Sodium Molybdate, Na₂MoO₄.2H₂O

Sodium Molybdate, Na₂MoO₄.2H₂O, may be prepared by fusing together requisite proportions of sodium carbonate and molybdenum trioxide and crystallising from the aqueous extract, or by neutralising a saturated aqueous solution of the trioxide or an acid molybdate by warm sodium carbonate solution. On evaporating in carbon dioxide-free air, lustrous plates, or rhombohedra are obtained. The solubility is as follows:

Temperature, ° C	0	4	6	9	10	15.5	18	32	51.5	100
Solubility	30.63	33.83	35.48	38.16	39.28	39.27	39.40	39.82	41.27	45.57

Solubility of sodium molybdate

the solid phase above 10° C. being Na₂MoO₄.2H₂O, and below 10° C., Na₂MoO₄.10H₂O, as is shown in fig. The solubility curve resembles that of sodium tungstate. The decahydrate is obtained in large striated (efflorescent) prisms, similar to the sulphate, when a solution is crystallised below 6° C. On gently heating, the crystals yield the dihydrate.

Further heating yields the anhydrous salt, which melts at 687° C., and apparently exists in four different modifications, since three transformation points have been observed between 410 and 640° C.