Iodic acid is a white water-soluble solid with the chemical formula . Its robustness contrasts with the instability of chloric acid and bromic acid. Iodic acid features iodine in the oxidation state +5 and is one of the most stable of the halogens. When heated, samples dehydrate to give iodine pentoxide. On further heating, the iodine pentoxide further decomposes, giving a mix of iodine, oxygen and lower oxides of iodine.
Preparation
Iodic acid can be produced by
oxidizing iodine with strong oxidizers such as
nitric acid, chlorine, chloric acid or hydrogen peroxide,
for example:
Iodic acid is also produced by the reaction of iodine monochloride with water:
Structure
Iodic acid crystallises from acidic solution as orthorhombic α- in
space group P2
12
12
1. The structure consists of pyramidal molecules linked by
hydrogen bonding and intermolecular iodine-oxygen interactions. The I=O
are 1.81 Å while the I–OH distance is 1.89 Å.
Several other polymorphs have been reported, including an orthorhombic γ form in space group
Pbca and an orthorhombic δ form in space group
P2
12
12
1.
All of the polymorphs contain pyramidal molecules, hydrogen bonding and I···O interactions, but differ in packing arrangement.
Properties
Iodic acid is a relatively
strong acid with a p
Ka of 0.75. It is strongly oxidizing in acidic solution, less so in basic solution. When iodic acid acts as oxidizer, then the product of the reaction is either iodine, or iodide ion. Under some special conditions (very low pH and high concentration of chloride ions, such as in concentrated hydrochloric acid), iodic acid is reduced to iodine trichloride, a golden yellow compound in solution and no further reduction occurs. In the absence of chloride ions, when there is an excess amount of reductant, then all iodate is converted to iodide ion. When there is an excess amount of iodate, then part of the iodate is converted to iodine.
Uses
Iodic acid is used as a
strong acid (though it is not truly a strong acid, but a weak acid that is very close to being a strong acid) in analytical chemistry. It may be used to standardize solutions of both
weak base and
, using
methyl red or
methyl orange as the
pH indicator.
Use in salt industry
Iodic acid can be used to synthesize
Sodium iodate or
potassium iodate for increasing iodine content of salt.
Other oxyacids
Iodic acid is part of a series of
in which iodine can assume
of −1, +1, +3, +5, or +7. A number of neutral
are also known.
