Potassium ferrocyanide

<h2 id="synthesis">Synthesis</h2>
In 1752, the French chemist <a href="/facts/Pierre_Macquer/x8NqwVtQ">Pierre Joseph Macquer</a> (1718–1784) first reported the preparation of Potassium hexacyanidoferrate(II), which he achieved by reacting <a href="/facts/Prussian_blue/IV0VooPH">Prussian blue</a> (iron(III) ferrocyanide) with <a href="/facts/Potassium_hydroxide/NBhPBuXc">potassium hydroxide</a>.<a class="footnote-ref" id="fnref:1" href="#fn:1">1</a><a class="footnote-ref" id="fnref:2" href="#fn:2">2</a>

<h3>Modern production</h3>
Potassium hexacyanidoferrate(II) is produced industrially from <a href="/facts/Hydrogen_cyanide/vNtA0Mxe">hydrogen cyanide</a>, <a href="/facts/Iron(II)_chloride/FNFq9GRl">iron(II) chloride</a>, and <a href="/facts/Calcium_hydroxide/Kz2KzRd8">calcium hydroxide</a>, 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 <a href="/facts/Potassium_carbonate/nLvuwSaT">potassium carbonate</a> to give the tetrapotassium salt.<a class="footnote-ref" id="fnref:3" href="#fn:3">3</a>

<h3>Historical production</h3>
Historically, the compound was manufactured from nitrogenous organic material, iron filings, and potassium carbonate.<a class="footnote-ref" id="fnref:4" href="#fn:4">4</a> Common <a href="/facts/Nitrogen/Nf1QpGDz">nitrogen</a> and <a href="/facts/Carbon/ukrgQexo">carbon</a> sources were <a href="/facts/Torrefaction/sGrvPK0w">torrified</a> horn, leather scrap, <a href="/facts/Offal/obWrHwDK">offal</a>, or dried blood. It was also obtained commercially from gasworks spent oxide (purification of city gas from hydrogen cyanide).

<h2 id="chemical-reactions">Chemical reactions</h2>
Treatment of potassium hexacyanidoferrate(II) with <a href="/facts/Nitric_acid/UDbwGp6s">nitric acid</a> gives H2[Fe(NO)(CN)5]. After neutralization of this intermediate with <a href="/facts/Sodium_carbonate/P6RHvIoK">sodium carbonate</a>, red crystals of <a href="/facts/Sodium_nitroprusside/EDgrE8E2">sodium nitroprusside</a> can be selectively crystallized.<a class="footnote-ref" id="fnref:5" href="#fn:5">5</a>
Upon treatment with <a href="/facts/Chlorine/y27UwYBM">chlorine</a> gas, potassium hexacyanidoferrate(II) converts to <a href="/facts/Potassium_hexacyanidoferrate(III)/czXTV4hd">potassium hexacyanidoferrate(III)</a>:

2 K4[Fe(CN)6] + Cl2 → 2 K3[Fe(CN)6] + 2 KCl
This reaction can be used to remove potassium hexacyanidoferrate(II) from a solution.
A famous reaction involves treatment with ferric salts, most commonly <a href="/facts/Iron(III)_chloride/a51pulq1">Iron(III) chloride</a>, to give <a href="/facts/Prussian_blue/IV0VooPH">Prussian blue</a>. In the reaction with Iron(III) chloride, producing <a href="/facts/Potassium_chloride/RgT2MdPa">Potassium chloride</a> as a side-product: 
3 K4[Fe(CN)6] + 4 FeCl3 → Fe4[Fe(CN)6]3 + 12 KCl
With the composition FeIII4[FeII(<a href="/facts/Cyanide/t0o8FMKI">CN</a>)6]3, this insoluble but deeply coloured material is the blue of <a href="/facts/Blueprint/wyJtXqKz">blueprinting</a>, as well as on many famous paintings such as <a href="/facts/The_Great_Wave_off_Kanagawa/cacLxPDe">The Great Wave off Kanagawa</a> and <a href="/facts/The_Starry_Night/uH7aM9Nb">The Starry Night</a>.

