Periodic acid ( ) is the highest oxoacid of

iodine Iodine is a chemical element with the Symbol (chemistry), symbol I and atomic number 53. The heaviest of the stable halogens, it exists as a semi-lustrous, non-metallic solid at standard conditions that melts to form a deep violet liquid at , ...

, in which the iodine exists in

oxidation state In chemistry, the oxidation state, or oxidation number, is the hypothetical charge of an atom if all of its bonds to different atoms were fully ionic. It describes the degree of oxidation (loss of electrons) of an atom in a chemical compound. C ...

+7. Like all periodates it can exist in two forms: orthoperiodic acid, with the

chemical formula In chemistry, a chemical formula is a way of presenting information about the chemical proportions of atoms that constitute a particular chemical compound or molecule, using chemical element symbols, numbers, and sometimes also other symbols, ...

, and metaperiodic acid, which has the formula . Periodic acid was discovered by Heinrich Gustav Magnus and C. F. Ammermüller in 1833.

Synthesis

Modern industrial scale production involves the

oxidation Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or ...

of a solution of sodium iodate under alkaline conditions, either

electrochemical Electrochemistry is the branch of physical chemistry concerned with the relationship between electrical potential difference, as a measurable and quantitative phenomenon, and identifiable chemical change, with the potential difference as an outc ...

ly on a

anode An anode is an electrode of a polarized electrical device through which conventional current enters the device. This contrasts with a cathode, an electrode of the device through which conventional current leaves the device. A common mnemonic is ...

, or by treatment with

chlorine Chlorine is a chemical element with the symbol Cl and atomic number 17. The second-lightest of the halogens, it appears between fluorine and bromine in the periodic table and its properties are mostly intermediate between them. Chlorine i ...

: : (

counter ion 160px, Polystyrene sulfonate, a cation-exchange resin, is typically supplied with as the counterion.">cation-exchange_resin.html" ;"title="Polystyrene sulfonate, a cation-exchange resin">Polystyrene sulfonate, a cation-exchange resin, is typical ...

s omitted for clarity) ''E''° = -1.6 V : Orthoperiodic acid can be dehydrated to give metaperiodic acid by heating to 100 °C under reduced pressure. : Further heating to around 150 °C gives iodine pentoxide () rather than the expected anhydride ''diiodine heptoxide'' (). Metaperiodic acid can also be prepared from various orthoperiodates by treatment with dilute

nitric acid Nitric acid is the inorganic compound with the formula . It is a highly corrosive mineral acid. The compound is colorless, but older samples tend to be yellow cast due to decomposition into oxides of nitrogen. Most commercially available ni ...

Properties

Orthoperiodic acid has a number of

acid dissociation constant In chemistry, an acid dissociation constant (also known as acidity constant, or acid-ionization constant; denoted ) is a quantitative measure of the strength of an acid in solution. It is the equilibrium constant for a chemical reaction :HA ...

s. The p''K''_a of metaperiodic acid has not been determined. :, p''K''_a = 3.29 :, p''K''_a = 8.31 :, p''K''_a = 11.60 There being two forms of periodic acid, it follows that two types of periodate salts are formed. For example, sodium metaperiodate, NaIO₄, can be synthesised from HIO₄ while sodium orthoperiodate, Na₅IO₆ can be synthesised from H₅IO₆.

Structure

Orthoperiodic acid forms

monoclinic In crystallography, the monoclinic crystal system is one of the seven crystal systems. A crystal system is described by three vectors. In the monoclinic system, the crystal is described by vectors of unequal lengths, as in the orthorhombic ...

crystals (

space group In mathematics, physics and chemistry, a space group is the symmetry group of an object in space, usually in three dimensions. The elements of a space group (its symmetry operations) are the rigid transformations of an object that leave it uncha ...

P2₁/''_n'') consisting of a slightly deformed octahedron interlinked via bridging hydrogens. Five I–O bond distances are in the range 1.87–1.91 Å and one I–O bond is 1.78 Å. The structure of metaperiodic acid also includes octahedra, however these are connected via ''cis''-edge-sharing with bridging oxygens to form one-dimensional infinite chains.

Reactions

Like all periodates periodic acid can be used to cleave various 1,2-difunctional compounds. Most notably periodic acid will cleave vicinal

diol A diol is a chemical compound containing two hydroxyl groups ( groups). An aliphatic diol is also called a glycol. This pairing of functional groups is pervasive, and many subcategories have been identified. The most common industrial diol is e ...

s into two

aldehyde In organic chemistry, an aldehyde () is an organic compound containing a functional group with the structure . The functional group itself (without the "R" side chain) can be referred to as an aldehyde but can also be classified as a formyl gro ...

ketone In organic chemistry, a ketone is a functional group with the structure R–C(=O)–R', where R and R' can be a variety of carbon-containing substituents. Ketones contain a carbonyl group –C(=O)– (which contains a carbon-oxygen double b ...

fragments (

Malaprade reaction In organic chemistry, the Malaprade reaction or Malaprade oxidation is a glycol cleavage reaction in which a vicinal diol is oxidized by periodic acid or a periodate salt to give the corresponding carbonyl functional groups. The reaction was firs ...

This can be useful in determining the structure of

carbohydrates In organic chemistry, a carbohydrate () is a biomolecule consisting of carbon (C), hydrogen (H) and oxygen (O) atoms, usually with a hydrogen–oxygen atom ratio of 2:1 (as in water) and thus with the empirical formula (where ''m'' may or m ...

as periodic acid can be used to open saccharide rings. This process is often used in labeling saccharides with fluorescent molecules or other tags such as

biotin Biotin (or vitamin B7) is one of the B vitamins. It is involved in a wide range of metabolic processes, both in humans and in other organisms, primarily related to the utilization of fats, carbohydrates, and amino acids. The name ''biotin'', bo ...

. Because the process requires vicinal diols, periodate oxidation is often used to selectively label the 3′-termini of RNA (

ribose Ribose is a simple sugar and carbohydrate with molecular formula C5H10O5 and the linear-form composition H−(C=O)−(CHOH)4−H. The naturally-occurring form, , is a component of the ribonucleotides from which RNA is built, and so this com ...

has vicinal diols) instead of DNA as deoxyribose does not have vicinal diols. Periodic acid is also used as an oxidising agent of moderate strength, as exemplified in the Babler oxidation of secondary

allyl alcohol Allyl alcohol ( IUPAC name: prop-2-en-1-ol) is an organic compound with the structural formula . Like many alcohols, it is a water-soluble, colourless liquid. It is more toxic than typical small alcohols. Allyl alcohol is used as a raw material ...

s which are oxidised to enones by stoichiometric amounts of orthoperiodic acid with

catalyst Catalysis () is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst (). Catalysts are not consumed in the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recyc ...

PCC.

Other oxyacids

Periodate is part of a series of

oxyacid An oxyacid, oxoacid, or ternary acid is an acid that contains oxygen. Specifically, it is a compound that contains hydrogen, oxygen, and at least one other element, with at least one hydrogen atom bonded to oxygen that can dissociate to produc ...

s in which iodine can assume

s of −1, +1, +3, +5, or +7. A number of neutral

iodine oxide Iodine oxides are chemical compounds of oxygen and iodine. Iodine has only two stable oxides which are isolatable in bulk, iodine tetroxide and iodine pentoxide, but a number of other oxides are formed in trace quantities or have been hypothesized ...

s are also known.

References