Ammonium molybdate tetrahydrate is marketed as a white, crystalline powder. It is also known as ammonium heptamolybdate. The tetrahydrate forms clear, colourless or slightly yellow-green crystals as six-sided prisms. When heated to over 90 °C the crystals release their water of crystallization. The salt dissolves well in the water to form an acidic solution.
Ammonium molybdate weathers in the air, releasing ammonia. With phosphate ions it forms a yellow precipitate after previous treatment of the sample with nitric acid in aqueous solution. It reacts with acids to form the corresponding ammonium salts and molybdenum (VI) oxide. With ascorbic acid it forms molybdenum blue in an aqueous solution after careful heating. This reaction detects the presence of molybdenum ions.
Ammonium heptamolybdate is produced quite simply by dissolving molybdenum (VI) oxide in an excess of aqueous ammonia and then evaporating the solution at room temperature. As the solution evaporates, the excess ammonia escapes. This method leads to the formation of six-sided transparent prisms of the tetrahydrate of ammonium heptamolybdate.
Solutions of ammonium heptamolybdate react with acids to form molybdenum(VI) oxide and the corresponding ammonium salts. The pH value of a concentrated solution is between 5 and 6.
Ammonium heptamolybdate is used in the laboratory for the detection of silicic acids, phosphoric acid, phosphates, arsenic, lead and sorbitol as well as for the analysis of seawater. In addition, it is used in a standardised procedure for measuring the immissions of hydrogen sulphide.
It is also used for the production of catalysts, as molybdenum fertilizer and on a large scale as an intermediate product for the extraction of molybdenum from molybdenum ores.
In biology and biochemistry, it is also used in electron microscopy as a contrast medium (negative imprint) in a concentration of 3 to 5% by volume, even if trehalose is present in the sample. In cryo electron microscopy it is also used - in saturated concentrations - as a contrast agent.