Phosphotungstic acid

Phosphotungstic acid (PTA), tungstophosphoric acid (TPA), is a heteropoly acid with the chemical formula H3PW12O40. It is normally present as a hydrate. EPTA is the name of ethanolic 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 H2O 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 (see 'other names' section of infobox).

Phosphotungstic acid
Names
Other names
Tungstophosphoric acid (TPA)
Phosphotungstic acid (PTA, PWA) 12-Phosphotungstic acid
12-Tungstophosphoric acid[1]
Dodecatungstophosphoric acid
Identifiers
3D model (JSmol)
ECHA InfoCard 100.108.885
UNII
Properties
H3PW12O40
Molar mass 2880.2 g/mol (anhydrous)
Melting point 89 °C (192 °F; 362 K) (hydrate)
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

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.[2] called it phospho-24-tungstic acid, formulating it as 3H2O·P2O5 24WO3·59H2O, (P2W24O80H6)·29H2O, which correctly identifies the atomic ratios of P, W and O. This formula was still quoted in papers as late as 1970.[3]

Phosphotungstic acid is used in histology as a component for staining of cell specimens, often together with haematoxylin as PTAH. It binds to fibrin, collagen, and fibres of connective tissues, and replaces the anions of dyes from these materials, selectively decoloring them.

Phosphotungstic acid is electron dense, opaque for electrons. It is a common negative stain for viruses, nerves, polysaccharides, and other biological tissue materials for imaging by a transmission electron microscope.

Structure
Structure of the phosphotungstate anion

Gouzerh[4] 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 1933[5] and then in 1934[6] and is generally known as the Keggin structure. 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.[7]

Preparation and chemical properties

Phosphotungstic acid can be prepared by the reaction of sodium tungstate, Na2WO4·2H2O, with phosphoric acid, H3PO4, acidified with hydrochloric acid, HCl.[8]

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:[9]

pHprincipal 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[PW9O34]9−
8.3PO43−, WO42−

The species [PW11O39]7− is a lacunary, or defective Keggin ion. The [P2W18O62]6− has a Dawson structure. At pH less than 8, the presence of ethanol or acetone stabilises the anion, [PW12O40]3−, reducing decomposition.[9]

Tungstophosphoric acid is thermally stable up to 400 °C, and is more stable than the analogous silicotungstic acid, H4SiW12O40.[10]

Large quantities of polar molecules such as pyridine are absorbed into the bulk phase and not simply on the surface. Solid state NMR studies of ethanol absorbed in the bulk phase show that both protonated dimers, ((C2H5OH)2H+) and monomers, (C2H5OH2+) are present.

Phosphotungstic acid is less sensitive to reduction than phosphomolybdic acid. Reduction with uric acid or iron(II) sulfate 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.[11]

Phosphotungstic acid is the strongest of heteropolyacids. Its conjugate base is the PW12O403− anion.[12] 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.[13] One estimate of the acidity is that the solid has an acidity stronger than H0 =−13.16,[10] which would qualify the compound as a superacid. 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.[14] It is in solution as a homogeneous catalyst, and as a heterogeneous catalyst "supported" on a substrate e.g. alumina, silica. Some acid catalysed reactions include:

Dyeing and pigments

Phosphotungstic acid has been used to precipitate different types of dyes as "lakes".[15] Examples are basic dyes and triphenylmethane dyes, e.g. pararosaniline derivatives.[16]

Histology

Phosphotungstic acid is used in histology for staining specimens, as a component of phosphotungstic acid haematoxylin, PTAH, and “trichrome” reagents, and as a negative stain for imaging by a transmission electron microscope.

