Chemistry:Iodic acid

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Iodic acid
Iodic acid
Ball-and-stick model of iodic acid
Space-filling model of iodic acid
Other names
Iodic(V) acid
3D model (JSmol)
Molar mass 175.91 g/mol
Appearance White solid
Density 4.62 g/cm3, solid
Melting point 110 °C (230 °F; 383 K)
269 g/100 mL (20 °C)
Acidity (pKa) 0.75
Conjugate base Iodate
−48.0·10−6 cm3/mol
Main hazards acid, corrosive, oxidant
GHS pictograms GHS03: OxidizingGHS05: Corrosive
GHS Signal word Danger
NFPA 704 (fire diamond)
Flash point Non-flammable
Related compounds
Other cations
Lithium iodate
Potassium iodate
Related halogen oxoacids
Chloric acid
Bromic acid
Related compounds
Hydroiodic acid
Iodine pentoxide
Periodic acid
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

Iodic acid, is a white water-soluble solid with the chemical formula HIO3. 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 oxo-acids 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.


Iodic acid can be produced by oxidizing iodine I2 with strong oxidizers such as nitric acid HNO3, chlorine Cl2, chloric acid HClO3 or hydrogen peroxide H2O2,[2] for example:

I2 + 6 H2O + 5 Cl2 ⇌ 2 HIO3 + 10 HCl


Iodic acid crystallises from acidic solution as orthorhombic α-HClO3 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.[3][4][5] Several other polymorphs have been reported: an orthorhombic γ form in space group Pbca,[6] an orthorhombic δ form in space group P212121,[7] and an orthorhombic β form also in 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 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 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.[8] It may be used in preparation of ionization to form alkyl halides.


Iodic acid is used as 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.

Use in salt industry

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

Other oxyacids

Iodate 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.

Iodine oxidation state −1 +1 +3 +5 +7
Name Hydrogen iodide Hypoiodous acid Iodous acid Iodic acid Periodic acid
Formula HI HIO HIO2 HIO3 HIO4 or H5IO6


  1. "Iodic acid" (in en). 
  2. Holleman, Arnold F.; Wiberg, Nils (2007) (in German). Lehrbuch der Anorganischen Chemie (102nd ed.). Berlin. ISBN 978-3-11-017770-1. 
  3. Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. p. 863. ISBN 978-0-08-037941-8. 
  4. Rogers, Max T.; Helmholz, Lindsay (1941). "The Crystal Structure of Iodic Acid". J. Am. Chem. Soc. 63 (1): 278–284. doi:10.1021/ja01846a068. 
  5. Ståhl, Kenny; Szafranski, Marek (1992). "A Single-Crystal Neutron Diffraction Study of HIO3 at 295 and 30 K and of DIO3 at 295 K". Acta Chem. Scand. 46: 1146–1148. doi:10.3891/acta.chem.scand.46-1146. 
  6. Fischer, Andreas; Lindsjö, Martin (2005). "γ-HIO3 – a Metastable, Centrosymmetric Polymorph of Iodic Acid". Z. Anorg. Allg. Chem. 631 (9): 1574–1576. doi:10.1002/zaac.200500099. 
  7. Wu, Tao; Zavalij, Peter Y.; Zachariah, Michael R. (2017). "Crystal structure of a new polymorph of iodic acid, δ-HIO3, from powder diffraction". Powder Diffraction 32 (4): 261–264. doi:10.1017/S0885715617000859. Bibcode2017PDiff..32..261W. 
  8. Ropp, Richard C. (2012). Encyclopedia of the Alkaline Earth Compounds.. pp. 96. ISBN 978-0-44-459550-8.