Ammonium Orthomolybdate – an Inorganic Compound

The inorganic compound ammonium orthomolybdate has the chemical formula (NH4)2MoO4. It is a white substance formed by reacting molybdenum trioxide with aqueous ammonia. Ammonia is lost when these solutions are heated, yielding ammonium heptamolybdate ((NH4)6Mo7O24.4H2O). It is employed in the detection of phosphorus.

It was utilized as a quantitative reagent in the analysis of phosphates, silicates, arsenates, and lead in aqueous solutions. (NH4)2MoO4 was utilized to perform cryo-negative staining and as a negative stain in biological electron microscopy at saturated concentrations. The crystallization of cell-free-expressed membrane proteins was studied using ammonium molybdate.

Properties

The compound is a white solid that is prepared by treating molybdenum trioxide with aqueous ammonia.

  • Chemical formula: (NH4)2MoO4
  • Molar mass: 196.02 g/mol
  • Appearance: colorless monoclinic crystals
  • Density: 2.276 g/cm3
  • Melting point: decomposes upon heating
  • Solubility in water: soluble
  • Solubility: insoluble in alcohol and liquid ammonia

Chemical reactions

Heating ammonium orthomolybdate solid or treatment with acid gives molybdenum trioxide. Such reactions proceed via ammonium dimolybdate. This equilibrium is exploited in the purification of molybdenum from its ores. Aqueous solutions of ammonium orthomolybdate react with hydrogen sulfide to give ammonium tetrathiomolybdate:

(NH4)3MoO4 + 4 H2S → (NH4)2MoS4 + 4 H2O

It reacts with arsenic acid upon heating to form a canary yellow precipitate of ammonium α-Keggin molybdoarsenate.

(NH4)2MoO4 + H3AsO4 → (NH4)2[As(Mo3O10)4] + 21NH4NO3 + 12H2O

The organic compound Ammonium orthomolybdate is treated with sodium peroxide(Na2O2). Phosphorus gets converted to sodium phosphate.

  • The aqueous extract is heated with concentrated nitric acid (HNO3) and ammonium molybdate {(NH4)2MO4)}.
  • A canary yellow precipitate of ammonium phosphomolybdate is collected.
  • This yellow precipitate formed shows that Phosphorus is present in the organic compound.

Reaction that take place during the process

In the first step, the reaction takes place as follows

Na3PO4+3HNO3⟶H3PO4+3NaNO3

In the second step, the reaction takes place as follows

H3PO4+12(NH4)2MoO4+21HNO3⟶(NH4)3PO4⋅12MoO3+ 21NH4NO3+12N2O

(NH4)3PO4⋅12MoO3 is the yellow precipitate formed.

Uses

Ammonium orthomolybdate is employed as a corrosion inhibitor and as an intermediary in various molybdenum extraction systems. It is also employed in ceramic decoration and colorimetric examination of phosphates and arensates.