Phosphotungstic acid (
PTA),
tungstophosphoric acid (
TPA), is a
heteropoly acidA heteropoly acid is a class of acid made up of a particular combination of hydrogen and oxygen with certain metals and non-metals. This type of acid is frequently used as a re-usable acid catalyst in chemical reactions....
with the chemical formula
31240. It normally present as a hydrate.
EPTA is the name of
ethanolEthanol, also called ethyl alcohol, pure alcohol, grain alcohol, or drinking alcohol, is a volatile, flammable, colorless liquid. It is a psychoactive drug, best known as the type of alcohol found in alcoholic beverages and in modern thermometers. Ethanol is one of the oldest recreational drugs...
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 H
2O 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 different names and acronyms including:-
- Phosphotungstic acid (PTA), (PWA)
- Tungstophosphoric acid (TPA)
- 12-phosphotungstic acid
- 12-tungstophosphoric acid (cited as the IUPAC standard in Cotton and Wilkinson, 2d edition, 1966)
- dodecatungstophosphoric acid
In the above the "12" or "dodeca" reflects the fact that the anion contains 12 tungsten atoms. Some early workers who did not know the structure, such as
Hsien WuHsien Wu was an early protein scientist who was the first to propose that protein denaturation was a purely conformational change, i.e., corresponded to protein unfolding and not to some chemical alteration of the protein...
, called it phospho-24-tungstic acid, formulating it as 3H
2O.P
2O
5 24WO
3.59H
2O, (P
2W
24O
80H
6).29H
2O, 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
histologyHistology is the study of the microscopic anatomy of cells and tissues of plants and animals. It is performed by examining a thin slice of tissue under a light microscope or electron microscope...
as a component for staining of cell specimens, often together with
haematoxylinHaematoxylin, hematoxylin, Natural Black 1, or C.I. 75290 is extracted from the wood of the logwood tree. When oxidized it forms haematein, a compound that forms strongly colored complexes with certain metal ions, notably Fe and Al salts. Metal-haematein complexes are used to stain cell nuclei...
as
PTAHIn Egyptian mythology, Ptah was the deification of the primordial mound in the Ennead cosmogony, which was more literally referred to as Ta-tenen , meaning risen land, or as Tanen, meaning submerged land, though Tatenen was a god in his own right before being assimilated with Ptah...
. It binds to
fibrinFibrin is a fibrous protein involved in the clotting of blood, and is non globular. It is a fibrillar protein that is polymerised to form a "mesh" that forms a hemostatic plug or clot over a wound site.Fibrin is made from fibrinogen, a soluble plasma glycoprotein that is synthesised by the liver...
,
collagenCollagen is the main protein of connective tissue in animals and the most abundant protein in mammals, making up about 25% to 35% of the whole-body protein content. It is naturally found exclusively in metazoa, including sponges. In muscle tissue it serves as a major component of endomysium...
, and fibres of
connective tissueConnective tissue is a form of fibrous tissue.. It is one of the four types of tissue in traditional classifications ....
s, and replaces the anions of dyes from these materials, selectively decoloring them.
Phosphotungstic acid is electron dense, opaque for
electronAn electron is a subatomic particle that carries a negative electric charge. It has no known substructure and is believed to be a point particle. An electron has a mass that is approximately 1836 times less than that of the proton. The intrinsic angular momentum of the electron is a half integer...
s. It is a common
negative stainNegative staining is an established method, often used in diagnostic microscopy, for contrasting a thin specimen with an optically opaque fluid....
for
virusA virus is an infectious agent too small to be seen directly with a light microscope. They are not made of cells and can only replicate inside the cells of another organism . Viruses infect all types of organisms, from animals and plants to bacteria and archaea...
es,
nerveA nerve is an enclosed, cable-like bundle of peripheral axons . A nerve provides a common pathway for the electrochemical nerve impulses that are transmitted along each of the axons. Nerves are found only in the peripheral nervous system...
s,
polysaccharidePolysaccharides are polymeric carbohydrate structures, formed of repeating units joined together by glycosidic bonds. These structures are often linear, but may contain various degrees of branching. Polysaccharides are often quite heterogeneous, containing slight modifications of the repeating unit...
s, and other biological tissue materials for imaging by a transmission electron microscope.
Structure
Gouzerh summarises the historical views on the structure of phosphotungstic acid leading up to Keggin's determination of the structure as:
- H7[P(W2O7)6] proposed by Miolati and further developed by Rosenheim
- H3[PO4W12O18(OH)36] (Pauling)
The structure was determined by J.F Keggin first published in 1933and then in 1934 and is generally known as the
Keggin structureKeggin 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 [XM12O40]n-, where X is the heteroatom , M is the addenda atom , and O represents...
. 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 tungstateSodium tungstate, Na2WO4, a tungstate of sodium, is useful as a source of tungsten. It is prepared from tungsten ores used to manufacture tungsten by reducing it....
