Periodic acid

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Periodic acid
Ortho-Periodsäure.svg
Orthoperiodic-acid-3D-balls.png
Orthoperiodic acid
Meta-Periodsäure.svg
Periodic-acid-3D-balls.png
Metaperiodic acid
Names
Other names
  • Paraperiodic acid
  • Iodic(VII) acid
  • Hydrogen periodate
Identifiers
10450-60-9 (orthoperiodic) N
13444-71-8 (metaperiodic) N
ChemSpider 23622 (orthoperiodic) N
58684 (metaperiodic) N
Jmol 3D model (orthoperiodic): Interactive image
(metaperiodic): Interactive image
PubChem 25289 (orthoperiodic)
65185 (metaperiodic)
  • (orthoperiodic): InChI=I=1S/H5IO6/c2-1(3,4,5,6)7/h(H5,2,3,4,5,6,7)
    Key: TWLXDPFBEPBAQB-UHFFFAOYSA-N
  • (metaperiodic): InChI=1/HIO4/c2-1(3,4)5/h(H,2,3,4,5)
    Key: KHIWWQKSHDUIBK-UHFFFAOYAH
  • (orthoperiodic): OI(=O)(O)(O)(O)O
  • (metaperiodic): OI(O)(O)(O)(O)=O
Properties
H5IO6 (orthoperiodic)
HIO4 (metaperiodic)
Molar mass 227.941 g/mol (H5IO6)
190.91 g/mol (HIO4)
Appearance Colourless crystals
Melting point 128.5 °C (263.3 °F; 401.6 K)[1]
Solubility soluble in water, alcohols
Vapor pressure {{{value}}}
Related compounds
Other anions
Other cations
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

Periodic acid (pronounced per-iodic /ˌpɜːr.aɪˈɒdᵻk/ PURR-eye-OH-dik, rather than /ˌpɪərɪˈɒdᵻk/ PEER-ee-OD-ik as in periodic table) 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.[2]

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[3]

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.[4]

H5IO6 → HIO4 + 2 H2O

Properties

Orthoperiodic acid has a number of acid dissociation constants.[5][6] The pKa of meta periodic acid has not been determined.

H5IO6 ⇌ H
4IO−
6 + H+,      pKa = 3.29
H
4IO−
6 ⇌ H
3IO2−
6 + H+,      pKa = 8.31
H
3IO2−
6 ⇌ H
2IO3−
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 IO6 octahedron interlinked via bridging hydrogens.[7] 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.[8]

Reactions

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Further information: Periodate cleavage 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.

Periodate cleavage.svg

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 HIO4 or H5IO6

See also

Compounds with a similar structure:

Compounds with similar chemistry:

References

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