The periodic acid it is an oxacid, which corresponds to oxidation state VII of iodine. It exists in two forms: the orthoperiodic (H5IO6) and metaperiodic acid (HIO4). It was discovered in 1838 by the German chemists H. G. Magnus and C. F. Ammermüller.
In dilute aqueous solutions, periodic acid occurs mainly in the form of metaperiodic acid and hydronium ion (H3OR+). Meanwhile, in concentrated aqueous solutions, periodic acid appears as orthoperiodic acid..
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 the H5IO6 and HIO4 are at first glance very different, the two are directly related to the degree of hydration.
The H5IO6 can be expressed as HIO4∙ 2HtwoOr, and therefore you have to dehydrate it to get the HIO4; the same happens in the opposite direction, when hydrating the HIO4 the H is produced5IO6.
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The upper image shows the molecular structure of metaperiodic acid, HIO4. 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 is the iodine atom (purple sphere), 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 able to donate 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 the 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 HIO molecule.4. In doing so, two octahedra IO are obtained6, linked by two I-O-I bonds in cis positions; that is, they are on the same side and are not separated by an angle of 180 °.
These octahedra IO6 they are linked in such a way that they end up creating infinite chains, that when interacting with each other they “arm” the HIO crystal4.
The image above shows the most stable and hydrated form of periodic acid: orthoperiodic, H5IO6. The colors for this model of bars and spheres is the same as for the HIO4 just explained. Here you can see directly what an octahedron IO looks like6.
Note that there are five OH groups, corresponding to the five H ions+ that could theoretically release the H molecule5IO6. However, due to increasing electrostatic repulsions, it can only release three of those five, establishing different dissociation equilibria..
These five OH groups allow H5IO6 accept several molecules of water, and it is for this reason that its crystals are hygroscopic; that is, they absorb the moisture present in the air. Also, these are responsible for its considerably high melting point for a compound of covalent nature..
Molecules of H5IO6 they form many hydrogen bonds with each other, and therefore provide such a directionality that also allows them to be arranged in an orderly space. As a result of said ordering, the H5IO6 forms monoclinic crystals.
-Metaperiodic acid: 190.91 g / mol.
-Orthoperiodic acid: 227.941 g / mol.
White or pale yellow solid, for HIO4, or colorless crystals, for H5IO6.
128 ºC (263.3 ºF, 401.6 ºF).
140 ºC.
Stable. Strong oxidant. Contact with combustible materials can cause fire. Hygroscopic. Incompatible with organic materials and strong reducing agents.
1.2 (solution of 100 g / L of water at 20 ºC).
Periodic acid is capable of breaking the bond of vicinal diols present in carbohydrates, glycoproteins, glycolipids, etc., originating molecular fragments with terminal groups aldehydes.
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 abbreviated as PAS.
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 the 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:
ItwoOR7 + HtwoO => 2HIO4
While for the formation of orthoperiodic acid, the ItwoOR7 must react with a higher quantity of water:
ItwoOR7 + 5HtwoO => 2H5IO6
Reacting with five water molecules instead of one.
The term ortho-, is used exclusively to refer to the H5IO6, and that is why periodic acid refers only to HIO4.
Other, less common names for periodic acid are:
-hydrogen tetraoxoiodate (VII).
-Tetraoxoiodic acid (VII)
Purple PAS stains obtained by the reaction of periodic acid with carbohydrates are used in the confirmation of 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 Sezany syndrome.
They are also used in the study of erythroleukemia, an immature red blood cell leukemia. Cells stain bright fuchsia. In addition, it is used in the study of infections with live fungi, staining the walls of the fungi a magenta color.
-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, 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 groups.two in adjacent positions. The periodic acid solution is used in conjunction with potassium carbonate. In this regard, serine is the simplest hydroxyamino acid.
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