<h2 id="applications">Applications</h2>
Potassium hexacyanidoferrate(II) finds many niche applications in industry. It and <a href="/facts/Sodium_ferrocyanide/HDBVu6ME">the related sodium salt</a> are widely used as anticaking agents for both road salt and table salt. The potassium and sodium hexacyanidoferrates(II) are also used in the purification of tin and the separation of copper from molybdenum ores. Potassium hexacyanidoferrate(II) is used in the production of wine and citric acid.<a class="footnote-ref" id="fnref:6" href="#fn:6">6</a>
In the EU, hexacyanidoferrates(II) (E 535–538) were, as of 2017, solely authorised in two food categories as salt additives.
It can also be used in animal feed.<a class="footnote-ref" id="fnref:7" href="#fn:7">7</a>
In the laboratory, potassium hexacyanidoferrate(II) is used to determine the concentration of <a href="/facts/Potassium_permanganate/nyjjuDlo">potassium permanganate</a>, a compound often used in <a href="/facts/Titration/gGUfvQcW">titrations</a> based on <a href="/facts/Redox/Pyd5RVcj">redox</a> reactions. Potassium hexacyanidoferrate(II) is used in a mixture with potassium ferricyanide and phosphate buffered solution to provide a buffer for <a href="/facts/Beta-galactosidase/fgP6xr9n">beta-galactosidase</a>, which is used to cleave <a href="/facts/X-Gal/mcfaArBG">X-Gal</a>, 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 hexacyanidoferrate(II) can be used as a fertilizer for plants.<a class="footnote-ref" id="fnref:8" href="#fn:8">8</a>
Prior to 1900, before the invention of the <a href="/facts/Castner_process/Q1xtGaMv">Castner process</a>, potassium hexacyanidoferrate(II) was the most important source of <a href="/facts/Alkali_metal/IkWbCLk2">alkali metal</a> <a href="/facts/Sodium_cyanide/CQfl206o">cyanides</a>.<a class="footnote-ref" id="fnref:9" href="#fn:9">9</a> In this historical process, <a href="/facts/Potassium_cyanide/xcoXn4I8">potassium cyanide</a> was produced by decomposing potassium hexacyanidoferrate(II):<a class="footnote-ref" id="fnref:10" href="#fn:10">10</a>
K4[Fe(CN)6] → 4 KCN + FeC2 + N2
Potassium ferrocyanide is a component of white gunpowder or Augendre's powder, a <a href="/facts/Blasting_powder/AEcB6HCR">blasting powder</a> of high energy.<a class="footnote-ref" id="fnref:11" href="#fn:11">11</a> It comprises 28 parts yellow potassium ferrocyanide, 23 parts cane sugar, and 49 parts of <a href="/facts/Potassium_chlorate/rARLS99c">potassium chlorate</a>, mixed under <a href="/facts/Absolute_alcohol/3lCLfBSP">absolute alcohol</a> to prevent premature detonation.<a class="footnote-ref" id="fnref:12" href="#fn:12">12</a><a class="footnote-ref" id="fnref:13" href="#fn:13">13</a>

<h2 id="structure">Structure</h2>
Like other metal cyanides, solid potassium hexacyanidoferrate(II), 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 <a href="/facts/Ligand/6etB3qeW">ligands</a>.<a class="footnote-ref" id="fnref:14" href="#fn:14">14</a> The K+---NC linkages break when the solid is dissolved in water.

<h3>Toxicity</h3>
The toxicity in rats is low, with <a href="/facts/Lethal_dose/Q2IDEBaS">lethal dose</a> (LD50) at 1.6—3.2g/kg.<a class="footnote-ref" id="fnref:15" href="#fn:15">15</a> The kidneys are the organ for ferrocyanide toxicity.<a class="footnote-ref" id="fnref:16" href="#fn:16">16</a>

<h2 id="see-also">See also</h2>
<ul><li><a href="/facts/Ferrocyanide/ccDHxjNN">Ferrocyanide</a></li>
<li><a href="/facts/Potassium_hexacyanidoferrate(III)/czXTV4hd">Potassium hexacyanidoferrate(III)</a><a class="footnote-ref" id="fnref:17" href="#fn:17">17</a></li>
<li><a href="/facts/Ferricyanide/JLsWWvvV">Ferricyanide</a></li></ul>

<h2 id="external-links">External links</h2>
<ul><li><a href="http://www.npi.gov.au/substances/cyanide/index.html">"Cyanide (inorganic) compounds fact sheet"</a>. National Pollutant Inventory Australia.</li>
<li><a href="http://blogs.rediff.com/thinko/2019/07/31/customers-can-rely-on-tata-salt-as-report-says-it-is-safe-to-consume/">"Potassium Ferrocyanide in Salt Is Entirely Safe To Consume"</a>. rediff.com.[permanent dead link‍]</li>
<li><a href="https://labphoto.tumblr.com/post/107528318273/a-large-crystal-of-potassium-ferrocyanide">Photo</a> of large potassium ferrocyanide crystals</li></ul>

