Dilute the filtrate from the determination of insoluble matter to 500
cc. with water free from carbon dioxide and thoroughly mix the solution.
Transfer 50 cc. of the solution to a 250 cc. beaker, add about 150 cc.
of water and 5 cc. of concentrated hydrochloric acid, and heat to
boiling. Add ammonium hydroxide in slight excess; when the solution has
been almost neutralized it is convenient to add a drop of methyl orange
indicator and then to add about 0.5 cc. of ammonium hydroxide after the
solution is neutral to the indicator. Digest at about 100° C. for a few
minutes and filter. Some analysts prefer to wash this gelatinous
precipitate with hot water by decantation, and some to wash it evenly
distributed over the surface of a filter paper; either method may be
used. It is difficult to free it completely from impurities and it is
not necessary to do so unless unusual quantities of calcium, magnesium,
sodium, or potassium are present. While the precipitate is being washed
do not allow it to become dry, as it then packs and can not be washed
clean. After most of the water has drained drying the filter may be
hastened by placing it on a sheet of blotting paper. If much iron is
present completely dry the precipitate, remove it from the paper, and
ignite the paper separately. Finally, blast the precipitate, with free
access of air to the crucible, for five or ten minutes, cool, and weigh
as oxides of iron and aluminium (Fe_{2}O_{3} + Al_{2}O_{3}).
Subtract the content of total iron, expressed as ferric oxide
(Fe_{2}O_{3}), from the weight of the combined oxides and report the
difference as aluminium oxide (Al_{2}O_{3}), in percentage.