PERIODIC ACID (HIO4): STRUCTURE, PROPERTIES AND USES - CHEMISTRY - 2025

The periodic acid is a oxyacid, which corresponds to the oxidation state VII of iodine. It exists in two forms: orthoperiodic (H ₅ IO ₆) and metaperiodic acid (HIO ₄). It was discovered in 1838 by the German chemists HG Magnus and CF Ammermüller.

In dilute aqueous solutions, periodic acid is mainly in the form of metaperiodic acid and hydronium ion (H ₃ O ⁺). Meanwhile, in concentrated aqueous solutions, periodic acid appears as orthoperiodic acid.

Hygroscopic crystals of orthoperiodic acid. Source: Leiem, from Wikimedia Commons

Both forms of periodic acid are present in a dynamic chemical equilibrium, the predominant form depending on the pH existing in the aqueous solution.

The upper image shows orthoperiodic acid, which consists of colorless hygroscopic crystals (for that reason they look wet). Although the formulas and structures between H ₅ IO ₆ and HIO ₄ are at first glance very different, the two are directly related to the degree of hydration.

H ₅ IO ₆ can be expressed as HIO ₄ ∙ 2H ₂ O, and therefore it must be dehydrated to obtain HIO ₄; the same happens in the opposite direction, when hydrating HIO ₄, H ₅ IO _{6 is produced}.

Structure of periodic acid

Metaperiodic acid. Source: Benjah-bmm27 via Wikipedia.

The upper image shows the molecular structure of metaperiodic acid, HIO ₄. This is the form that is most explained in chemistry texts; however, it is the least thermodynamically stable.

As can be seen, it consists of a tetrahedron in the center of which the iodine atom (purple sphere) is located, and the oxygen atoms (red spheres) at its vertices. Three of the oxygen atoms form a double bond with iodine (I = O), while one of them forms a single bond (I-OH).

This molecule is acidic due to the presence of the OH group, being capable of donating an H ⁺ ion; and even more so when the positive partial charge of H is greater due to the four oxygen atoms bound to the iodine. Note that HIO ₄ can form four hydrogen bonds: one through the OH (donut) and three through its oxygen atoms (accepts).

Crystallographic studies have shown that iodine can in fact accept two oxygens from a neighboring HIO ₄ molecule. In doing so, two IO ₆ octahedra are obtained, linked by two IOI bonds in cis positions; that is, they are on the same side and are not separated by an angle of 180 °.

These IO ₆ octahedra are linked in such a way that they end up creating infinite chains, that when they interact with each other they “arm” the HIO ₄ crystal.

Orthoperiodic acid

Orthoperiodic acid. Source: Benjah-bmm27 via Wikipedia.

The image above shows the most stable and hydrated form of periodic acid: orthoperiodic acid, H ₅ IO ₆. The colors for this model of bars and spheres is the same as for the HIO ₄ just explained. Here you can see directly what an IO ₆ octahedron looks like.

Note that there are five OH groups, corresponding to the five H ⁺ ions that could theoretically release the H ₅ IO ₆ molecule. However, due to increasing electrostatic repulsions, it can only release three of those five, establishing different equilibria of dissociation.

These five OH groups allow H ₅ IO _{6 to} accept various water molecules, and it is for this reason that its crystals are hygroscopic; that is, they absorb the moisture present in the air. They are also responsible for its considerably high melting point for a compound of a covalent nature.

The H ₅ IO ₆ molecules form many hydrogen bonds with each other, and therefore they grant such a directionality that also allows them to be arranged in an orderly space. As a result of this arrangement, H ₅ IO ₆ forms monoclinic crystals.

Properties

Molecular weights

-Metaperiodic acid: 190.91 g / mol.

-Orthoperiodic acid: 227.941 g / mol.

Physical appearance

White or pale yellow solid, for HIO ₄, or colorless crystals, for H ₅ IO ₆.

Melting point

128 ° C (263.3 ° F, 401.6 ° F).

Ignition point

140 ° C.

Stability

Stable. Strong oxidizer. Contact with combustible materials can cause fire. Hygroscopic. Incompatible with organic materials and strong reducing agents.

pH

1.2 (solution of 100 g / L of water at 20 ºC).

