Phosphotungstic acid (PTA) or tungstophosphoric acid (TPA), is a heteropoly acid with the chemical formula . It forms hydrates . It is normally isolated as the ''n'' = 24 hydrate but can be

desiccated Desiccation () is the state of extreme dryness, or the process of extreme drying. A desiccant is a hygroscopic (attracts and holds water) substance that induces or sustains such a state in its local vicinity in a moderately sealed container. ...

to the hexahydrate (''n'' = 6). EPTA is the name of

ethanol Ethanol (abbr. EtOH; also called ethyl alcohol, grain alcohol, drinking alcohol, or simply alcohol) is an organic compound. It is an alcohol with the chemical formula . Its formula can be also written as or (an ethyl group linked to a ...

ic phosphotungstic acid, its alcohol solution used in biology. It has the appearance of small, colorless-grayish or slightly yellow-green crystals, with melting point 89 °C (24 hydrate). It is odorless and soluble in water (200 g/100 ml). It is not especially toxic, but is a mild acidic irritant. The compound is known by a variety of names and acronyms (see 'other names' section of infobox). In these names the "12" or "dodeca" reflects the fact that the anion contains 12 tungsten atoms. Some early workers who did not know the structure called it phospho-24-tungstic acid, formulating it as 3H₂O·P₂O₅ 24WO₃·59H₂O, , which correctly identifies the atomic ratios of P, W and O. This formula was still quoted in papers as late as 1970. Phosphotungstic acid is used in

histology Histology, also known as microscopic anatomy or microanatomy, is the branch of biology which studies the microscopic anatomy of biological tissues. Histology is the microscopic counterpart to gross anatomy, which looks at larger structures v ...

as a component for

staining Staining is a technique used to enhance contrast in samples, generally at the microscopic level. Stains and dyes are frequently used in histology (microscopic study of biological tissues), in cytology (microscopic study of cells), and in th ...

of cell specimens, often together with haematoxylin as

PTAH Ptah ( egy, ptḥ, reconstructed ; grc, Φθά; cop, ⲡⲧⲁϩ; Phoenician: 𐤐𐤕𐤇, romanized: ptḥ) is an ancient Egyptian deity, a creator god and patron deity of craftsmen and architects. In the triad of Memphis, he is the hu ...

. It binds to

fibrin Fibrin (also called Factor Ia) is a fibrous, non-globular protein involved in the clotting of blood. It is formed by the action of the protease thrombin on fibrinogen, which causes it to polymerize. The polymerized fibrin, together with pl ...

collagen Collagen () is the main structural protein in the extracellular matrix found in the body's various connective tissues. As the main component of connective tissue, it is the most abundant protein in mammals, making up from 25% to 35% of the whol ...

, and fibres of

connective tissue Connective tissue is one of the four primary types of animal tissue, along with epithelial tissue, muscle tissue, and nervous tissue. It develops from the mesenchyme derived from the mesoderm the middle embryonic germ layer. Connective tiss ...

s, and replaces the anions of dyes from these materials, selectively decoloring them. Phosphotungstic acid is electron dense, opaque for

electron The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family, and are generally thought to be elementary particles because they have n ...

s. It is a common

negative stain In microscopy, negative staining is an established method, often used in diagnostic microscopy, for contrasting a thin specimen with an optically opaque fluid. In this technique, the background is stained, leaving the actual specimen untouched, and ...

for

virus A virus is a submicroscopic infectious agent that replicates only inside the living cells of an organism. Viruses infect all life forms, from animals and plants to microorganisms, including bacteria and archaea. Since Dmitri Ivanovsk ...

es,

nerve A nerve is an enclosed, cable-like bundle of nerve fibers (called axons) in the peripheral nervous system. A nerve transmits electrical impulses. It is the basic unit of the peripheral nervous system. A nerve provides a common pathway for the ...

polysaccharide Polysaccharides (), or polycarbohydrates, are the most abundant carbohydrates found in food. They are long chain polymeric carbohydrates composed of monosaccharide units bound together by glycosidic linkages. This carbohydrate can react with w ...

s, and other biological tissue materials for imaging by a

transmission electron microscope Transmission electron microscopy (TEM) is a microscopy technique in which a beam of electrons is transmitted through a specimen to form an image. The specimen is most often an ultrathin section less than 100 nm thick or a suspension on a gr ...

Structure

Gouzerh summarises the historical views on the structure of phosphotungstic acid leading up to Keggin's determination of the structure as: * proposed by Miolati and further developed by Rosenheim * (Pauling) The structure was determined by J.F Keggin first published in 1933 and then in 1934 and is generally known as the

Keggin structure Keggin structure is the best known structural form for heteropoly acids. It is the structural form of α-Keggin anions, which have a general formula of , where X is the heteroatom (most commonly are pentavalent phosphorus PV, tetravalent silico ...

