Periodic acid

 ( Halogen Oxoacids ) 

Periodic acid ( ) is an oxoacid of iodine. It can exist in two forms: orthoperiodic acid, with the chemical formula , and metaperiodic acid, which has the formula . Periodic acids are colourless crystals. Periodic acid features iodine in the highest oxidation state of +7.

Periodic acid was discovered by Heinrich Gustav Magnus and C. F. Ammermüller in 1833.

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Synthesis Modern industrial scale production involves the oxidation of a solution of sodium iodate under Alkali, either on a Lead dioxide anode, or by treatment with chlorine:

(1997). 9780750633659, Butterworth-Heinemann. ISBN 9780750633659

( omitted for clarity) E° = −1.6 V

A standard laboratory preparation involves treating a mixture of tribarium dihydrogen orthoperiodate with nitric acid. Upon concentrating the mixture, the barium nitrate, which is less soluble, is separated from periodic acid:

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Properties Orthoperiodic acid has a number of acid dissociation constants.

founded by A.F. Holleman; continued by Egon Wiberg; translated by Mary Eagleson, William Brewer; revised by Bernhard J. Aylett (2025). 9780123526519, Academic Press, W. de Gruyter.. ISBN 9780123526519

Jean-Louis Burgot (2012). 9781441983824, Springer. ISBN 9781441983824

The p K_a of metaperiodic acid has not been determined.

, p K_a1 = 3.29

, p K_a2 = 8.31

, p K_a3 = 11.60

There being two forms of periodic acid, it follows that two types of periodate salts are formed. For example, sodium periodate, , can be synthesised from while sodium periodate, can be synthesised from .

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Structure Orthoperiodic acid forms monoclinic crystals (space group P2₁/ _n) consisting of a slightly deformed octahedron interlinked via bridging hydrogens. Five I–O bond distances are in the range 1.87–1.91 Å and one I–O bond is 1.78 Å. The structure of metaperiodic acid also includes octahedra, however these are connected via cis-edge-sharing with bridging oxygens to form one-dimensional infinite chains.

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Reactions Orthoperiodic acid can be dehydrated to give metaperiodic acid by heating to 100 °C under reduced pressure.

Further heating to around 150 °C gives iodine pentoxide () rather than the expected anhydride diiodine heptoxide (). Metaperiodic acid can also be prepared from various orthoperiodates by treatment with dilute nitric acid.

Like all periodic acid can be used to cleave various 1,2-difunctional compounds. Most notably periodic acid will cleave vicinal into two aldehyde or ketone fragments (Malaprade reaction).

This can be useful in determining the structure of carbohydrates as periodic acid can be used to open saccharide rings. This process is often used in labeling saccharides with fluorescent molecules or other tags such as biotin. Because the process requires vicinal diols, periodate oxidation is often used to selectively label the 3′-termini of RNA (ribose has vicinal diols) instead of DNA as deoxyribose does not have vicinal diols.

Periodic acid is also used as an oxidising agent of moderate strength, as exemplified in the Babler oxidation of secondary which are oxidised to enones by stoichiometric amounts of orthoperiodic acid with catalytic PCC.

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Other oxyacids Periodic acid is part of a series of in which iodine can assume of −1, +1, +3, +5, or +7. A number of neutral are also known.

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See also Compounds with a similar structure:

• Perchloric acid, perbromic acid, the related perhalogenic acids
• Telluric acid and perxenic acid, the isoelectronic oxoacids of tellurium and xenon

Compounds with similar chemistry:

• Lead tetraacetate (Criegee oxidation)

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