Ammonium heptamolybdate

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Ammonium heptamolybdate
Heptamolybdenan amonný.JPG
Names
IUPAC name
Ammonium docosaoxoheptamolybdate(6–)
Other names
Ammonium molybdate
Ammonium paramolybdate
(see text)
Identifiers
3D model (Jmol)
ChemSpider
ECHA InfoCard 100.031.553
EC Number 234-320-9
UNII
Properties
(NH4)6Mo7O24
Molar mass 1163.9 g/mol
1235.86 g/mol (tetrahydrate)
Appearance white to yellow-green crystalline solid
Density 2.498 g/cm3
Melting point ~90 ˚C (loses water molecule)
190 °C (decomp.)
43 g/100 ml (tetrahydrate)
Hazards
Main hazards Irritant
Safety data sheet External MSDS
NFPA 704
Flammability code 0: Will not burn. E.g., water Health code 2: Intense or continued but not chronic exposure could cause temporary incapacitation or possible residual injury. E.g., chloroform Reactivity code 0: Normally stable, even under fire exposure conditions, and is not reactive with water. E.g., liquid nitrogen Special hazards (white): no codeNFPA 704 four-colored diamond
Flash point Non-flammable
Related compounds
Other anions
Ammonium orthomolybdate
Ammonium dimolybdate
Other cations
Potassium paramolybdate
Related compounds
Molybdenum(VI) oxide
Molybdic acid
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

Ammonium heptamolybdate is the inorganic compound, normally encountered as the tetrahydrate, whose chemical formula is (NH4)6Mo7O24·4H2O. It is a colorless solid, often referred to as ammonium paramolybdate or simply as ammonium molybdate, although "ammonium molybdate" can also refer to ammonium orthomolybdate, (NH4)2MoO4, and several other compounds. Potassium heptamolybdate, also obtained as the tetrahydrate, is very similar to the ammonium salt.[1]

Synthesis[edit]

Ammonium heptamolybdate is easily prepared by dissolving molybdenum trioxide in an excess of aqueous ammonia and evaporating the solution at room temperature. While the solution evaporates, the excess of ammonia escapes. This method results in the formation of six-sided transparent prisms of the tetrahydrate of ammonium heptamolybdate.[2]

Solutions of ammonium paramolybdate react with acids to form molybdic acid and an ammonium salt. The pH value of a concentrated solution will lie between 5 and 6.

Structure[edit]

The compound was first analyzed crystallographically by Lindqvist, but has been reanalyzed.[1] All Mo centers are octahedral. Some oxide ligands are terminal, some doubly bridging, and a few are triply bridging ligands.

The salt contains the heptamolybdate hexaanion.

Uses[edit]

  • as an analytical reagent to measure the amount of phosphates, silicates, arsenates and lead in aqueous solution (e.g. pigments, river water, sea water etc.)[3]
  • in the production of molybdenum metal and ceramics
  • in the preparation of dehydrogenation and desulphurisation catalysts
  • in the fixing of metals
  • in electroplating
  • in fertilizers for crops.
  • as a negative stain in biological electron microscopy, typically in the 3–5% (vol/vol) concentration range and in the presence of trehalose;[4] or at saturated concentration to perform cryo-negative staining.[5][6]
  • For the detection of recreational drugs as a component of the froehde reagent

Safety[edit]

Ammonium paramolybdate is harmful if swallowed or inhaled. It causes irritation to the eyes, skin, and respiratory tract. It affects kidneys and blood.

References[edit]

  1. ^ a b Evans, H.T., Jr.; Gatehouse, B. M.; Leverett, P. "Crystal Structure of the Heptamolybdate(VI) (paramolybdate) ion, (Mo7O24)6−, in the ammonium and potassium tetrahydrate salts" Journal of the Chemical Society. Dalton Transactions, Inorganic Chemistry1975, p.505-p514.
  2. ^ L. Svanberg & H. Struve, J. pr. Ch. 44 [1848], p. 282; cited in Gmelin's Handbuch für Anorganische Chemie, 53, p. 255.
  3. ^ Parsons, T.; Maita, V. & Lalli, C. (1984). A manual of chemical and biological methods for seawater analysis. Oxford: Pergamon.
  4. ^ Harris, J. R. and Horne, R. W. 1991. "Negative staining", in Harris J. R. (Ed.), Electron Microscopy in Biology, Oxford University Press, Oxford.
  5. ^ Adrian, M.; Dubochet, J.; Fuller, S. D. and Harris, J. R. 1998. "Cryo-negative Staining". Micron 29, p. 145–160; doi:10.1016/S0968-4328(97)00068-1.
  6. ^ De Carlo, S.; El-Bez, C.; Alvarez-Rúa, C.; Borge, J. and Dubochet, J. 2002. "Cryo-negative staining reduces electron-beam sensitivity of vitrified biological particles". J. Struct. Biol. 138, p. 216–226; doi:10.1016/S1047-8477(02)00035-7; PMID 12217660.

See also[edit]

Phosphate test aka Deniges' method links to here.