. The anion has full tetrahedral symmetry and comprises a cage of twelve tungsten atoms linked by oxygen atoms with the phosphorus atom at its centre. The picture on the right shows the octahedral coordination of oxygen atoms around the tungsten atoms, and that the surface of the anion has both bridging and terminal oxygen atoms. Further investigation showed that the compound was a hexahydrate not a pentahydrate as Keggin had proposed.

Preparation and chemical properties

Phosphotungstic acid can be prepared by the reaction of sodium tungstate dihydrate, , with

phosphoric acid Phosphoric acid (orthophosphoric acid, monophosphoric acid or phosphoric(V) acid) is a colorless, odorless phosphorus-containing solid, and inorganic compound with the chemical formula . It is commonly encountered as an 85% aqueous solution, w ...

, , acidified with

hydrochloric acid Hydrochloric acid, also known as muriatic acid, is an aqueous solution of hydrogen chloride. It is a colorless solution with a distinctive pungent smell. It is classified as a strong acid. It is a component of the gastric acid in the dige ...

, HCl. This procedure has been updated. Phosphotungstic acid solutions decompose as the pH is increased. A step-wise decomposition has been determined and the approximate compositions at various pH values are as follows: : The species is a lacunary, or defective Keggin ion. The has a Dawson structure. At pH less than 8, the presence of ethanol or acetone stabilises the anion, , reducing decomposition. Tungstophosphoric acid is thermally stable up to 400 °C, and is more stable than the analogous

silicotungstic acid Silicotungstic acid or tungstosilicic acid is a heteropoly acid with the chemical formula . It forms hydrates . In freshly prepared samples, ''n'' is approximately 29, but after prolonged desiccation, ''n'' = 6. It is a white solid although impu ...

, .''Oxide catalysts in solid state chemistry'' T Okuhara, M Misono Encyclopedia of Inorganic chemistry Editor R Bruce King (1994) John Wiley and Sons Large quantities of polar molecules such as

pyridine Pyridine is a basic heterocyclic organic compound with the chemical formula . It is structurally related to benzene, with one methine group replaced by a nitrogen atom. It is a highly flammable, weakly alkaline, water-miscible liquid w ...

are absorbed into the bulk phase and not simply on the surface. Solid state NMR studies of

absorbed in the bulk phase show that both protonated dimers, , and monomers, , are present. Phosphotungstic acid is less sensitive to reduction than phosphomolybdic acid. Reduction with

uric acid Uric acid is a heterocyclic compound of carbon, nitrogen, oxygen, and hydrogen with the formula C5H4N4O3. It forms ions and salts known as urates and acid urates, such as ammonium acid urate. Uric acid is a product of the metabolic breakdown ...

iron(II) sulfate Iron(II) sulfate (British English: iron(II) sulphate) or ferrous sulfate denotes a range of salts with the formula Fe SO4·''x''H2O. These compounds exist most commonly as the heptahydrate (''x'' = 7) but several values for x are know ...

produces a brown coloured compound. the related silicotungstic acid when reduced forms a similar brown compound where one of the four units in the Keggin structure becomes a metal-metal bonded cluster of three edge shared octahedra. Phosphotungstic acid is the strongest of heteropolyacids. Its conjugate base is the anion. Its acidity in acetic acid has been investigated and shows that the three protons dissociate independently rather than sequentially, and the acid sites are of the same strength. One estimate of the acidity is that the solid has an acidity stronger than H₀ = −13.16, which would qualify the compound as a

superacid In chemistry, a superacid (according to the classical definition) is an acid with an acidity greater than that of 100% pure sulfuric acid (), which has a Hammett acidity function (''H''0) of −12. According to the modern definition, a superaci ...

. This acidic strength means that even at low pH the acid is fully dissociated.

Uses

Catalyst

In common with the other heteropolyacids phosphotungstic acid is a catalyst and its high acidity and thermal stability make it a catalyst of choice according to some researchers. It is in solution as a homogeneous catalyst, and as a

heterogeneous catalyst In chemistry, heterogeneous catalysis is catalysis where the phase of catalysts differs from that of the reactants or products. The process contrasts with homogeneous catalysis where the reactants, products and catalyst exist in the same phase. ...

"supported" on a substrate e.g. alumina,

silica Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is ...

. Some acid catalysed reactions include: *the

homogeneous catalysis In chemistry, homogeneous catalysis is catalysis by a soluble catalyst in a solution. Homogeneous catalysis refers to reactions where the catalyst is in the same phase as the reactants, principally in solution. In contrast, heterogeneous catalysi ...

of the hydrolysis of propene to give 2-propanol *the

of the

Prins reaction The Prins reaction is an organic reaction consisting of an electrophilic addition of an aldehyde or ketone to an alkene or alkyne followed by capture of a nucleophile or elimination of an H+ ion. The outcome of the reaction depends on reaction c ...

*the

heterogeneous catalysis In chemistry, heterogeneous catalysis is catalysis where the phase of catalysts differs from that of the reactants or products. The process contrasts with homogeneous catalysis where the reactants, products and catalyst exist in the same phase. ...

of the dehydration of 2-propanol to propene and

methanol Methanol (also called methyl alcohol and wood spirit, amongst other names) is an organic chemical and the simplest aliphatic alcohol, with the formula C H3 O H (a methyl group linked to a hydroxyl group, often abbreviated as MeOH). It is ...

to hydrocarbons.

