SOILS	257
the solution and the outside thoroughly rinsed. It can then be taken in
the hand and the interior rinsed. A policeman may have to be used if silicic
acid adheres to the crucible. Do not use metal crucible tongs for removing
crucibles from solutions, especially if the latter are acid, as in this case.
Evaporate the liquid to dryness over a steam bath, or keep the casserole
in. constant motion over a low flame. Heat for 15 minutes at 120° in an
oven constructed of material that will not be damaged by acid vapors, or to
just below redness over a flame. When cool, add 5 cc of concentrated
hydrochloric acid and 75 cc of water, heat until soluble salts are dissolved,
filter off the silica and wash the paper and silica with hot water until free
from acid. Repeat the evaporation of the filtrate and washings and treat
as before, using a different filter paper. Save the filtrates and washings for
the determination of other inorganic constituents.
Rurn both filter papers in one platinum crucible (which need not be
weighed previously) then ignite over a blast lamp, cool and weigh. Add a
few drops of sulphuric acid and about 5 cc of hydrofluoric acid (pouring the
latter directly from the bottle) to the material in the platinum crucible and
volatilize the silicon tetrafluoride and acids by evaporation to dryness under
a hood. Ignite the residue and weigh. The loss in weight represents
silica. Calculate the per cent.
The residue in the crucible consists of oxides of iron and aluminium. Add
about 1 gm of potassium pyrosulphate and heat, gradually raising the tem-
perature to redness, until solution of the oxides is complete. Cool and
dissolve the fusion in hot water. Precipitate the metals as hydroxides, as
directed below, wash and discard the filtrate and washings. Preserve the
precipitate on the paper, so that it may be burned in the same crucible as
the main precipitate of iron and aluminium hydroxides, the total oxides
being weighed together.
X>etermination of Iron and Aluminium: Direct Method for Iron, Indirect
Method for Aluminium.—Dilute the filtrate from the determination of
silica to about 75 cc. Add a drop of methyl red and then add dilute ammo-
nium hydroxide until the solution is distinctly basic, avoiding an undue excess.
Boil for 5 minutes or until the odor of ammonia is faint, but without
prolonging the boiling until the solution becomes acid in reaction. Filter the
precipitate through an extracted paper and wash with hot water two or three
times. Return the precipitate to the first beaker and dissolve in warm water
containing a small amount of hydrochloric acid. Reprecipitate, filter and
wash the precipitate free from chlorides. Save the filtrate and washings
from both precipitations for the determination of calcium and magnesium.
Burn the paper at a low temperature in a weighed platinum crucible,
inclining the crucible to facilitate oxidation. When most of the carbon has
been removed, add the paper containing the iron and aluminium hydroxides
from the silica determination (see above) and burn this in the same manner.
Finally heat to bright redness, cool in a desiccator and weigh as oxides of iron
(ferric), aluminium, titanium and phosphorus.
Unless the residue of oxides is white, add about 2 gm of potassium
pyrosulphate and heat* gradually raising the temperature to bright redness.
17