Orthoperiodic acid
Other names	Paraperiodic acid
Identifiers
CAS number	10450-60-9
Properties
Molecular formula	H5IO6
Molar mass	227.941 g/mol
Appearance	Colorless crystals
Melting point	122 °C, 395 K, 252 °F
Hazards
MSDS	External MSDS
EU classification	Oxidizer (O), Toxic (T), Corrosive (C)
R-phrases	R23 R24 R25 R34 R41
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Periodic acid,^[1] or iodic(VII) acid^[2] is an oxoacid of iodine having chemical formula HIO₄ or H₅IO₆.

In dilute solution, periodic acid exists as H⁺ and IO₄⁻. When more concentrated, orthoperiodic acid, H₅IO₆, is formed. This can be obtained as a crystalline solid. Orthoperiodic acid can be dehydrated to metaperiodic acid, HIO₄. Further heating gives diiodine pentoxide (I₂O₅) and oxygen. The anhydride diiodine heptoxide does not exist in nature but can be formed synthetically.

Thus, two forms of periodates exist. One relating to the acid HIO₄, the other relating to H₅IO₆. The former results in metaperiodates (meta- meaning less water) and the latter, orthoperiodates (ortho- meaning more water). Metaperiodates have solubilities and chemical properties similar to perchlorates (similar but larger ion size) though they are less oxidizing than perchlorates.

Periodic acid is also used in organic chemistry for structural analysis. Periodic acid will cleave a vicinal diol into two aldehyde or ketone fragments. This can be useful in determining the structure of carbohydrates.

Ball-and-stick model of metaperiodic acid

[edit] Notes and references

1. ^ The name is not derived from "period", but from "iodine": per-iodic acid (compare iodic acid, perchloric acid), and it should thus be pronounced per-iodic and not as in the usual meaning of periodic.
2. ^ Alan Isaacs, John Daintith, Elizabeth Martin, ed (1984). Concise Science Dictionary. Oxford: Oxford University Press. pp. 356. ISBN 0192115936.