Periodic Acid Oxidation

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Periodic acid is the highest oxoacid of iodine, in which the iodine exists in oxidation state VII. Like all periodates it can exist in two forms: orthoperiodic acid, with the chemical formula H₅IO₆ and metaperiodic acid, which has the formula HIO₄. Periodic acid was discovered by Heinrich Gustav Magnus and C. F. Ammermüller in 1833.

Orthoperiodic Acid Metaperiodic Acid

By NEUROtiker (Own work) [Public domain], via Wikimedia Commons

By Benjah-bmm27 (Own work) [Public domain], via Wikimedia Commons

Synthesis

Modern industrial scale production involves the electrochemical oxidation of iodic acid, on a PbO2anode, with the following standard electrode potential:

H₅IO₆ + H⁺ + 2 e⁻ → IO⁻³ + 3 H₂O E° = 1.6 V

Orthoperiodic acid can be dehydrated to give metaperiodic acid by heating to 100 °C

HIO₄ + 2 H₂O ⇌ H₅IO₆

Further heating to around 150 °C gives iodine pentoxide (I₂O₅) rather than the expected anhydride diiodine heptoxide (I₂O₇). Metaperiodic acid can also be prepared by from various orthoperiodates by treatment with dilute nitric acid.

H₅IO₆ → HIO₄ + 2 H₂O

Properties

Orthoperiodic acid has a number of acid dissociation constants.The pKa of meta periodic acid has not been determined.

H₅IO₆ ⇌ H₄IO⁻₆ + H⁺, pKa = 3.29

H₄IO⁻₆ ⇌ H₃IO²⁻₆ + H⁺, pKa = 8.31

H₃IO²⁻₆ ⇌ H₂IO³⁻₆ + H⁺, pKa = 11.60

There being two forms of periodic acid, it follows that two types of periodate salts are formed. For example, sodium metaperiodate, NaIO₄, can be synthesised from HIO₄ while sodium orthoperiodate, Na₅IO₆ can be synthesised from H₅IO₆.

Structure

Orthoperiodic acid forms monoclinic crystals (space group P2₁_/n) consisting of slightly deformed IO₆octahedron interlinked via bridging hydrogens. The crystal structure of metaperiodic acid also includes IO₆ octahedron, however these are connected via cis-edge-sharing with bridging oxygens to form one-dimensional infinite chains.

Reactions

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

By Project Osprey (Own work) [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons

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 in as an oxidising agent of moderate strength.

Other oxyacids

Periodate is part of a series of oxyacids in which iodine can assume oxidation states of −1, +1, +3, +5, or +7. A number of neutral iodine oxides are also known.

Iodine Oxidation State	-1	+1	+3	+5	+7
Name	Hydrogen iodide	Hypoiodous acid	Iodous acid	Iodic acid	Periodic acid
Formula	HI	HIO	HIO₂	HIO₃	HIO₄or H₅IO₅

References

Aylett, founded by A.F. Holleman ; continued by Egon Wiberg ; translated by Mary Eagleson, William Brewer ; revised by Bernhard J. (2001). Inorganic chemistry (1st English ed., [edited] by Nils Wiberg. ed.). San Diego, Calif. : Berlin: Academic Press, W. de Gruyter. p. 453. ISBN 0123526515.
Ammermüller, F.; Magnus, G. (1833). "Ueber eine neue Verbindung des Jods mit Sauerstoff, die Ueberjodsäure". Annalen der Physik und Chemie (in German). 104 (7): 514–525. doi:10.1002/andp.18331040709.
Parsons, Roger (1959). Handbook of electrochemical constants. Butterworths Scientific Publications Ltd. p. 71.
Riley, edited by Georg Brauer; translated by Scripta Technica, Inc. Translation editor Reed F. (1963). Handbook of preparative inorganic chemistry. Volume 1 (2nd ed.). New York, N.Y.: Academic Press. pp. 323–324. ISBN 012126601X.
Aylett, founded by A.F. Holleman; continued by Egon Wiberg; translated by Mary Eagleson, William Brewer; revised by Bernhard J. (2001). Inorganic chemistry (1st English ed., [edited] by Nils Wiberg. ed.). San Diego, Calif. : Berlin: Academic Press, W. de Gruyter. p. 454. ISBN 0123526515.
Burgot, Jean-Louis. Ionic equilibria in analytical chemistry. New York: Springer. p. 358. ISBN 1441983821.
Feikema, Y. D. (10 June 1966). "The crystal structures of two oxy-acids of iodine. I. A study of orthoperiodic acid, H5IO6, by neutron diffraction". Acta Crystallographica. 20 (6): 765–769. doi:10.1107/S0365110X66001828.
Kraft, Thorsten; Jansen, Martin (1 September 1997). "Crystal Structure Determination of Metaperiodic Acid, HIO4, with Combined X-Ray and Neutron Diffraction". Angewandte Chemie International Edition in English. 36 (16): 1753–1754. doi:10.1002/anie.199717531

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