Skip to main content

Ammonium Heptamolybdate Tetrahydrate

(NH4)6Mo7O24·4H2O salt
Molar Mass
1235.860 g/mol
Calculate in Molar Mass Calculator →

Properties

StateSolid
ColorColorless to pale yellow-green
SolubilityVery soluble in water (430 g/L at 20 °C); insoluble in ethanol
Melting Point90 °C (decomposes to release NH3 + H2O)

About Ammonium Heptamolybdate Tetrahydrate

Ammonium heptamolybdate tetrahydrate looks like a dense, almost-colorless crystal but its formula tells a more interesting story: the polyatomic anion is a seven-octahedron polyoxometalate cluster about 8 Å across, made up of edge- and corner-sharing MoO6 units that arrange into a compact ellipsoidal shape. The cluster, [Mo7O24]⁶⁻, is the dominant form of molybdate(VI) in solution between roughly pH 3 and 6. Above pH 6 the cluster falls apart into the simple tetrahedral MoO4²⁻ orthomolybdate; below pH 2 it reorganizes into octa- and dodecamolybdate species, ultimately precipitating as MoO3·H2O. That pH-dependent speciation is one of the cleanest demonstrations of polyoxometalate chemistry available — early-transition-metal d⁰ cations like Mo(VI), W(VI), and V(V) all show analogous behavior, and the resulting clusters drive a substantial fraction of supported-catalyst preparation chemistry. The single most-used analytical application is the phosphomolybdate-blue method for phosphate quantification: phosphate forms a heteropolyanion with twelve molybdates around it, mild reduction generates a deep-blue mixed-valence Mo(V)/Mo(VI) species that absorbs at 880 nm, and the absorbance maps quantitatively onto phosphate concentration. The same chemistry, in slightly modified form, supports silicate, arsenate, and germanate analysis.

Where you'll encounter it

If you've run a Murphy-Riley phosphate analysis in environmental or clinical chemistry — EPA Method 365.1 or any of its variants — ammonium molybdate is the active reagent. The procedure is simple enough to teach in an undergraduate analytical lab: acidified molybdate plus phosphate plus ascorbic acid gives the blue color in about ten minutes. The same compound is the standard precursor for industrial molybdenum-disulfide (MoS2) hydrodesulfurization catalysts used in petroleum refining; refineries impregnate alumina supports with aqueous AHM, calcine to MoO3, then sulfide in H2S to give the active MoS2 phase. In agriculture, molybdenum is a trace nutrient required by nitrogen-fixing legume symbionts (the nitrogenase active site contains a Mo cofactor), and AHM-coated seed treatments at gram-per-hectare doses are sometimes the cheapest way to correct molybdenum-deficient acidic soils.

Common Uses

  • Phosphomolybdate-blue colorimetric phosphate determination
  • Precursor for MoS2 hydrodesulfurization catalyst preparation
  • Mo trace-nutrient seed treatment for legumes and brassicas
  • Cooling-water and ceramic-glaze corrosion inhibitor
  • Heteropoly-acid synthesis for selective oxidation catalysis

Safety Information

Chronic-inhalation respiratory toxicant — repeated exposure to airborne dust causes 'molybdenosis' with cough, fatigue, and disturbed copper metabolism. The OSHA PEL is 5 mg/m³ for soluble molybdenum compounds, achievable with standard fume-hood handling but not with open-bench powder transfers. Eye irritant on direct contact. GHS H319, H372. Wear N95 dust respirator when scooping the dry solid.

This safety summary is for educational reference only and may not be complete. It is not a substitute for Safety Data Sheets (SDS), medical advice, or professional chemical safety guidance. Always consult appropriate SDS and qualified professionals before handling chemicals.

Constituent Elements

N — Nitrogen H — Hydrogen Mo — Molybdenum O — Oxygen

Frequently Asked Questions

What is the molar mass of ammonium heptamolybdate tetrahydrate?
1235.86 g/mol for the tetrahydrate, 1163.80 g/mol for the anhydrous form. The hydrated salt is what ships commercially, so check the bottle label before doing molarity calculations. Sum 6(NH4) at 18.04, 7(Mo) at 95.95, 24(O) at 15.999, plus 4(H2O) at 18.015, giving 108.23 + 671.65 + 383.98 + 72.06 = 1235.92 (the small rounding difference from 1235.86 reflects which Mo and N atomic-mass standards a given source uses).
Why does molybdate form polynuclear clusters?
Mo(VI) is a small (~0.6 Å), highly charged d⁰ cation that strongly prefers tetrahedral or octahedral oxo coordination. In basic solution the deprotonated tetrahedral MoO4²⁻ dominates, with each Mo holding four terminal oxo ligands and no protons available for condensation. As pH drops, sequential protonation gives HMoO4⁻ and H2MoO4 species, and the protonated oxos start sharing across metal centers — first edges, then corners — to build up the [Mo7O24]⁶⁻ cluster around pH 3–6. The same pattern shows up in W(VI) and V(V) chemistry, with cluster size and shape varying by metal.
How is the phosphomolybdate blue test used for phosphate?
Phosphate condenses with twelve molybdates in dilute acid to form 12-phosphomolybdic heteropolyanion [PMo12O40]³⁻. Mild reduction with ascorbic acid (or stannous chloride for higher sensitivity) reduces some of the Mo(VI) centers to Mo(V), giving the intensely blue mixed-valence species that absorbs strongly at 880 nm. Absorbance is linear in phosphate concentration up to several mg/L, the chemistry tolerates a wide range of typical interferents, and the method is robust enough that it remains the standard phosphate assay in environmental and clinical labs decades after its development.