8 julio, 2024

Periodic acid: what it is, structure, properties, uses

What is periodic acid?

He periodic acid It is an oxacid, which corresponds to the VII oxidation state of iodine. It exists in two forms: orthoperiodic acid (H5IO6) and metaperiodic acid (HIO4). It was discovered in 1838 by the German chemists HG Magnus and CF Ammermüller.

In dilute aqueous solutions, periodic acid is found mainly in the form of metaperiodic acid and hydronium ion (H3O+). While, in concentrated aqueous solutions, periodic acid occurs as orthoperiodic acid.

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

H5IO6 can be expressed as HIO4∙2H2O, and therefore must be dehydrated to obtain HIO4; the same happens in the opposite direction, when hydrating the HIO4 the H5IO6 is produced.

Periodic Acid Structure

The structure of periodic acid consists of a tetrahedron in whose center the iodine atom is located, and at its vertices the oxygen atoms. 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 partial positive charge of H is larger due to the four oxygen atoms bonded to the iodine. HIO4 can form four hydrogen bonds: one through the OH (donates) and three through its oxygen atoms (accepts).

Crystallographic studies have shown that iodine can in fact accept two oxygens from a neighboring HIO4 molecule. In doing so, two IO6 octahedrons are obtained, joined 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 IO6 octahedrons are linked in such a way that they end up creating infinite chains, which when interacting with each other “arm” the HIO4 crystal.

orthoperiodic acid

Orthoperiodic acid (H5IO6) is the most stable and hydrated form of periodic acid.

There are five OH groups, corresponding to the five H+ ions that the H5IO6 molecule could theoretically release. However, due to increasing electrostatic repulsions, it can only release three of those five, establishing different dissociation equilibria.

These five OH groups allow H5IO6 to accept several water molecules, and it is for this reason that its crystals are hygroscopic, that is, they absorb the humidity present in the air. Likewise, these are responsible for its considerably high melting point for a compound of covalent nature.

The H5IO6 molecules form many hydrogen bonds with each other, and, therefore, provide such a directionality that also allows them to arrange themselves in an orderly manner in space. As a result of this arrangement, H5IO6 forms monoclinic crystals.

Properties

Molecular Weights: tometaperiodic acid: 190.91 g/mol. Orthoperiodic acid: 227.941 g/mol.
Physical appearance: yeswhite or pale yellow solid, for HIO4or colorless crystals, for H5IO6.
Melting point: 128°C (263.3°F, 401.6°F).
Ignition point: 140°C
Stability: andstable. Strong oxidant. Contact with combustible materials may cause fire. Hygroscopic. Incompatible with organic materials and strong reducing agents.
pH: 1.2 (solution of 100 g/L of water at 20 °C).
Reactivity:ePeriodic acid is capable of breaking the bond of neighboring diols present in carbohydrates, glycoproteins, glycolipids, etc., originating molecular fragments with aldehyde terminal 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 are colored purple). Periodic acid and Schiff’s reagent are coupled into a reagent that is abbreviated as PAS.

Nomenclature

Traditional

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

In chemistry books they always place HIO4 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:

I2O7 + H₂O => 2HIO4

While for the formation of orthoperiodic acid, the I2O7 must react with a higher amount of water:

I2O7 + 5H2O => 2H5IO6

Reacting with five water molecules instead of one.

The term ortho- is used exclusively to refer to H5IO6, and therefore periodic acid refers only to HIO4.

Systematic and stock

Other, less common, names for periodic acid are:

hydrogen tetraoxoiodate(VII).
tetraoxoiodic acid (VII).

Applications

doctors

PAS purple stains obtained by the reaction of periodic acid with carbohydrates are used in confirmation of glycogen storage disease. For example, von Gierke’s disease.
It is used in the following medical conditions: Paget’s disease, alveolar soft part sarcoma, detection of lymphocyte clumps in mycosis fungoides, and Sezany syndrome.
It is also used in the study of erythroleukemia, a leukemia of immature red blood cells. Cells stain bright fuchsia. In addition, infections with live fungi are used in the study, staining the walls of the fungi in 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 cause the release of acetaldehyde and higher aldehydes. Additionally, it can release formaldehyde for detection and isolation, as well as release ammonia from hydroxyamino acids.
Periodic acid solutions are used in the study of the presence of amino acids that have OH and NH2 groups in adjacent positions. Periodic acid solution is used in conjunction with potassium carbonate. In this regard, serine is the simplest hydroxyamino acid.

References

Periodic acid. Retrieved from en.wikipedia.org.

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