, Na
2WO
4.2H
2O, with
phosphoric acidPhosphoric acid, also known as orthophosphoric acid or phosphoric acid, is a mineral acid having the chemical formula H3PO4. Orthophosphoric acid molecules can combine with themselves to form a variety of compounds which are also referred to as phosphoric acids, but in a more...
, H
3PO
4, acidified with
hydrochloric acidHydrochloric acid is the solution of hydrogen chloride in water. It is a highly corrosive, strong mineral acid and has major industrial uses. It is found naturally in gastric acid....
, HCl.
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:
| pH | principal components |
|---|
| 1.0 |
[PW12O40]3− |
| 2.2 |
[PW12O40]3−, [P2W21O71]6−, [PW11O39]7− |
| 3.5 |
[PW12O40]3−, [P2W21O71]6−, [PW11O39]7−, [P2W18O62]6−, [P2W19O67]10−
|
| 5.4 |
[P2W21O71]6−, [PW11O39]7−, [P2W18O62]6− |
| 7.3 |
PW9O349− |
| 8.3 |
PO43−, WO42− |
The species [PW
11O
39]
7− is a lacunary, or defective Keggin ion. The [P
2W
18O
62]
6− has a
Dawson structureIn chemistry, a polyoxometalate is a polyatomic ion, usually an anion, that consists of three or more transition metal oxyanions linked together by shared oxygen atoms to form a large, closed 3-dimensional framework....
. At pH less than 8, the presence of ethanol or acetone stabilises the anion, [PW
12O
40]
3−, reducing decomposition.
Tungstophosphoric acid is thermally stable up to 400°C, and is more stable than the analogous silicotungstic acid, H
4SiW
12O
40.
Large quantities of polar molecules such as pyridine are absorbed into the bulk phase and not simply on the surface. Solid state
NMRNMR may refer to:Applications of Nuclear Magnetic Resonance:* Nuclear Magnetic Resonance* NMR Spectroscopy* Solid-state nuclear magnetic resonance* Protein nuclear magnetic resonance spectroscopy* Proton NMR* Carbon-13 NMR...
studies of
ethanolEthanol, also called ethyl alcohol, pure alcohol, grain alcohol, or drinking alcohol, is a volatile, flammable, colorless liquid. It is a psychoactive drug, best known as the type of alcohol found in alcoholic beverages and in modern thermometers. Ethanol is one of the oldest recreational drugs...
absorbed in the bulk phase show that both protonated dimers, ((C
2H
5OH)
2H
+) and monomers, (C
2H
5OH
2+) are present.
Phosphotungstic acid is less sensitive to reduction than phosphomolybdic acid. Reduction with
uric acidUric acid is an organic compound of carbon, nitrogen, oxygen and hydrogen with the formula C
5H
4N
4O
3.-Biology:...
or
iron(II) sulfateIron sulfate or ferrous sulfate is the chemical compound with the formula , known since ancient times as copperas...
produces a brown coloured compound. the related silicotungstic acid when reduced forms a similar brown compound where one of the four W3 units in the Keggin structure becomes a metal-metal bonded cluster of three edge shared W(IV) octahedra.
Phosphotungstic acid is the strongest of heteropolyacids. Its conjugate base is the PW
12O
403− 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
H0The Hammett acidity function is a measure of acidity that is used for very concentrated solutions of strong acids, including superacids. In such solutions, simple approximations such as the Henderson-Hasselbalch equation are no longer valid due to the variations of the activity coefficients in...
=
−13.16, which would qualify the compound as a
superacidA superacid is an acid with an acidity greater than that of 100% pure sulfuric acid, which has a Hammett acidity function of −12. Commercially available superacids include trifluoromethanesulfonic acid , also known as triflic acid, and fluorosulfonic acid , both of which are about a thousand...
. This acidic strength means that even at low pH the acid is fully dissociated.
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 catalystHomogeneous catalysis is a chemistry term which describes catalysis where the catalyst is in the same state , as the reactants. It is the opposite to heterogeneous catalysis.Notice that two liquids can be different phases...
, and as a
heterogeneous catalystHeterogeneous catalysis is a chemistry term which describes catalysis where the catalyst is in a different phase to the reactants. Heterogeneous catalysts provide a surface on which the reaction may take place.In order for the reaction to occur, one or more of the reactants must diffuse to the...
"supported" on a substrate e.g.
aluminaAluminium oxide is an amphoteric oxide of aluminium with the chemical formula
23. It is also commonly referred to as alumina, corundum, sapphire, ruby or aloxite in the mining, ceramic and materials science communities. It is produced by the Bayer process from bauxite...
,
silicaThe chemical compound silicon dioxide, also known as silica , is an oxide of silicon with a chemical formula of ' and has been known for its hardness since antiquity. Silica is most commonly found in nature as sand or quartz, as well as in the cell walls of diatoms...
