Iodic acid

Iodic acid can be produced by oxidizing iodine with strong oxidizers such as nitric acid, chlorine, chloric acid or hydrogen peroxide, for example:

I₂ + 6 H₂O + 5 Cl₂ ⇌ 2 HIO₃ + 10HCl

Iodic acid is also produced by the hydrolysis of iodine chlorides,

5 ICl + 3 H₂O → 5 HCl + HIO₃ + 2 I₂

5 ICl₃ + 9 H₂O → 15 HCl + 3 HIO₃ + I₂

although all methods involving chlorine in acidic conditions produce tetrachloroiodic acid as a byproduct:

HIO₃ + 6 HCl ⇌ HICl₄ + Cl₂ + 3 H₂O

Iodic acid crystallises from acidic solution as orthorhombic α-HIO
₃ in space group P2₁2₁2₁. The structure consists of pyramidal molecules linked by hydrogen bonding and intermolecular iodine-oxygen interactions. The I=O bond lengths 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 P2₁2₁2₁. All of the polymorphs contain pyramidal molecules, hydrogen bonding and I···O interactions, but differ in packing arrangement.

Iodic acid is a relatively strong acid with a pK_a 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 elemental iodine or the 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 which is present in solution as tetrachloroiodic acid. 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.

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 and strong bases, using methyl red or methyl orange as the indicator.

Iodic acid can be used to synthesize sodium or potassium iodate for increasing iodine content of salt.

Iodic acid 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.