<h2 id="references">References</h2>

<ol>
<li id="fn:1">Macquer (1752). "Éxamen chymique de bleu de Prusse" [Chemical examination of Prussian blue]. Histoire de l'Académie Royale des Sciences …, § Mémoires de l'Académie royale des Sciences (in French): 60–77. 
 From pp. 63-64: "Après avoir essayé ainsi inutilement de décomposer le bleu de Prusse par les acides, … n'avoit plus qu'une couleur jaune un peu rousse." (After having tried so vainly to decompose Prussian blue by acids, I made recourse to alkalies. I put a half ounce of this [Prussian] blue in a flask, and I poured on it ten ounces of a solution of nitre fixed by tartar [i.e., potassium nitrate (nitre) which is mixed with crude cream of tartar and then ignited, producing potassium carbonate]. As soon as these two substances had been mixed together, I saw with astonishment that, without the aid of heat, the blue color had entirely disappeared; the powder [i.e., precipitate] at the bottom of the flask had only a rather gray color: having put this vessel on a sand bath in order to heat the solution until it simmered, this gray color also disappeared entirely, and all that was contained in the flask, both the powder [i.e., precipitate] and the solution, had only a yellow color [that was] a little red.) <a href="https://www.biodiversitylibrary.org/item/88112#page/250/mode/1up" target="_blank">https://www.biodiversitylibrary.org/item/88112#page/250/mode/1up</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></li>
<li id="fn:2">Munroe, Charles E.; Chatard, Thomas M. (1902). "Manufactures: Chemicals and Allied Products". Twelfth Census of the United States: Bulletins (210): 1–306.; see p. 31. <a href="https://books.google.com/books?id=8UIUAQAAMAAJ&pg=RA1-PA31" target="_blank">https://books.google.com/books?id=8UIUAQAAMAAJ&pg=RA1-PA31</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></li>
<li id="fn:3">Gail, E.; Gos, S.; Kulzer, R.; Lorösch, J.; Rubo, A.; Sauer, M.; Kellens, R.; Reddy, J.; Steier, N.; Hasenpusch, W. (October 2011). "Cyano Compounds, Inorganic". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. doi:10.1002/14356007.a08_159.pub3. ISBN 978-3527306732. <a href="978-3527306732" target="_blank">978-3527306732</a> <a href="#fnref:3" class="footnote-back-ref">↩</a></li>
<li id="fn:4">Von Wagner, Rudolf (1897). Manual of chemical technology. New York: D. Appleton & Co. p. 474 & 477. <a href="https://archive.org/details/manualofchemical00wagnuoft" target="_blank">https://archive.org/details/manualofchemical00wagnuoft</a> <a href="#fnref:4" class="footnote-back-ref">↩</a></li>
<li id="fn:5">Seel, F. (1965). "Sodium nitrosyl cyanoferrate". In Brauer, G. (ed.). Handbook of Preparative Inorganic Chemistry. Vol. 2 (2nd ed.). New York: Academic Press. p. 1768. LCCN 63-14307. Archived from the original on 2010-03-07. Retrieved 2017-09-10. <a href="https://web.archive.org/web/20100307011411/http://www.scribd.com/doc/27443280/Handbook-of-Preparative-Inorganic-Chemistry-Vol-2-2d-Ed-George-Brauer" target="_blank">https://web.archive.org/web/20100307011411/http://www.scribd.com/doc/27443280/Handbook-of-Preparative-Inorganic-Chemistry-Vol-2-2d-Ed-George-Brauer</a> <a href="#fnref:5" class="footnote-back-ref">↩</a></li>
<li id="fn:6">Gail, E.; Gos, S.; Kulzer, R.; Lorösch, J.; Rubo, A.; Sauer, M.; Kellens, R.; Reddy, J.; Steier, N.; Hasenpusch, W. (October 2011). "Cyano Compounds, Inorganic". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. doi:10.1002/14356007.a08_159.pub3. ISBN 978-3527306732. <a href="978-3527306732" target="_blank">978-3527306732</a> <a href="#fnref:6" class="footnote-back-ref">↩</a></li>
<li id="fn:7">"EuSalt Expert Meeting on E 535 and E 536 as Feed Additives". EUSalt. Archived from the original on 2019-05-12. Retrieved 2018-12-06. <a href="https://web.archive.org/web/20190512013808/https://eusalt.com/events/eusalt-expert-meeting-e-535-and-e-536-feed-additives" target="_blank">https://web.archive.org/web/20190512013808/https://eusalt.com/events/eusalt-expert-meeting-e-535-and-e-536-feed-additives</a> <a href="#fnref:7" class="footnote-back-ref">↩</a></li>