Dyeing and pigments

Phosphotungstic acid has been used to precipitate different types of dyes as "

lakes A lake is an area filled with water, localized in a basin, surrounded by land, and distinct from any river or other outlet that serves to feed or drain the lake. Lakes lie on land and are not part of the ocean, although, like the much large ...

". Examples are basic dyes and triphenylmethane dyes, e.g. pararosaniline derivatives.

Histology

Phosphotungstic acid is used in

for staining specimens, as a component of phosphotungstic acid haematoxylin,

, and “trichrome” reagents, and as a

for imaging by a

. ;Phosphotungstic acid haematoxylin (

) :Mallory described the reagent now generally known as PTAH in 1897. PTAH stains tissues either reddish brown or blue depending on their type. This property of simultaneously staining two different colours is different from other haematoxylin reagents e.g. alum-haematoxylin. The role of phosphotungstic acid and the mechanism of staining is not fully understood. The active component of haematoxylin is the oxidised form, haematin, although this rarely acknowledged in the literature which refer to haematoxylin staining. Phosphotungstic acid forms a lake with haematin. The make –up of the reagent is uncertain, examination of a year old sample showed there to be three coloured components, blue, red and yellow. These were not identified. Some investigations of “model” systems, reacting various compounds such as

amino acid Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha ...

purine Purine is a heterocyclic aromatic organic compound that consists of two rings ( pyrimidine and imidazole) fused together. It is water-soluble. Purine also gives its name to the wider class of molecules, purines, which include substituted purines ...

pyrimidine Pyrimidine (; ) is an aromatic, heterocyclic, organic compound similar to pyridine (). One of the three diazines (six-membered heterocyclics with two nitrogen atoms in the ring), it has nitrogen atoms at positions 1 and 3 in the ring. The othe ...

s and

amine In chemistry, amines (, ) are compounds and functional groups that contain a basic nitrogen atom with a lone pair. Amines are formally derivatives of ammonia (), wherein one or more hydrogen atoms have been replaced by a substituent ...

s with PTAH show that they give rise to different colours. ;Trichrome reagents :In these reagents two or three basic dyes are used with phosphotungstic acid, in either a one step or multi-stage procedure. These reagents colour different tissue types different colours. Again the mechanism of staining is not fully understood. Some explanations include the proposal that phosphotungstic acid acts as a

mordant A mordant or dye fixative is a substance used to set (i.e. bind) dyes on fabrics by forming a coordination complex with the dye, which then attaches to the fabric (or tissue). It may be used for dyeing fabrics or for intensifying stains in ...

to bind the dye to the tissue or that alternatively it binds to tissue blocking it to dye molecules. ;Negative staining :Adsorption onto tissue or the surface of viruses and its electron density are the bases of phosphotungstic acids action as a

. This electron density arises from the presence of the 12 tungsten atoms which each have an

atomic number The atomic number or nuclear charge number (symbol ''Z'') of a chemical element is the charge number of an atomic nucleus. For ordinary nuclei, this is equal to the proton number (''n''p) or the number of protons found in the nucleus of ever ...

of 74. The mechanism of the adsorption onto tissue has been proposed as being electrostatic rather than involving hydrogen bonding, as adsorption is not affected by pH.

Analysis

The potassium salt is only slightly soluble, unlike most other phosphotungstate salts, and has been proposed as a method for the

gravimetric analysis Gravimetric analysis describes a set of methods used in analytical chemistry for the quantitative determination of an analyte (the ion being analyzed) based on its mass. The principle of this type of analysis is that once an ion's mass has been ...

of potassium.

Precipitation of proteins

In a number of analytical procedures one of the roles of phosphotungstic acid is to precipitate out proteins. It has been termed a "universal" precipitant for polar proteins. Further studies showed that no precipitation occurred with α-amino groups but did occur with guanidino, ε-amino and imidazole groups.

Medicinal

Very little work appears to have been carried out in this area. One example relates to liver necrosis in rats.

Composite proton exchange membranes

The heteropoly acids, including phosphotungstic acid, are being investigated as materials in composite

proton exchange membrane A proton-exchange membrane, or polymer-electrolyte membrane (PEM), is a semipermeable membrane generally made from ionomers and designed to conduct protons while acting as an electronic insulator and reactant barrier, e.g. to oxygen and hydrogen g ...

s, such as

Nafion Nafion is a brand name for a sulfonated tetrafluoroethylene based fluoropolymer-copolymer discovered in the late 1960s by Dr. Walther Grot of DuPont. Nafion is a brand of the Chemours company. It is the first of a class of synthetic polymers with ...

. The interest lies in the potential of these composite materials in the manufacture of fuel cells as they have improved operating characteristics.

References

{{Tungsten compounds Tungstic acids Phosphates Oxides Staining Electron microscopy stains Coordination complexes Heteropoly acids