Periodic acid, or iodic(VII) acid is an oxoacid of iodine having chemical formula HIO4 or H5IO6.
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+
Orthoperiodic acid can be obtained as a crystalline solid that can be dehydrated to metaperiodic acid, HIO4. Further heating gives diiodine pentoxide (I2O5) 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 whilst 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.
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