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Iodic acid

Names
Other names Iodic(V) acid
Identifiers
CAS Number	7782-68-5
3D model (JSmol)	Interactive image
ChEBI	CHEBI:24857
ChEMBL	ChEMBL1161636
ChemSpider	22761
ECHA InfoCard	100.029.056
PubChem CID	24345
UNII	6U8J18JSBM
CompTox Dashboard (EPA)	DTXSID2064812
InChI InChI=1S/HIO3/c2-1(3)4/h(H,2,3,4)Key: ICIWUVCWSCSTAQ-UHFFFAOYSA-N InChI=1/HIO3/c2-1(3)4/h(H,2,3,4)Key: ICIWUVCWSCSTAQ-UHFFFAOYAT
SMILES O()
Properties
Chemical formula	HIO3
Molar mass	175.91 g/mol
Appearance	White solid
Density	4.62 g/cm, solid
Melting point	110 °C (230 °F; 383 K)
Solubility in water	269 g/100 mL (20 °C)
Acidity (pKa)	0.75
Conjugate base	Iodate
Magnetic susceptibility (χ)	−48.0·10 cm/mol
Hazards
Occupational safety and health (OHS/OSH):
Main hazards	acid, corrosive, oxidant
GHS labelling:
Pictograms
Signal word	Danger
NFPA 704 (fire diamond)	3 0 1OX
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 , 100 kPa). Y verify (what is  ?) 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.

Preparation

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

I₂ + 6H₂O + 5Cl₂ ⇌ 2HIO₃ + 10HCl

Iodic acid is also produced by the reaction of iodine monochloride with water:

5ICl + 3H₂O → 5HCl + HIO₃ + 2I₂

Structure

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.

Properties

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 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 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

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.

References

1. Perrin, D. D., ed. (1982) . Ionisation Constants of Inorganic Acids and Bases in Aqueous Solution. IUPAC Chemical Data (2nd ed.). Oxford: Pergamon (published 1984). Entry 127. ISBN 0-08-029214-3. LCCN 82-16524.
2. "Iodic acid" (PDF). Archived from the original (PDF) on 2014-09-26. Retrieved 2022-01-30.
3. Holleman, Arnold F.; Wiberg, Nils (2007). Lehrbuch der Anorganischen Chemie (in German) (102nd ed.). Berlin. ISBN 978-3-11-017770-1.{{cite book}}: CS1 maint: location missing publisher (link)
4. Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. p. 863. ISBN 978-0-08-037941-8.
5. Rogers, Max T.; Helmholz, Lindsay (1941). "The Crystal Structure of Iodic Acid". J. Am. Chem. Soc. 63 (1): 278–284. doi:10.1021/ja01846a068.
6. 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.
7. 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.
8. 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. Bibcode:2017PDiff..32..261W. doi:10.1017/S0885715617000859. S2CID 104100313.

• Analytical standards
• Halogen oxoacids
• Iodates
• Iodine compounds
• Mineral acids
• Oxidizing acids