Skip to main content

Ammonium Metavanadate

NH4VO3 salt
Molar Mass
116.978 g/mol
Calculate in Molar Mass Calculator →

Properties

StateSolid
ColorPale yellow to off-white
SolubilitySlightly soluble in cold water; more soluble in hot water and ammonia
Melting Point200 °C (decomposes to V2O5)

About Ammonium Metavanadate

Ammonium metavanadate is the standard commercial source of vanadium(V), and most of what gets done with vanadium in a lab starts with this pale-yellow salt. The most common transformation is also the simplest: heat NH4VO3 in air at 200–400 °C and it decomposes to vanadium pentoxide (V2O5) plus ammonia and water — the textbook industrial route to V2O5, which is itself the active component of the catalyst in the contact process for sulfuric acid manufacture and the starting material for vanadium redox-flow battery electrolytes. The aqueous chemistry of vanadate is a richer story than the simple formula suggests. V(V) is a small, highly charged d⁰ cation that behaves much like P(V) in forming oxoanions: in strongly basic solution the dominant species is the orthovanadate VO4³⁻; below pH 13 the protonation cascade gives HVO4²⁻, then H2VO4⁻; in mildly acidic solution (pH 2–6) the chemistry shifts dramatically as polyvanadates form, with the bright orange decavanadate cluster [V10O28]⁶⁻ appearing as the dominant species and giving the characteristic orange color of acidic vanadate solutions; below pH 2 the pervanadyl cation VO2⁺ takes over. That pH-dependent speciation is one of the cleanest examples of polyoxometalate chemistry in early-transition-metal aqueous systems, and it's the basis for several analytical methods that exploit the visible-color shifts during titration.

Where you'll encounter it

If you work on vanadium redox-flow batteries — currently a leading candidate for grid-scale stationary energy storage — ammonium metavanadate is the starting material for preparing the V²⁺/V³⁺ and VO²⁺/VO2⁺ electrolyte couples. The compound is also the standard analytical reagent in the Walkley–Black method for soil organic-carbon determination, where vanadate catalyzes dichromate oxidation of organic matter and the unconsumed dichromate is back-titrated to give carbon content. In an inorganic teaching lab, ammonium metavanadate is the cleanest way to demonstrate vanadate-pH speciation: a single solid dissolved in water at varying pH gives a striking color sequence from yellow at high pH through orange (decavanadate) to brown (mixed species) to pale yellow (VO2⁺) at low pH.

Common Uses

  • Precursor for V2O5 used in contact-process H2SO4 catalysts
  • Starting material for vanadium-redox-flow-battery electrolyte preparation
  • Walkley–Black soil-carbon analysis catalyst
  • Aniline-black textile-dye mordant in traditional formulations
  • Visible-color reference for polyoxovanadate teaching demonstrations

Safety Information

Vanadium compounds are respiratory toxicants — chronic inhalation produces 'vanadium-tongue' (a green-stained tongue) and progressive bronchitis at exposures well below acute toxicity. The OSHA PEL is 0.05 mg/m³ measured as V2O5 dust, low enough that any dry handling needs respiratory PPE. Acute oral toxicity is moderate (Category 3); skin and eye contact are irritating. Aquatic toxicity is significant, so spill cleanup must avoid drain disposal. GHS H301, H315, H319, H332, H372, H410.

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 V — Vanadium O — Oxygen

Frequently Asked Questions

What is the molar mass of ammonium metavanadate?
116.978 g/mol. Sum 14.007 for the nitrogen, 4(1.008) for the four hydrogens, 50.942 for the vanadium, and 3(15.999) for the three oxygens, giving 116.98. The number is useful for redox-flow-battery electrolyte preparation, where 1 M vanadium solutions are prepared by dissolving 116.98 g/L of NH4VO3 in sulfuric acid and reducing electrolytically to the V(IV) and V(III) working states.
How is V2O5 made from NH4VO3?
Thermal decomposition in air: 2 NH4VO3 → V2O5 + 2 NH3 + H2O, run at 200–400 °C in an open crucible or rotary kiln. The reaction goes essentially to completion above 250 °C and gives an orange-brown solid that's pure enough for catalytic applications without further purification. Industrial production for sulfuric-acid contact-process catalyst uses exactly this route, with the resulting V2O5 supported on silica or kieselguhr and promoted with potassium sulfate.
Why does aqueous vanadate chemistry depend on pH?
V(V) is small (~0.5 Å), highly charged, and d⁰, which makes it electrophilic at the metal center and prone to forming oxo bridges through condensation. At high pH (>13) the deprotonated orthovanadate VO4³⁻ dominates because there are no protons to drive condensation. As pH drops, sequential protonation (HVO4²⁻, H2VO4⁻) precedes condensation; below pH 6 the protonated species condense via μ-O bridges into the decavanadate cluster [V10O28]⁶⁻, and below pH 2 the cluster breaks apart into the small pervanadyl ion VO2⁺. The whole sequence is driven by the same forces that drive phosphate condensation chemistry, just at a different pH range.