# Periodic acid

Periodic acid (/ˌpɜːraɪˈɒdɪk/ per-eye-OD-ik) is the highest oxoacid of iodine, in which the iodine exists in oxidation state +7. It can exist in two forms: orthoperiodic acid, with the chemical formula H$_5$IO$_6$, and metaperiodic acid, which has the formula HIO$_4$.
Periodic acid was discovered by Heinrich Gustav Magnus and C. F. Ammermüller in 1833.

## Synthesis

Modern industrial scale production involves the oxidation of a solution of sodium iodate under alkaline conditions, either electrochemically on a PbO$_2$ anode, or by treatment with chlorine:
    IO−3 + 6 HO$^−$ - 2 e$^−$ → IO5−6 + 3 H$_2$O (counter ions omitted for clarity) E° = -1.6 V
    IO−3 + 6 HO$^−$ + Cl$_2$ → IO5−6 + 2 Cl$^−$ + 3 H$_2$O
A standard laboratory preparation involves treating a mixture of barium periodate with nitric acid. Upon concentrating the mixture, the barium nitrate, which is less soluble, is separated from periodic acid:
    Ba$_3$H$_4$(IO$_6$)$_2$ + 6 HNO$_3$ → 3 Ba(NO$_3$)$_2$ + 2H$_5$IO$_6$

## Properties

Orthoperiodic acid has a number of acid dissociation constants. The pK$_a$ of metaperiodic acid has not been determined.
    H$_5$IO$_6$ ⇌ H$_4$IO−6 + H$^+$, pK$_a$ = 3.29
    H$_4$IO−6 ⇌ H$_3$IO2−6 + H$^+$, pK$_a$ = 8.31
    H$_3$IO2−6 ⇌ H$_2$IO3−6 + 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, NaIO$_4$, can be synthesised from HIO$_4$ while sodium orthoperiodate, Na$_5$IO$_6$ can be synthesised from H$_5$IO$_6$.

### Structure

Orthoperiodic acid forms monoclinic crystals (space group P2$_1$/$_n$) consisting of a slightly deformed IO$_6$ 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 IO$_6$ octahedra, however these are connected via cis-edge-sharing with bridging oxygens to form one-dimensional infinite chains.

## Reactions

Orthoperiodic acid can be dehydrated to give metaperiodic acid by heating to 100 °C under reduced pressure.
    H$_5$IO$_6$ ⇌ HIO$_4$ + 2 H$_2$O
Further heating to around 150 °C gives iodine pentoxide (I$_2$O$_5$) rather than the expected anhydride diiodine heptoxide (I$_2$O$_7$). Metaperiodic acid can also be prepared from various orthoperiodates by treatment with dilute nitric acid.
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 (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 allyl alcohols which are oxidised to enones by stoichiometric amounts of orthoperiodic acid with catalyst PCC.

## Other oxyacids

Periodic acid 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 | HIO$_2$ | HIO$_3$ | HIO$_4$ or H$_5$IO$_6$ |