Ammonium Molybdate Tetrahydrate, Ammonium Heptamolybdate Tetrahydrate, Ammonium Paramolybdate Tetrahydrate, 12054-85-2

Ammonium Molybdate Tetrahydrate, Ammonium Heptamolybdate Tetrahydrate, Ammonium Paramolybdate Tetrahydrate, 12054-85-2

Ammonium Molybdate Tetrahydrate

Molecular Formula: (NH₄)₆Mo₇O₂₄ • 4H₂O 
CAS Number: 12054-85-2 
Molecular Weight: 1235.86 g/mol 

General Information and Description

Ammonium Molybdate Tetrahydrate, often referred to as Ammonium Heptamolybdate Tetrahydrate or Ammonium Paramolybdate, is the most common hydrated form of Ammonium Molybdate . It is a white, crystalline solid that is highly soluble in water. Chemically, it is a complex salt containing the heptamolybdate ion (Mo₇O₂₄⁶⁻), six ammonium ions (NH₄⁺), and four water molecules of hydration. It serves as a primary source of molybdenum in various industrial, agricultural, and research applications .

It is distinct from simple molybdates due to its polymeric structure, which forms when molybdenum trioxide (MoO₃) is dissolved in excess ammonia and crystallized .

Other Names (Synonyms)

• Ammonium Heptamolybdate Tetrahydrate 

• Ammonium Paramolybdate Tetrahydrate 

• Molybdic Acid Ammonium Salt Tetrahydrate 

• Hexaammonium Heptamolybdate Tetrahydrate 

Production Method

Ammonium Molybdate Tetrahydrate is typically produced through a hydrometallurgical process. It is manufactured by dissolving molybdenum trioxide (MoO₃) or roasted molybdenite concentrate in an excess of aqueous ammonia solution . The resulting solution is then evaporated at room temperature, allowing the pure tetrahydrate salt to crystallize out . The overall reaction can be simplified as:
7 MoO₃ + 6 NH₄OH + H₂O → (NH₄)₆Mo₇O₂₄ • 4H₂O

Physical and Chemical Properties

• Physical Appearance: White to slightly yellow or green crystalline powder .

• Odor: Odorless .

• pH Value: An aqueous solution is slightly acidic, with a pH typically ranging from 4.0 to 5.5 .

• Solubility: Highly soluble in water (approx. 400 g/L at 20°C). It is insoluble in alcohol .

• Density: 2.498 g/mL at 25°C .

• Thermal Stability (Melting/Decomposition): It loses its water of crystallization around 90-100°C . It decomposes upon further heating to 190°C and above, eventually forming molybdenum trioxide (MoO₃) .

• Purity: Available in various grades, from technical grade (~99%) to high-purity ACS reagent grade (81.0-83.0% as MoO₃, or 99.98% trace metals basis) .

Applications and Uses

Thanks to its high solubility and molybdenum content, Ammonium Molybdate Tetrahydrate is utilized across a wide range of fields .

1. Agriculture (Micronutrient Fertilizer):

   • It is used to correct molybdenum deficiency in soils. Molybdenum is an essential trace element for plants, particularly legumes, as it is a crucial component of the enzyme nitrogenase, which is necessary for biological nitrogen fixation .

2. Analytical Chemistry:

   • It serves as a key reagent for the quantitative determination of phosphates, silicates, arsenates, and lead. It forms characteristic heteropoly acids (e.g., ammonium phosphomolybdate) with these ions, which are used in colorimetric analysis .

3. Catalysis:

   • It acts as a precursor and a catalyst in the chemical industry. It is used in the production of dehydrogenation and desulphurization catalysts for petroleum refining. It also catalyzes oxidation reactions in organic synthesis .

4. Material Science and Metal Preparation:

   • It is used as a precursor to produce high-purity molybdenum metal powder, molybdenum trioxide (MoO₃), and other molybdenum compounds .

   • It is utilized in the synthesis of molybdenum-based alloys and ceramics .

   • Corrosion Inhibition: Molybdate ions are investigated as a less toxic alternative to chromates in corrosion-inhibiting coatings for metals .

5. Biochemistry and Research:

   • In biological electron microscopy, it can be used as a negative stain .

   • It is employed in enzyme studies to understand the role of molybdenum in biological systems .

   • It serves as a precursor for synthesizing other metal molybdates, such as calcium molybdate (CaMoO₄) .

Safety and Handling

• Classification: It is generally considered an irritant. It is harmful if swallowed and can cause irritation to the skin, eyes, and respiratory tract .

• Hazard Codes: Xi (Irritant), Xn (Harmful) .

• GHS Signal Word: Warning .

• Hazard Statements: H302 (Harmful if swallowed), H315 (Causes skin irritation), H319 (Causes serious eye irritation), H335 (May cause respiratory irritation) .

• Precautions: Use only in well-ventilated areas or under a fume hood. Avoid breathing dust. Wear appropriate protective gloves, clothing, and eye/face protection. Wash thoroughly after handling. Avoid release to the environment .

• Stability: Stable under normal conditions. Incompatible with strong acids (liberates toxic fumes) and strong oxidizing agents