Periodic acid

Orthoperiodic acid
Other names Paraperiodic acid
Identifiers
CAS number	10450-60-9 N
PubChem	65185
ChemSpider	58684 Y
ChEBI	CHEBI:29149 Y
ChEMBL	CHEMBL1161637 Y
Jmol-3D images	Image 1
SMILES OI(O)(O)(O)(O)=O
InChI InChI=1S/HIO4/c2-1(3,4)5/h(H,2,3,4,5) Y Key: KHIWWQKSHDUIBK-UHFFFAOYSA-N Y InChI=1/HIO4/c2-1(3,4)5/h(H,2,3,4,5) Key: KHIWWQKSHDUIBK-UHFFFAOYAH
Properties
Molecular formula	H5IO6
Molar mass	227.941 g/mol
Appearance	Colourless crystals
Melting point	122 °C; 252 °F; 395 K
Hazards
MSDS	External MSDS
EU classification	Oxidizer (O), Toxic (T), Corrosive (C)
R-phrases	R23 R24 R25 R34 R41
NFPA 704	0 3 0 OX
N (verify) (what is: Y/N?) Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
Infobox references

Metaperiodic acid
Other names Periodic acid
Identifiers
CAS number	13444-71-8 Y
PubChem	65185
ChemSpider	58684 Y
ChEBI	CHEBI:29149 Y
ChEMBL	CHEMBL1161637 Y
Jmol-3D images	Image 1
SMILES OI(=O)(=O)=O
InChI InChI=1S/HIO4/c2-1(3,4)5/h(H,2,3,4,5) Y Key: KHIWWQKSHDUIBK-UHFFFAOYSA-N Y InChI=1/HIO4/c2-1(3,4)5/h(H,2,3,4,5) Key: KHIWWQKSHDUIBK-UHFFFAOYAH
Properties
Molecular formula	HIO4
Molar mass	190.91 g/mol
Appearance	Colourless crystals
Melting point	122 °C; 252 °F; 395 K
Hazards
MSDS	External MSDS
EU classification	Oxidizer (O), Toxic (T), Corrosive (C)
R-phrases	R23 R24 R25 R34 R41
NFPA 704	0 3 0 OX
N (verify) (what is: Y/N?) Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
Infobox references

Periodic acid, or iodic(VII) acid^[1] is an oxoacid of iodine having chemical formula HIO₄ or, in the more common hydrated form, H₅IO₆.^[2] "Periodic acid" is not derived from "period", but from "iodine": per-iodic acid (compare iodic acid, perchloric acid), and it is thus pronounced per-iodic /ˌpɜr.aɪˈɒdɨk/ PURR-eye-OH-dik, and not as /ˌpɪərɪˈɒdɨk/ PEER-ee-OD-ik.

In dilute aqueous solution, periodic acid exists as discrete hydronium (H₃O⁺) and metaperiodate (IO₄⁻) ions. When more concentrated, orthoperiodic acid, H₅IO₆, is formed; this dissociates into hydronium and orthoperiodate (IO₆⁵⁻) ions. In practice, the metaperiodate and orthoperiodate ions co-exist in a pH-dependent chemical equilibrium:

IO₄⁻ + 6 H₂O IO₆⁵⁻ + 4 H₃O⁺

The hydrated form, orthoperiodic acid, is a crystalline solid. This can be dehydrated to the metaperiodic acid, HIO₄ (pronounced as "meh-tah-purr-eye-OH-dik"):

HIO₄ + 2H₂O H₅IO₆

Further heating gives diiodine pentoxide (I₂O₅), water and oxygen rather than the expected anhydride diiodine heptoxide; this anhydride does not occur in nature but can be formed synthetically.

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. Metaperiodates have solubilities and chemical properties similar to perchlorates (similar but larger ion size) though they are less oxidizing than perchlorates.

Periodic acid is used in organic chemistry for structural analysis. Periodic acid will cleave vicinal diols into two aldehyde or ketone fragments. This can be useful in determining the structure of carbohydrates. It is also used in organic synthesis as an oxidising agent of moderate strength.

See also

• Periodic acid-Schiff stain
• Perchloric acid, perbromic acid and perastatic acid, the related perhalogenic acids
• Telluric acid and perxenic acid, the isoelectronic oxoacids of tellurium and xenon

References