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Yellow Prussiate of Potash is used in the production of pharmaceutical products, in metallurgy, agriculture, in combination with other substances as a paper fungicide, as an anti-caking agent and food additive, and in printing and photography. It is the starting point in the manufacture of almost all cyanogen compounds, and the basis of the ferric ferrocyanate, Prussian Blue (see below), used in paint and the textile dyeing industry.
Prussian Blue. The original name of the pigment - Berlin Blue - came from its use as a dye in the German Army uniforms. Often called the first of the modern or artificial pigments, Prussian blue was introduced in the early part of the nineteenth century. A German dyer in Berlin named Diesbach, accidentally discovered the black-blue of Prussian blue in 1704, thinking that his pigment would be red since it was made from cattle blood. Prussian blue was the earliest of the modern synthetic colors.
C19th recipe for Prussian Blue: Dissolve sulphate of iron (copperas, green vitriol) in water and boil the solution. Add nitric acid until red fumes cease to come off, and enough sulphuric acid to render the liquor clear. This is the persulphate of iron. To this add a solution of ferrocyanide of potassium (yellow prussiate of potash), as long as any precipitate is produced. Wash this precipitate thoroughly with water acidulated with sulphuric acid, and dry in a warm place].
Prussian blue (Preußisch Blau, Berliner Blau) is a dark blue pigment used in paints and formerly in blueprints. It has several different chemical names, these being iron(III) ferrocyanide, ferric ferrocyanide, iron(III) hexacyanoferrate(II), and ferric hexacyanoferrate. Prussian blue is a strong color, and tends towards black and dark purple when mixed with other oil paints.
The chemical formula is Fe4[Fe(CN)6]3.
The intense blue color of Prussian blue is caused by the transfer of electrons from one iron atom to another within the molecule. Red light at 680 nm is absorbed, causing an electron to transfer from an Fe(II) atom to a neighboring Fe(III) atom. The transmitted light looks blue as a result.
Despite the presence of the cyanide ion, prussian blue, like other ferro- and ferri-cyanides, is not especially toxic due to the strong binding between the cyanide and ferrous or ferric (iron) ions. However, treatment with acids can liberate the cyanide in the form of hydrogen cyanide, which is extremely toxic. In fact, the toxicity of cyanide, like carbon monoxide, is due to its forming strong complexes with the iron in certain vital compounds in the body such as hemoglobin. Because cyanoferrates already contain cyanide bound to iron, they are not especially toxic.
Suspended as a colloid in water, Prussian blue is the basis for laundry bluing.
As engineer's blue it is mixed with an oily material, and rubbed onto a metallic surface. This is then rubbed with another surface, and the removal of the pigment indicates the position of high-spots. Thus it can be used to indicate the flatness of a surface or the trueness of a bearing assembly.
Joseph Whitworth popularized the first practical method of making accurate flat surfaces, during the 1830s, by using engineer's blue and scraping techniques on three trial surfaces. Up until his introduction of the scraping technique, the same three plate method was employed using polishing techniques, giving less accurate results. This led to an explosion of development of precision instruments using these flat surface generation techniques as a basis for further construction of precise shapes.
Prussian blue may also be mixed with methylated spirits to create a quick drying stain. This stain is used in the marking out operation in metalworking, to transfer design layouts to the workpiece prior to machining.
The role of a Prussian blue test to detect levels of cyanide was a key component of the Errol Morris film Mr. Death: The Rise and Fall of Fred A. Leuchter, Jr.. The formation of Prussian blue is an example of a "wet" chemical test for cyanide.
Prussian blue is the pigment formed on cyanotypes, giving them their name "blueprint".
Prussian blue can act as a chelating agent, and is used as a treatment for heavy metals poisoning. In particular, pharmaceutical Prussian blue (not artists' pigment!) is used for patients who have ingested radioactive caesium or thallium (also non-radioactive thallium). According to the IAEA an adult man can eat 10 grams per day without serious harm.