Iodic acid

Iodic acid

Iodic acid, HIO3, can be obtained as a white solid. It dissolves in water very well, but it also exists in the pure state, as opposed to chloric acid or bromic acid. Iodic acid contains iodine in the oxidation state +5 and it is one of the most stable oxo-acids of the halogens in its pure state. When iodic acid is carefully heated, then it loses water, and iodine pentoxide remains behind. On stronger heating, the iodine pentoxide further decomposes, giving a mix of iodine, oxygen and lower oxides of iodine.


Iodic acid can be produced by oxidizing I2 with chlorine in an aqueous solution.


Iodic acid is a relatively strong acid with a pKa 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 ion, e.g. 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 reductor, 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 standard strong acid in analytical chemistry. It may be used to standardize solutions of both weak and strong bases, with methyl red or methyl orange as the indicator.


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