Phosphotungstic acid haematoxylin (PTAH)
Mallory described the reagent now generally known as PTAH in 1897.[17] 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.[18] 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.[19] These were not identified. Some investigations of “model” systems, reacting various compounds such as amino acids, purines, pyrimidines and amines with PTAH show that they give rise to different colours.[20]
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 to bind the dye to the tissue[21] or that alternatively it binds to tissue blocking it to dye molecules.[22]
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. This electron density arises from the presence of the 12 tungsten atoms which each have an atomic number 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.[3]

Analysis

The potassium salt is only slightly soluble, unlike most other phosphotungstate salts, and has been proposed as a method for the gravimetric analysis of potassium.[23]

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.[24] Further studies showed that no precipitation occurred with α-amino groups but did occur with guanidino, ε-amino and imidazole groups.[25]

Medicinal

Very little work appears to have been carried out in this area. One example relates to liver necrosis in rats.[26]

Composite proton exchange membranes

The heteropoly acids, including phosphotungstic acid, are being investigated as materials in composite proton exchange membranes, such as Nafion. The interest lies in the potential of these composite materials in the manufacture of fuel cells as they have improved operating characteristics.[27]

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See also

References
  1. Cotton FA, Wilkinson G (1966). Advanced inorganic chemistry: a comprehensive text (2nd ed.). New York: Wiley. ISBN 0470175583. LCCN 66020662. OCLC 85904497.
  2. Hsein W (1920). "Contribution to the chemistry of phosphomolybdic acids, phosphotungstic acids and allied substances". Journal of Biological Chemistry. 43: 189–220.
  3. Quintarelli G, Zito R, Cifonelli JA (November 1971). "On phosphotungstic acid staining. I". The Journal of Histochemistry and Cytochemistry. 19 (11): 641–7. doi:10.1177/19.11.641. PMID 4107745.
  4. Gouzerh P, Che M (2006). "From Scheele and Berzelius to Müller Polyoxometalates (POMs) revisited and the "missing link" between the bottom up and top down approaches". L'Actualité Chimique. 298: 9–22 – via Semantic Scholar.
  5. Keggin JF (1933). "Structure of the Molecule of 12-Phosphotungstic Acid". Nature. 131 (3321): 908–909. doi:10.1038/131908b0. ISSN 0028-0836.
  6. Keggin JF (1934). "The structure and formula of 12-phosphotungstic acid". Proceedings of the Royal Society A. 144 (851): 75–100. Bibcode:1934RSPSA.144...75K. doi:10.1098/rspa.1934.0035. ISSN 0950-1207.
  7. Brown GM, Noe-Spirlet MR, Busing WR, Levy HA (1977). "Dodecatungstophosphoric acid hexahydrate, (H5O2+)3(PW12O403−). The true structure of Keggin's 'pentahydrate' from single-crystal X-ray and neutron diffraction data". Acta Crystallographica Section B. 33 (4): 1038–1046. doi:10.1107/s0567740877005330. ISSN 0567-7408.
  8. Bailar, Jr., John C.; Booth, H. S.; Grennert, M. (1939). "Phosphotungstic Acid". Inorganic Syntheses. 1: 132–133. doi:10.1002/9780470132326.ch49. ISBN 9780470132326.
  9. Zhu Z, Tain R, Rhodes C (2003). "A study of the decomposition behaviour of 12-tungstophosphate heteropolyacid in solution". Canadian Journal of Chemistry. 81 (10): 1044–1050. doi:10.1139/v03-129.
  10. Oxide catalysts in solid state chemistry T Okuhara, M Misono Encyclopedia of Inorganic chemistry Editor R Bruce King (1994) John Wiley and Sons ISBN 0-471-93620-0