<li id="fn:8">Kopras, Elizabeth J. (2012). Bingham, Eula; Cohrssen, Barbara (eds.). Patty's Toxicology. Vol. 1 (6 ed.). Hoboken, NJ: Wiley. p. 990. ISBN 978-0-470-41081-3. <a href="978-0-470-41081-3" target="_blank">978-0-470-41081-3</a> <a href="#fnref:8" class="footnote-back-ref">↩</a></li>
<li id="fn:9">Gail, E.; Gos, S.; Kulzer, R.; Lorösch, J.; Rubo, A.; Sauer, M.; Kellens, R.; Reddy, J.; Steier, N.; Hasenpusch, W. (October 2011). "Cyano Compounds, Inorganic". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. doi:10.1002/14356007.a08_159.pub3. ISBN 978-3527306732. <a href="978-3527306732" target="_blank">978-3527306732</a> <a href="#fnref:9" class="footnote-back-ref">↩</a></li>
<li id="fn:10">Von Wagner, Rudolf (1897). Manual of chemical technology. New York: D. Appleton & Co. p. 474 & 477. <a href="https://archive.org/details/manualofchemical00wagnuoft" target="_blank">https://archive.org/details/manualofchemical00wagnuoft</a> <a href="#fnref:10" class="footnote-back-ref">↩</a></li>
<li id="fn:11">Fay, Albert (1918). A Glossary of the Mining and Mineral Industry (1949 ed.). Washington, DC: United States Bureau of Mines. p. 741. <a href="/wiki/United_States_Bureau_of_Mines" target="_blank">/wiki/United_States_Bureau_of_Mines</a> <a href="#fnref:11" class="footnote-back-ref">↩</a></li>
<li id="fn:12">Chemischen Technologie [A Handbook of Chemical Technolgie] (in German). Translated by Crookes, Willliam (1872 ed.). New York: D. Appleton & Company. p. 155. <a href="/wiki/William_Crookes" target="_blank">/wiki/William_Crookes</a> <a href="#fnref:12" class="footnote-back-ref">↩</a></li>
<li id="fn:13">Brannt, William T. (William Theodore); Wahl, William H. (William Henry) (1919). The Techno-chemical receipt book. New York: Henry Carey Baird & Co. p. 30. <a href="https://archive.org/details/technochemicalre00bran/page/30/mode/1up" target="_blank">https://archive.org/details/technochemicalre00bran/page/30/mode/1up</a> <a href="#fnref:13" class="footnote-back-ref">↩</a></li>
<li id="fn:14">Willans, Mathew J.; Wasylishen, Roderick E.; McDonald, Robert (2009-05-18). "Polymorphism of Potassium Ferrocyanide Trihydrate as Studied by Solid-State Multinuclear NMR Spectroscopy and X-ray Diffraction". Inorganic Chemistry. 48 (10): 4342–4353. doi:10.1021/ic802134j. ISSN 0020-1669. PMID 19425611. <a href="https://pubs.acs.org/doi/10.1021/ic802134j" target="_blank">https://pubs.acs.org/doi/10.1021/ic802134j</a> <a href="#fnref:14" class="footnote-back-ref">↩</a></li>
<li id="fn:15">Kopras, Elizabeth J. (2012). Bingham, Eula; Cohrssen, Barbara (eds.). Patty's Toxicology. Vol. 1 (6 ed.). Hoboken, NJ: Wiley. p. 990. ISBN 978-0-470-41081-3. <a href="978-0-470-41081-3" target="_blank">978-0-470-41081-3</a> <a href="#fnref:15" class="footnote-back-ref">↩</a></li>
<li id="fn:16">Peter Aggett, Fernando Aguilar, Riccardo Crebelli, Birgit Dusemund, Metka Filipič, Maria Jose Frutos, Pierre Galtier, David Gott, Ursula Gundert-Remy, Gunter Georg Kuhnle, Claude Lambré, Jean-Charles Leblanc, Inger Therese Lillegaard, Peter Moldeus, Alicja Mortensen, Agneta Oskarsson, Ivan Stankovic, Ine Waalkens-Berendsen, Rudolf Antonius Woutersen, Matthew Wright and Maged Younes. (2018). "Re-evaluation of sodium ferrocyanide (E 535), potassium ferrocyanide (E 536) and calcium ferrocyanide (E 538) as food additives". EFSA Journal. 16 (7): 5374. doi:10.2903/j.efsa.2018.5374. PMC 7009536. PMID 32626000.{{cite journal}}: CS1 maint: multiple names: authors list (link) <a href="https://www.efsa.europa.eu/en/efsajournal/pub/5374" target="_blank">https://www.efsa.europa.eu/en/efsajournal/pub/5374</a> <a href="#fnref:16" class="footnote-back-ref">↩</a></li>
<li id="fn:17">Kosugi, Nobuhiro; Yokoyama, Toshihiko; Kuroda, Haruo (May 1986). "Polarization dependence of XANES of square-planar Ni(CN)2−4 ion. A comparison with octahedral Fe(CN)4−6 and Fe(CN)3−6 ions". Chemical Physics. 104 (3): 449–453. doi:10.1016/0301-0104(86)85034-0. ISSN 0301-0104. <a href="https://dx.doi.org/10.1016/0301-0104(86)85034-0" target="_blank">https://dx.doi.org/10.1016/0301-0104(86)85034-0</a> <a href="#fnref:17" class="footnote-back-ref">↩</a></li>
</ol>

Potassium ferrocyanide open-in-new

Potassium ferrocyanide