To avoid errors arising from the solvent action of ammoniacal liquids upon glass, the iron precipitate should be filtered without unnecessary delay.]

[Note 5:  The washings from the ferric hydroxide are acidified with nitric acid, before testing with silver nitrate, to destroy the ammonia which is a solvent of silver chloride.

The use of suction to promote filtration and washing is permissible, though not prescribed.  The precipitate should not be allowed to dry during the washing.]

!Ignition of the Iron Precipitate!

Heat a platinum or porcelain crucible, cool it in a desiccator and weigh, repeating until a constant weight is obtained.

Fold the top of the filter paper over the moist precipitate of ferric hydroxide and transfer it cautiously to the crucible.  Wipe the inside of the funnel with a small fragment of washed filter paper, if necessary, and place the paper in the crucible.

Incline the crucible on its side, on a triangle supported on a ring-stand, and stand the cover on edge at the mouth of the crucible.  Place a burner below the front edge of the crucible, using a low flame and protecting it from drafts of air by means of a chimney.  The heat from the burner is thus reflected into the crucible and dries the precipitate without danger of loss as the result of a sudden generation of steam within the mass of ferric hydroxide.  As the drying progresses the burner may be gradually moved toward the base of the crucible and the flame increased until the paper of the filter begins to char and finally to smoke, as the volatile matter is expelled.  This is known as “smoking off” a filter, and the temperature should not be raised sufficiently high during this process to cause the paper to ignite, as the air currents produced by the flame of the blazing paper may carry away particles of the precipitate.

When the paper is fully charred, move the burner to the base of the crucible and raise the temperature to the full heat of the burner for fifteen minutes, with the crucible still inclined on its side, but without the cover (Note 1).  Finally set the crucible upright in the triangle, cover it, and heat at the full temperature of a blast lamp or other high temperature burner.  Cool and weigh in the usual manner (Note 2).  Repeat the strong heating until the weight is constant within 0.0003 gram.

From the weight of ferric oxide (Fe_{2}O_{3}) calculate the percentage of iron (Fe) in the sample (Note 3).

[Note 1:  These directions for the ignition of the precipitate must be closely followed.  A ready access of atmospheric oxygen is of special importance to insure the reoxidation to ferric oxide of any iron which may be reduced to magnetic oxide (Fe_{3}O_{4}) during the combustion of the filter.  The final heating over the blast lamp is essential for the complete expulsion of the last traces of water from the hydroxide.]

[Note 2:  Ignited ferric oxide is somewhat hygroscopic.  On this account the weighings must be promptly completed after removal from the desiccator.  In all weighings after the first it is well to place the weights upon the balance-pan before removing the crucible from the desiccator.  It is then only necessary to move the rider to obtain the weight.]