Molybdenum blue hydrate

Sulfides interfere because they too reduce molybdate to molybdenum blue. Thiosulfates likewise give a blue coloration, but this fades completely on the addition of hydrazine hydrate. 

Molybdenum Arsenates and Molybdo-arsenates.—Molybdous Arsenate, Mo(HAs04)2.wH20( ), is said1 to be formed as a grey precipitate when molybdous chloride is treated with sodium monohydrogen arsenate the precipitate first redissolves, hut afterwards becomes permanent. Molybdic arsenate, obtained in a similar manner from molybdic chloride, has been described by Berzelius, who also considered that an acid salt was produced on dissolving the hydrate of molybdenum dioxide in excess of arsenic acid, since the solution turned blue on standing.1... 

The pentachloride is a little unstable in air when heated to about 1380° C. or less it leaves a residue of molybdenum when heated in hydrogen at 250° C. it is reduced to amorphous molybdenum trichloride. Its aqueous solution is unstable in air, especially on warming, when hydrogen chloride is more rapidly evolved and the blue oxide (p. 131) remains. Decomposition of its solution in hydrochloric acid also readily takes place. In alcohol and ether it dissolves to a green solution in sulphuric acid its solution is bluish green and in nitric acid colourless alkalies dissolve it with production of the hydrated dioxide and a molybdate. ... 

The blue oxide is best obtained by allowing powdered molybdenum to remain for a long time at ordinary temperatures in contact with an aqueous suspension of the trioxide, filtering, and then digesting with a further quantity of molybdenum. The solution is evaporated in vacuo. Cold water used for washing the solid should previously be rendered air-free. Another good method of preparation consists in precipitating in the cold, by means of excess of a solution of hydrated molybdenum tetrachloride, a solution of ammonium molybdate in hydrochloric acid the precipitate is w ashed with air-free w ater in an atmosphere of carbon dioxide, and is dried m vacuo. 

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