. Some acid catalysed reactions include:
- the homogeneous catalysis
Homogeneous catalysis is a chemistry term which describes catalysis where the catalyst is in the same state , as the reactants. It is the opposite to heterogeneous catalysis.Notice that two liquids can be different phases...
of the hydrolysis of propene to give 2-propanol
- the homogeneous catalysis
Homogeneous catalysis is a chemistry term which describes catalysis where the catalyst is in the same state , as the reactants. It is the opposite to heterogeneous catalysis.Notice that two liquids can be different phases...
of the Prins reactionThe 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 . The outcome of the reaction depends on reaction conditions . With water and a protic acid such as sulfuric acid as the reaction...
- the heterogeneous catalysis
Heterogeneous catalysis is a chemistry term which describes catalysis where the catalyst is in a different phase to the reactants. Heterogeneous catalysts provide a surface on which the reaction may take place.In order for the reaction to occur, one or more of the reactants must diffuse to the...
of the dehydration of 2-propanol to propene and methanolMethanol, also known as methyl alcohol, wood alcohol, wood naphtha or wood spirits, is a chemical with formula CH3OH . It is toxic: drinking 10 ml will cause blindness, and as little as 100 ml will cause death...
to hydrocarbons.
Dyeing and pigments
Phosphotungstic acid has been used to precipitate different types of dyes as "
lakesA Lake pigment is a pigment manufactured by precipitating a dye with an inert binder, usually a metallic salt. Manufacturers and suppliers to artists and industry frequently omit the lake designation in the name...
". Examples are basic dyes and triphenylmethane dyes, e.g.
pararosanilinePararosaniline, Magenta 0, Basic Red 9, or C.I. 42500 is a magenta dye having chemical formula 19183. It is one of the four components of basic fuchsine. Pararosaniline, which is sold as a single dye, may make the best Schiff's reagent...
derivatives.
Histology
Phosphotungstic acid is used in
histologyHistology is the study of the microscopic anatomy of cells and tissues of plants and animals. It is performed by examining a thin slice of tissue under a light microscope or electron microscope...
for staining specimens, as a component of phosphotungstic acid haematoxylin,
PTAHIn Egyptian mythology, Ptah was the deification of the primordial mound in the Ennead cosmogony, which was more literally referred to as Ta-tenen , meaning risen land, or as Tanen, meaning submerged land, though Tatenen was a god in his own right before being assimilated with Ptah...
, and “trichrome” reagents, and as a
negative stainNegative staining is an established method, often used in diagnostic microscopy, for contrasting a thin specimen with an optically opaque fluid....
for imaging by a transmission electron microscope.
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. Interestingly 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 e.g. amino acid
Amino acids are molecules containing an amine group, a carboxylic acid group and one of the twenty R-groups. These molecules are particularly important in biochemistry, where this term refers to alpha-amino acids with the general formula H
2NCHRCOOH, where R is an organic substituent...
s, purineA purine is a heterocyclic aromatic organic compound, consisting of a pyrimidine ring fused to an imidazole ring. Purines, including substituted purines and their tautomers, are the most widely distributed kind of nitrogen-containing heterocycle in nature....
s, pyrimidinePyrimidine is a heterocyclic aromatic organic compound similar to benzene and pyridine, containing two nitrogen atoms at positions 1 and 3 of the six-member ring...
s and amineAmines are organic compounds and functional groups that contain a basic nitrogen atom with a lone pair. Amines are derivatives of ammonia, wherein one or more hydrogen atoms have been replaced by a substituent such as an alkyl or aryl group. Important amines include amino acids, biogenic amines,...
s. with PTAHIn Egyptian mythology, Ptah was the deification of the primordial mound in the Ennead cosmogony, which was more literally referred to as Ta-tenen , meaning risen land, or as Tanen, meaning submerged land, though Tatenen was a god in his own right before being assimilated 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 is a substance used to set dyes on fabrics or tissue sections 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 cell or tissue preparations. A mordant is always a polyvalent metal ion...
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 negative stain
Negative staining is an established method, often used in diagnostic microscopy, for contrasting a thin specimen with an optically opaque fluid....
. This electron density arises from the presence of the 12 tungsten atoms which each have an atomic numberIn chemistry and physics, the atomic number is the number of protons found in the nucleus of an atom and therefore identical to the charge number of the nucleus. It is conventionally represented by the symbol Z. The atomic number uniquely identifies a chemical element...
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 analysisGravimetric analysis describes a set of methods in analytical chemistry for the quantitative determination of an analyte based on the mass of a solid...
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 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 membraneA proton exchange membrane or polymer electrolyte membrane is a semipermeable membrane generally made from ionomers and designed to conduct protons while being impermeable to gases such as oxygen or hydrogen...
s, such as
NafionNafion is a sulfonated tetrafluoroethylene based fluoropolymer-copolymer discovered in the late 1960s by Walther Grot of DuPont. It is the first of a class of synthetic polymers with ionic properties which are called ionomers...
. The interest lies in the potential of these composite materials in the manufacture of fuel cells as they have improved operating characteristics.