Molybdates yield sparingly soluble orange-yellow molybdyl 'oxinate' with oxine solution; the pH of the solution should be between the limits 3.3-7.6. The complex differs from other 'oxinates' in being insoluble in organic solvents and in many concentrated inorganic acids. The freshly precipitated compound dissolves only in concentrated sulphuric acid and in hot solutions of caustic alkalis. This determination is of particular interest, as it allows a complete separation of molybdenum from rhenium.
Neutralise the solution of alkali molybdate, containing up to 0.1 g of Mo, to methyl red, and then acidify with a few drops of 1M sulphuric acid. Add 5mL 2M ammonium acetate, dilute to 50-100 mL, and heat to boiling. Precipitate the molybdenum by the addition of a 3 per cent solution of oxine in 8% acetic acid, until the supernatant liquid becomes perceptibly yellow. Boil gently and stir for 3 minutes, filter through a filtering crucible (sintered-glass or porcelain), wash with hot water until free from the reagent, and dry to constant weight at 130-140 °C. Weigh as MoO2(C9H6ON)2.