Identifying Aluminum Minerals

Precipitation by Ammonium Hydroxide
Aluminum is precipitated in the form of aluminum hydroxide, A1 (OH)₃, when an excess of ammonium hydroxide is added to an acid solution. The precipitate is flocculent in form and colorless or white. It is precipitate under the same conditions as ferric hydroxide (see page 111), and since the latter has a d ark color a small amount of aluminum hydroxide might be overlooked in a mixture of the two. To make a further test under these conditions, filter off t he precipitate and treat it with a hot solution of sodium hydroxide, which will dissolve any aluminum hydroxide present but will not affect the ferric hydroxide. Filter, and to the filtrate add hydrochloric acid in slight excess, and then make alkaline with ammonium hydroxide again. This will precipitate any aluminum that may be present as pure aluminum hydroxide.
Blowpipe Test with Cobalt Nitrate
Light colored and infusible aluminum minerals when moistened with a drop of cobalt nitrate and heated intensely before the blowpipe assume a dark blue color. Zinc silicates will also yield a blue color under similar conditions.

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