Reactivity

Periodic acid is capable of breaking the bond of vicinal diols present in carbohydrates, glycoproteins, glycolipids, etc., originating molecular fragments with terminal aldehydes groups.

This property of periodic acid is used to determine the structure of carbohydrates, as well as the presence of substances related to these compounds.

The aldehydes formed by this reaction can react with Schiff's reagent, detecting the presence of complex carbohydrates (they turn purple). Periodic acid and Schiff's reagent are coupled into a reagent that is abbreviated PAS.

Nomenclature

Traditional

Periodic acid has its name because iodine works with the highest of its valences: +7, (VII). This is the way of naming it according to the old nomenclature (the traditional one).

In chemistry books they always place HIO ₄ as the only representative of periodic acid, being synonymous with metaperiodic acid.

Metaperiodic acid owes its name to the fact that iodic anhydride reacts with a water molecule; that is, its degree of hydration is the lowest:

I ₂ O ₇ + H ₂ O => 2HIO ₄

While for the formation of orthoperiodic acid, I ₂ O ₇ must react with a higher quantity of water:

I ₂ O ₇ + 5H ₂ O => 2H ₅ IO ₆

Reacting with five water molecules instead of one.

The term ortho-, is used exclusively to refer to H ₅ IO ₆, and therefore periodic acid refers only to HIO ₄.

Systematics and stock

Other, less common names for periodic acid are:

-Hydrogen tetraoxoiodate (VII).

-Tetraoxoiodic acid (VII)

Applications

Doctors

PAS staining. Source: No machine-readable author provided. KGH assumed (based on copyright claims).

Purple PAS stains obtained by the reaction of periodic acid with carbohydrates are used in the confirmation of a glycogen storage disease; for example, Von Gierke's disease.

They are used in the following medical conditions: Paget's disease, sarcoma of the soft part when seeing, detection of lymphocyte aggregates in mycosis fungoides and in Sezany syndrome.

They are also used in the study of erythroleukemia, an immature red blood cell leukemia. Cells stain bright fuchsia. In addition, live fungal infections are used in the study, staining the walls of the fungi a magenta color.

At the laboratory

-It is used in the chemical determination of manganese, in addition to its use in organic synthesis.

-Periodic acid is used as a selective oxidant in the field of organic chemistry reactions.

-Periodic acid can produce the release of acetaldehyde and higher aldehydes. Additionally, periodic acid can release formaldehyde for detection and isolation, as well as the release of ammonia from hydroxyamino acids.

-Periodic acid solutions are used in the study of the presence of amino acids that have OH and NH _{2 groups} in adjacent positions. Periodic acid solution is used in conjunction with potassium carbonate. In this regard, serine is the simplest hydroxyamino acid.

References

1. Gavira José M Vallejo. (October 24, 2017). Meaning of the prefixes meta, pyro and ortho in the old nomenclature. Recovered from: triplenlace.com
2. Gunawardena G. (March 17, 2016). Periodic acid. Chemistry LibreTexts. Recovered from: chem.libretexts.org
3. Wikipedia. (2018). Periodic acid. Recovered from: en.wikipedia.org
4. Kraft, T. and Jansen, M. (1997), Crystal Structure Determination of Metaperiodic Acid, HIO4, with Combined X-Ray and Neutron Diffraction. Angew. Chem. Int. Ed. Engl., 36: 1753-1754. doi: 10.1002 / anie.199717531
5. Shiver & Atkins. (2008). Inorganic chemistry. (Fourth edition). Mc Graw Hill.
6. Martin, AJ, & Synge, RL (1941). Some applications of periodic acid to the study of the hydroxyamino-acids of protein hydrolysates: The liberation of acetaldehyde and higher aldehydes by periodic acid. 2. Detection and isolation of formaldehyde liberated by periodic acid. 3. Ammonia split from hydroxyamino-acids by periodic acid. 4. The hydroxyamino-acid fraction of wool. 5.; Hydroxylysine 'With an Appendix by Florence O. Bell Textile Physics Laboratory, University of Leeds. The Biochemical journal, 35 (3), 294-314.1.
7. Asima. Chatterjee and SG Majumdar. (1956). Use of Periodic Acid for Detecting and Locating Ethylenic Unsaturation. Analytical Chemistry 1956 28 (5), 878-879. DOI: 10.1021 / ac60113a028.