Ammonium heptamolybdate
Colorless to green - yellowish crystals
Fixed
2.498 g · cm -3
90 ° C ( dec.)
- In water 430 g/l-1
- Soluble in ethanol
Attention
333 mg · kg -1 ( LD50, rat, oral)
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Ammonium forms colorless to green- yellowish crystals which decompose at about 90 ° C with loss of crystal water. It typically forms a tetrahydrate having the chemical formula (NH4) 6Mo7O24 · 4 H2O. It is often referred to only as ammonium molybdate, ammonium molybdate, although can be described as good Ammoniumorthomolybdat ( NH4) 2MoO4 and other molybdates.
Representation
Ammonium is quite easily prepared by dissolving molybdenum ( VI) oxide in an excess of aqueous ammonia and subsequent evaporation of the solution at room temperature. While the solution evaporated, escapes the excess ammonia. This method leads to the formation of six-sided transparent prisms of ammonium heptamolybdate tetrahydrate of. Solutions of ammonium heptamolybdate react with acids back to molybdenum (VI ) oxide and the corresponding ammonium salts. The pH of a concentrated solution is 5 to 6
Use
Ammonium heptamolybdate is used in the laboratory for the detection of silicic acid, phosphoric acid, phosphates, arsenic, lead and sorbitol as well as for the analysis of sea water.
With phosphates it forms after previous treatment of the material sample with nitric acid a yellow precipitate of molybdenum yellow / Ammoniumdodecamolybdatophosphat ( NH4) 3 [P ( Mo3O10 ) 4]. The addition of ascorbic acid as a mild reducing agent, intense blue color (formation of molybdenum blue ) takes place. At lower concentrations of molybdate no precipitation, but only color change of the solution takes place.
Taking into account that the heptamolybdate in aqueous solution an equilibrium is received:
Results in the following equation:
These reactions are also used for the photometric determination of molybdate or phosphate in the trace range.
It also serves for the production of catalysts, as a Molybdenum-based fertilizer and a large scale as an intermediate for the production of molybdenum from molybdenum ores.
In biology, biochemistry and it is furthermore in the electron as a contrast agent (negative mold ) at a concentration of 3-5 vol - % is used, even when trehalose is present in the sample. Also in the cryo-electron microscopy it serves - in saturating concentrations - as a contrast agent.