Hypoiodous acid

Structural formula
General
Surname	Hypoiodous acid
other names	Hypoiodic acid (obsolete) Iodine (I) acid
Molecular formula	HIO
Brief description	only known in aqueous solution, cannot be shown in pure form
External identifiers / databases
CAS number 14332-21-9 PubChem 123340 ChemSpider 109942 Wikidata Q414664
properties
Molar mass	143.91 g mol −1
pK s value	10.64 (25 ° C)
solubility	soluble in water
safety instructions
GHS hazard labeling no classification available
As far as possible and customary, SI units are used. Unless otherwise noted, the data given apply to standard conditions .

The oxygen acid of iodine in the +1 oxidation state is called hypoiodous acid . It has the molecular formula HIO and its salts are called hypoiodites . Hypoiodous acid only exists briefly in aqueous solution and cannot be isolated as a pure substance .

Classification

Hypoiodous acid belongs to the substance group of halogenated oxygen acids and belongs to the subgroup of iodoic acids (empirical formula type HIO _n with n = 1, 2, 3, 4 and H ₅ IO ₆ ) or to the subgroup of hypohalous acids (empirical formula type HXO with X = F, Cl, Br, I, At).

presentation

Analogous to the hypochlorous acid HClO and the hypobromous acid HBrO, an aqueous solution of hypoiodous acid can be prepared by utilizing the disproportionation of elemental iodine I ₂ (iodine in the oxidation state 0) in water to form hydroiodic acid HI (iodine in the oxidation state −1) and hypoiodic acid HIO (iodine in the +1 oxidation state) take place:

${\displaystyle \mathrm {{\overset {\pm 0}{I}}\!{}_{2}+H_{2}O\ \rightleftharpoons \ H\!{\overset {-1}{I}}+{\overset {+1}{HIO}}} }$

In order to shift the reaction equilibrium, which is largely on the left side ( equilibrium constant 2.0 · 10 ⁻¹³ at pH = 7 and 25 ° C) to the right, the reaction is carried out in the presence of a slurry of mercury (II) oxide HgO the resulting hydroiodic acid HI is intercepted as a result of the formation of poorly soluble basic mercury (II) iodide (in the following simply formulated as HgI ₂ · 2 HgO):

${\displaystyle \mathrm {2\ I_{2}+3\ HgO+H_{2}O\ \longrightarrow \ HgI_{2}\cdot 2\ HgO+2\ HIO} }$

In order to slow down the decomposition of the hypoiodous acid, work must be carried out at low temperatures.

properties

Acid-base behavior

Hypoiodous acid HIO is only a very weak acid , its acidity ( p K _S value of 10.64 at 25 ° C) is about three orders of magnitude smaller than that of the hypochlorous acid HClO and the hypobromous acid HBrO. From the acidic to the weakly alkaline pH range, it is therefore present almost exclusively in the form of undissociated HIO molecules; only at increasingly alkaline pH values ​​are increasing proportions of its corresponding base, the hypoiodite anion IO ^- , in equilibrium .

Instability, redox behavior

Hypoiodous acid HIO is a very unstable chemical compound and quickly decomposes in a multi-stage reaction to iodine I ₂ and iodic acid HIO ₃ ( iodic acid HIO ₂ also occurs as an intermediate ):

1) Disproportionation of hypoiodous acid to hydriodic acid and iodic acid

${\displaystyle \mathrm {3\ {\overset {+1}{HIO}}\ \longrightarrow \ 2\ H\!{\overset {-1}{I}}+{\overset {+5}{HIO}}{}_{3}} }$

2) Comproportionation of hydriodic acid and iodic acid to iodine

${\displaystyle \mathrm {5\ H\!{\overset {-1}{I}}+{\overset {+5}{HIO}}{}_{3}\ \longrightarrow \ 3\ {\overset {\pm 0}{I}}\!{}_{2}+3\ H_{2}O} }$

3) Overall reaction: disproportionation of hypoiodous acid to iodine and iodic acid

${\displaystyle \mathrm {5\ {\overset {+1}{HIO}}\ \longrightarrow \ 2\ {\overset {\pm 0}{I}}\!{}_{2}+{\overset {+5}{HIO}}{}_{3}+2\ H_{2}O} }$

Like the Disproportionierungsneigung shows can hypoiodous acid HIO or the hypoiodite anion IO ^- both as an oxidant and as a reducing agent react ( Redox Amphoteric Behavior ). The oxidizing effect is more pronounced in acidic than in alkaline, which is expressed in a higher redox potential :

pH = 0		pH = 14
Redox couple	Normal potential	Redox couple	Normal potential
${\displaystyle \mathrm {{\overset {-1}{I}}\!{}^{-}\ |\ {\overset {+1}{HIO}}} }$	+0.99 V	${\displaystyle \mathrm {{\overset {-1}{I}}\!{}^{-}\ |{\overset {+1}{\ I}}\!O^{-}} }$	+0.48 V.
${\displaystyle \mathrm {{\overset {\pm 0}{I}}\!{}_{2}\ |\ {\overset {+1}{HIO}}} }$	+1.44 V	${\displaystyle \mathrm {{\overset {\pm 0}{I}}\!{}_{2}\ |{\overset {+1}{\ I}}\!O^{-}} }$	+0.42 V
${\displaystyle \mathrm {{\overset {+1}{HIO}}\ |\ {\overset {+5}{HIO}}{}_{3}} }$	+1.13V	${\displaystyle \mathrm {{\overset {+1}{\ I}}\!O^{-}\,|{\overset {+5}{\ I}}\!O_{3}^{-}} }$	+0.15 V.

use

Probably hypoiodous acid, the active species of the iodine solution used in medicine for the disinfection Iodwasser .

Individual evidence

1. ↑ ^{a b c d e f g h i} A. F. Holleman , E. Wiberg , N. Wiberg : Textbook of Inorganic Chemistry . 102nd edition. De Gruyter, Berlin 2007, ISBN 978-3-11-017770-1 , p. 463–465, 468, 474, 2008 ( limited preview of the 101st edition in Google Book Search).
2. ↑ This substance has either not yet been classified with regard to its hazardousness or a reliable and citable source has not yet been found.

Web links

• Kiel University: Iodine (PDF file; 165 kB)