175an essential feature, because periodic acid is so readily reduced.It is essential that the hot nitric acid shall not come into contact with rubber, in order that no nitrous acid may be formed. To avoid this, the top of the tube ground into the cover is sealed off, and the side-arm delivery tube lengthened to at least 25 em. The side arm is then inserted 7 to 8 em. below the water-cooled part of the condenser tube, which should allow of very little clearance, in order that no nitric acid may come in contact with the rubber stopper joining the two.The still is conveniently heated by placing on an asbestos pad over an electric hot plate with adjustable temperature control. If much refluxing occurs, an asbestos shield should be formed around the still.Two methods are in common use for the production of metallic rhenium. The first of these involves a direct reduction of commercial potassium perrhenate (procedure A) and yields a product that usually contains a small amount of alkali but is pure enough for most preparative purposes. The second method (procedure B) is slightly more complicated in that a preliminary precipitation of rhenium heptasulfide is followed by conversion to ammonium perrhenate which is subsequently reduced to the metal.lv2 Metallic rhenium so produced is usually purer than that prepared by direct reduction of the potassium salt.

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