Prussian blue

Prussian blue is a very dark blue pigment made from a special chemical reaction. It was first created in the early 1700s and was the first blue color made by people to use in art and painting. Because it mixes well with water, it became popular for painting, drawing, and making blue prints for architects and engineers.

Besides art, Prussian blue is also important in medicine. If someone swallows certain dangerous metals by accident, doctors can give them Prussian blue to help take those metals out of the body. This can be a lifesaving treatment and it is one of the most important medicines in the world.

The blue color comes from tiny particles in the paint, and even small changes in these particles can change how the blue looks. Artists in Japan used Prussian blue a lot in the 1800s for their colorful woodblock prints, making it a favorite color for many years.

History

Prussian blue is very important because it was the first lasting and strong blue paint used widely since people lost how to make Egyptian blue. Before this, artists in Europe used paints like indigo dye, smalt, and Tyrian purple, as well as very expensive ultramarine from lapis lazuli. Artists in Japan and those making woodblock prints also did not have a blue paint that lasted well until they started bringing Prussian blue from Europe.

Prussian blue was first made by a paint maker named Johann Jacob Diesbach in Berlin around 1706. It happened by accident when he used potash that had some blood in it to try to make a red dye from cochineal. Instead of red, the blood, potash, and iron created a blue compound. This blue paint quickly replaced the costly ultramarine and became famous. Letters between Johann Leonhard Frisch and Gottfried Wilhelm Leibniz from 1708 to 1716 talk about this new blue paint. The oldest known painting using Prussian blue is from 1709.

Military symbol

Starting in the 1700s, Prussian blue was the main color for uniforms in the Prussian Army. Known as Dunkelblau (dark blue), this color became very symbolic and was worn by most German soldiers for special events and when not fighting until World War I. After that, they started wearing a greenish-gray color instead.

Synthesis

Prussian blue is made by changing special white solids that contain iron and carbon. These solids have a chemical makeup of M₂Fe[Fe(CN)₆], where M can be Na⁺ or K⁺. When this white solid is changed using hydrogen peroxide or sodium chlorate, it creates Prussian blue.

There is also a type of Prussian blue that can dissolve in water. This is made by mixing potassium ferrocyanide with iron(III). Another way is to use potassium ferricyanide and iron(II), which also makes the same solution. When extra iron(III) is added, it makes the blue color that does not dissolve. Even though it is made from chemicals that include cyanide, Prussian blue is not harmful because the cyanide is tightly held by the iron.

Turnbull's blue

In the past, mixing iron(II) with ferricyanide was thought to make a different blue color called Turnbull's blue. But we now know that Turnbull's blue and Prussian blue are the same. The small differences in color come from how they are made.

Prussian white

Prussian white, also called Berlin white or Everett's salt, is made when all the iron in Prussian blue is in the form of Fe(II). It is a sodium hexacyanoferrate and has the formula Na₂Fe[Fe(CN)₆]. Prussian white looks and behaves differently from Prussian blue. It is being studied for use in batteries.

Properties

Prussian blue is a very fine blue powder that does not dissolve in water. Even though it doesn’t dissolve, tiny pieces of it can move through very fine filters. This blue color comes from a special mix of iron and other elements.

The exact way Prussian blue is made affects its look. Its strong blue color happens because of how its tiny parts share energy. When mixed into paints, it can look very dark blue or even black, depending on how it is prepared. The blue color can even change to colorless under certain conditions, losing the special energy sharing that creates the blue look.

Use

Prussian blue is a dark blue color that is easy to make, cheap, and safe. People have used it for a long time in paints, watercolors, and dyes. It is also used to make black and blue inks, and it is found in some crayons and laundry products.

In medicine, a special form of Prussian blue helps remove certain harmful metals from the body. It is used when someone has swallowed dangerous metals. It can also be used to show if there is iron in samples taken from the body during tests.

Machinists use Prussian blue to mark metal pieces so they can be shaped correctly. It is also used in some tests to measure certain chemicals in a lab.

Research

Battery materials

Prussian blue has been studied since 1978 for use in batteries. It can store and release electrical energy in solutions with certain metals, especially potassium. This works well because potassium fits nicely into the structure of Prussian blue. Other metals like sodium and lithium are too big and don’t work as well.

Scientists have also tried replacing the iron in Prussian blue with other metals such as manganese, cobalt, nickel, and zinc. These changed versions can be used in lithium-ion, sodium-ion, and potassium-ion batteries to help store energy.