  11. Polyoxoanions M.T.Pope, Encyclopedia of Inorganic Chemistry Editor R Bruce King (1994) John Wiley and Sons ISBN 0-471-93620-0
  12. Acid Catalysis Archived 2011-08-11 at the Wayback Machine, Davis Group, Department of Chemical Engineering, University of Virginia. Retrieved 2009-06-02.
  13. Farcasiu D, Li JQ (March 1995). "Acidity measurements on a heteropolyacid hydrate in acetic acid solution: a case of three hydrons ionizing independently, rather than consecutively". Journal of Catalysis. 152 (1): 198–203. doi:10.1006/jcat.1995.1073.
  14. Devassy BM, Lefebvre F, Halligudi SG (2005). "Zirconia-supported 12-tungstophosphoric acid as a solid catalyst for the synthesis of linear alkyl benzenes". Journal of Catalysis. 231 (1): 1–10. doi:10.1016/j.jcat.2004.09.024. ISSN 0021-9517.
  15. Non-staining pigments and their use US patent: 2999026 Issue date: Sep 1961, Inventor: Chester Davis
  16. Hunger K, Herbst W (2000). "Pigments, Organic". In Bohnet M (ed.). Ullmann's encyclopedia of industrial chemistry (6 ed.). Weinheim, Germany: Wiley InterScience. doi:10.1002/14356007.a20_371. ISBN 9783527306732. OCLC 751968805.
  17. Mallory FB (September 1897). "On certain improvements in histological technique: I. A differential stain for amoeligbæ coli. II. phosphotungstic-acid-hæmatoxylin stain for certain tissue elements. III. A method of fixation for neuroglia fibres". The Journal of Experimental Medicine. 2 (5): 529–33. doi:10.1084/jem.2.5.529. PMC 2132861. PMID 19866848.
  18. Terner JY, Gurland J, Gaer F (May 1964). "Phosphotungstic acid-hematoxylin; spectrophotometry of the lake in solution and in stained tissue". Stain Technology. 39 (3): 141–53. doi:10.3109/10520296409061220. PMID 14157455.
  19. Puchtler H, Waldrop FS, Meloan SN (1980). "On the mechanism of Mallory's phosphotungstic acid-haematoxylin stain". Journal of Microscopy. 119 (3): 383–390. doi:10.1111/j.1365-2818.1980.tb04109.x. ISSN 0022-2720. PMID 6157822.
  20. Terner JY (April 1966). "Phosphotungstic acid-hematoxylin, reactivity in vitro". The Journal of Histochemistry and Cytochemistry. 14 (4): 345–51. doi:10.1177/14.4.345. PMID 4164215.
  21. Kiernan JA (2006). "Dyes and other colorants in microtechnique and biomedical research". Color. Technol. 122 (1): 1–21. doi:10.1111/j.1478-4408.2006.00009.x. ISSN 1472-3581.
  22. Everett MM, Miller WA (1974). "The role of phosphotungstic and phosphomolybdic acids in connective tissue staining I. Histochemical studies". The Histochemical Journal. 6 (1): 25–34. doi:10.1007/bf01011535. PMID 4130630.
  23. Rieben WK, Van Slyke DD (1944). "Gravimetric determination of potassium as phospho-12-tungstate". Journal of Biological Chemistry. 156: 765–76.
  24. Scott JE (November 1971). "Phosphotungstate: a "universal" (nonspecific) precipitant for polar polymers in acid solution". The Journal of Histochemistry and Cytochemistry. 19 (11): 689–91. doi:10.1177/19.11.689. PMID 5121870.
  25. Sternberg MZ (January 1970). "The separation of proteins with heteropolyacids". Biotechnology and Bioengineering. 12 (1): 1–17. doi:10.1002/bit.260120102. PMID 5433882.
  26. Uskoković-Marković S, Milenković M, Topić A, Kotur-Stevuljević J, Stefanović A, Antić-Stanković J (2007). "Protective effects of tungstophosphoric acid and sodium tungstate on chemically induced liver necrosis in wistar rats". Journal of Pharmacy & Pharmaceutical Sciences. 10 (3): 340–9. PMID 17727797.
  27. Alberti G, Casciola M (2003). "Composite Membranes for Medium-Temperature PEM Fuel Cells". Annual Review of Materials Research. 33 (1): 129–154. Bibcode:2003AnRMS..33..129A. doi:10.1146/annurev.matsci.33.022702.154702. ISSN 1531-7331.
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