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, water-soluble salt. It was first described soon after the discovery of rhenium. 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, water-soluble salt. It was first described soon after the discovery of rhenium. The crystal structure of APR is that of scheelite, in which the atomic cation is replaced by the ammonium molecular cation. 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. NH4ReO4 can be regarded as the prototype structure of a family of ammonium scheelites, which include the pertechnetate (NH4TcO4), periodate (NH4IO4), tetrachlorothallate (NH4TlCl4) and tetrachloroindate (NH4InCl4). 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. Pure rhenium powder can be produced from APR by heating it in the presence of hydrogen: Heating must be done slowly because ammonium perrhenate decomposes to volatile Re2O7 starting at 250 °C. When heated in a sealed tube at 500 °C, APR decomposes to rhenium dioxide: