Periodic acid

Synthesis

Modern industrial scale production involves the electrochemical oxidation of iodic acid, on a PbO₂ anode, 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 ⇌ H
4IO−
6 + H⁺,      pK_a = 3.29H
4IO−
6 ⇌ H
3IO2−
6 + H⁺,      pK_a = 8.31H
3IO2−
6 ⇌ H
2IO3−
6 + H⁺,      pK_a = 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 IO₆ octahedron 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.

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.