therefore, the deep blue of Prussian blue is essentially a ceaseless electron transition between two iron atoms that takes place on a picosecond scale.
The most legendary role transformation of Prussian blue is undoubtedly its journey from the artist's palette to the doctor's prescription pad.
When Prussian blue is taken orally, it is not absorbed by the human body but safely passes through the digestive tract. At this point, the potassium ions that were loosely bound in the lattice become exchangeable sites. The lethal thallium ions or radioactive cesium ions, due to their similar ionic radius and charge to potassium ions, are more strongly "locked" into these cavities. Once captured, they are excreted out of the body along with the insoluble Prussian blue carrier through feces, thereby blocking the lethal "enterohepatic circulation".
This has made Prussian blue an effective antidote for acute thallium poisoning and internal contamination by radioactive cesium. After the Chernobyl and Fukushima nuclear accidents, it was widely used to treat affected individuals, reducing the biological half-life of radioactive cesium in the human body from about 110 days to approximately 30 days. An ancient pigment has thus, due to its unique molecular structure, become a life-saving "blue antidote".
Today, the journey of Prussian blue is far from over. Its open framework and adjustable redox properties are breathing new life into it in multiple cutting-edge technological fields. The story of Prussian blue began with a pursuit of red, but by following the fundamental laws of self-assembly of matter and electron transfer, it unexpectedly opened a door to a blue miracle. It started from the canvas, crossed the battlefield of medicine, and finally integrated into the waves of energy, environment, and information technology. This story tells us that great scientific discoveries may originate from chance, but their enduring vitality across centuries must be rooted in a deep understanding and ingenious thinking of the underlying structure of the material world. That blue that settled in the flask three hundred years ago still illuminates the path of human exploration today.