Ammonium Perrhenate

Ammonium perrhenate (APR) is the ammonium salt of perrhenic acid, NH₄ReO₄. It is the most common form in which rhenium is traded. It is a white salt; soluble in ethanol and water, and mildly soluble in NH₄Cl. It was first described soon after the discovery of rhenium.

Structure

The crystal structure of APR resembles that of scheelite, with atomic cation is replaced by ammonium. The pertechnetate (NH₄TcO₄), periodate (NH₄IO₄), tetrachlorothallate (NH₄TlCl₄), and tetrachloroindate (NH₄InCl₄) follow this motif. It undergoes a molecular orientational ordering transition on cooling without change of space group, but with a highly anisotropic change in the shape of the unit cell, resulting in the unusual property of having a positive temperature and pressure Re NQR coefficient. APR does not give hydrates.

Preparation

Ammonium perrhenate may be prepared from virtually all common sources of rhenium. The metal, oxides, and sulfides can be oxidized with nitric acid and the resulting solution treated with aqueous ammonia. Alternatively an aqueous solution of Re₂O₇ can be treated with ammonia followed by crystallisation.

Reactions

Ammonium perrhenate is weak oxidizer. It slowly reacts with hydrochloric acid:
:NH₄ReO₄ + 6 HCl → NH₄ eCl₄O+ Cl₂ ↑ + 3H₂O.
It is reduced to metallic Re upon heating under hydrogen:
:2 NH₄ReO₄ + 7 H₂ → 2 Re + 8 H₂O + 2 NH₃
Ammonium perrhenate decomposes to volatile Re₂O₇ starting at 250 °C. When heated in a sealed tube at 500 °C, It decomposes to rhenium dioxide:
:2NH₄ReO₄ → 2ReO₂ + N₂ + 4 H₂O
The ammonium ion can be displaced with some concentrated nitrates e.g. potassium nitrate,, silver nitrate, etc.:
:NH₄ReO₄ + KNO₃ → KReO₄ ↓ + NH₄NO₃
It can be reduced to nonahydridorhenate with sodium in ethanol:
:NH₄ReO₄ + 18Na + 13C₂H₅OH → Na₂ eH₉+ 13NaC₂H₅O + 3NaOH + NH₃•H₂O.

References

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{{DEFAULTSORT:Ammonium Perrhenate
Inorganic compounds
Perrhenates
Ammonium compounds