chemistry

Potassium ferrocyanide

Potassium ferrocyanide is a chemistry topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Potassium ferrocyanide rather than just read about it. In short: Potassium ferrocyanide is the inorganic compound with formula K4[Fe(CN)6]·3H2O. It is the potassium salt of the coordination complex [Fe(CN)6]4−.

Key takeaways

Potassium ferrocyanide belongs to chemistry; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Potassium ferrocyanide to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Potassium ferrocyanide from memory before moving on to harder problems.

Reference excerpt

Potassium ferrocyanide is the inorganic compound with formula K4[Fe(CN)6]·3H2O. It is the potassium salt of the coordination complex [Fe(CN)6]4−. This salt forms lemon-yellow monoclinic crystals.

Synthesis In 1752, the French chemist Pierre Joseph Macquer (1718–1784) first reported the preparation of potassium ferrocyanide, which he achieved by reacting Prussian blue (iron(III) ferrocyanide) with potassium hydroxide.

Modern production Potassium ferrocyanide is produced industrially from hydrogen cyanide, iron(II) chloride, and calcium hydroxide, the combination of which affords Ca2[Fe(CN)6]·11H2O. This solution is then treated with potassium salts to precipitate the mixed calcium-potassium salt CaK2[Fe(CN)6], which in turn is treated with potassium carbonate to give the tetrapotassium salt.

Historical production Historically, the compound was manufactured from nitrogenous organic material, iron filings, and potassium carbonate. Common nitrogen and carbon sources were torrified horn, leather scrap, offal, or dried blood. It was also obtained commercially from gasworks spent oxide (purification of city gas from hydrogen cyanide).

Chemical reactions Treatment of potassium ferrocyanide with nitric acid gives H2[Fe(NO)(CN)5]. After neutralization of this intermediate with sodium carbonate, red crystals of sodium nitroprusside can be selectively crystallized. Upon treatment with chlorine gas, potassium ferrocyanide converts to potassium ferricyanide:

2 K4[Fe(CN)6] + Cl2 → 2 K3[Fe(CN)6] + 2 KCl This reaction can be used to remove potassium ferrocyanide from a solution. A famous reaction involves treatment with ferric salts, most commonly iron(III) chloride, to give Prussian blue. In the reaction with iron(III) chloride, producing potassium chloride as a side-product: 3 K4[Fe(CN)6] + 4 FeCl3 → Fe4[Fe(CN)6]3 + 12 KCl With the composition FeIII4[FeII(CN)6]3, this insoluble but deeply coloured material is the blue of blueprinting, as well as on many famous paintings such as The Great Wave off Kanagawa and The Starry Night.

Applications Potassium ferrocyanide finds many niche applications in industry. It and the related sodium salt are widely used as anticaking agents for both road salt and table salt. The potassium and sodium ferrocyanides are also used in the purification of tin and the separation of copper from molybdenum ores. Potassium ferrocyanide is used in the production of wine and citric acid. In the EU, potassium ferrocyanides (E 535–538) were, as of 2017, solely authorised in two food categories as salt additives. It can also be used in animal feed. In the laboratory, potassium ferrocyanide is used to determine the concentration of potassium permanganate, a compound often used in titrations based on redox reactions. Potassium ferrocyanide is used in a mixture with potassium ferricyanide and phosphate buffered solution to provide a buffer for beta-galactosidase, which is used to cleave X-Gal, giving a bright blue visualization where an antibody (or other molecule), conjugated to Beta-gal, has bonded to its target. On reacting with Fe(3) it gives a Prussian blue colour. Thus it is used as an identifying reagent for iron in labs. Potassium ferrocyanide can be used as a fertilizer for plants. Prior to 1900, before the invention of the Castner process, potassium ferrocyanide was the most important source of alkali metal cyanides. In this historical process, potassium cyanide was produced by decomposing potassium ferrocyanide:

K4[Fe(CN)6] → 4 KCN + FeC2 + N2 Potassium ferrocyanide is a component of white gunpowder or Augendre's powder, a blasting powder of high energy. It comprises 28 parts yellow potassium ferrocyanide, 23 parts cane sugar, and 49 parts of potassium chlorate, mixed under absolute alcohol to prevent premature detonation. Potassium Ferrocyanide is a main component of the traditional formula for the 19th century Cyanotype photographic process.

Structure Like other metal cyanides, solid potassium ferrocyanide, both as the hydrate and anhydrous salts, has a complicated polymeric structure. The polymer consists of octahedral [Fe(CN)6]4− centers crosslinked with K+ ions that are bound to the CN ligands. The K+---NC linkages break when the solid is dissolved in water.

Toxicity The toxicity in rats is low, with lethal dose (LD50) at 1.6—3.2g/kg. The kidneys are the organ for ferrocyanide toxicity.

See also Ferrocyanide Potassium ferricyanide Ferricyanide

References

External links "Cyanide (inorganic) compounds fact sheet". National Pollutant Inventory Australia. "Potassium Ferrocyanide in Salt Is Entirely Safe To Consume". rediff.com. Photo of large potassium ferrocyanide crystals

Illustrations

Worked examples

Example 1 — a first encounter with Potassium ferrocyanide

Start with the simplest possible case. Write down what Potassium ferrocyanide claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In chemistry, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Potassium ferrocyanide before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Potassium ferrocyanide ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Potassium ferrocyanide

In research: Potassium ferrocyanide appears in chemistry research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Potassium ferrocyanide in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Potassium ferrocyanide is common in secondary-school and first-year university syllabi. It links to neighbouring topics Cyano complexes, E-number additives, Iron(II) compounds, so understanding it makes those chapters shorter.
In everyday life: Look for Potassium ferrocyanide outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Potassium ferrocyanide in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Potassium ferrocyanide means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
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Frequently asked questions

What is Potassium ferrocyanide in simple terms?

Potassium ferrocyanide is the inorganic compound with formula K4[Fe(CN)6]·3H2O. It is the potassium salt of the coordination complex [Fe(CN)6]4−.

Why does Potassium ferrocyanide matter?

Because it connects several chemistry ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Potassium ferrocyanide?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Potassium ferrocyanide.