Sodium molybdate

Sodium molybdate, Na₂MoO₄, is useful as a source of molybdenum. This white, crystalline salt is often encountered as the dihydrate, Na₂MoO₄·2H₂O.

Preparation

Dissolution of MoO₃ in sodium hydroxide at 50–70 °C followed by crystallizing the filtered product. If crystallized below 10 °C, the decahydrate forms. Above 10 °C, the dihydate crystallizes. The anhydrous salt is obtained by heating this product at 100 °C.

MoO₃ + 2NaOH + H₂O → Na₂MoO₄·2H₂O

Uses

The agriculture industry uses 500 tons per year as a fertilizer. In particular, its use has been suggested for treatment of whiptail in broccoli and cauliflower in molybdenum-deficient soils. However, care must be taken because at a level of 0.3 ppm sodium molybdate can cause copper deficiencies in animals, particularly cattle.

It is used in industry for corrosion inhibition, as it is a non-oxidizing anodic inhibitor. The addition of sodium molybdate significantly reduces the nitrite requirement of fluids inhibited with nitrite-amine, and improves the corrosion protection of carboxylate salt fluids. In industrial water treatment applications where galvanic corrosion is a potential due to bimetallic construction, the application of sodium molybdate is preferred over sodium nitrite. Sodium molybdate has the advantage in that the dosing of lower ppm's of molybdate allow for lower conductivity of the circulating water. Sodium molybdate at levels of 50-100 ppm offer the same levels of corrosion inhibition as sodium nitrite at levels of 800+ ppm. By utilizing lower concentrations of sodium molybdate, conductivity is kept at a minimum and thus galvanic corrosion potentials are decreased.

Reactions

When treated with sodium borohydride, molybdate is reduced to molybdenum(IV) oxide:

Na₂MoO₄ + NaBH₄ + 2H₂O → NaBO₂ + MoO₂ + 2NaOH + 3H₂

Sodium molybdate reacts with the acids of dithiophosphates:

Na₂MoO₄ + (R = Me, Et)(RO)₂PS₂H → [MoO₂(S₂P(OR)₂)₂]

which further reacts to form [MoO₃(S₂P(OR)₂)₄].

Structure

In aqueous solution, sodium molybdate features dissociated sodium ions and tetrahedral molybdate (MoO₄^2-), which adopts a sulfate-like structure. The solid dihydrate material has a complex structure typical for alkali metal salts of oxyanions. The MoO₄^2- subunits are tetrahedral with Mo-O distances near 178 pm.

Safety

Sodium molybdate supports the biosynthesis of molybdoenzymes, which are found in all higher forms of life. The LC50 for freshwater fish ranges from 60 to 7630 mg/L. The toxicity of soluble molybdate to marine organisms has also been reported.

References