QUANTITATIVE DETERMINATIONS 75
the value of the solution in terms of iron. Dilute to make 1 cc equivalent
to 0.005 gm of iron.
Instead of weighing four portions of ferrous ammonium sulphate a
standard solution may be made by dissolving ten times the required amount,
adding 50 cc of dilute sulphuric acid and diluting to 500 cc with recently
boiled and cooled water. Portions of 50 cc are then measured and titrated.
The solution oxidizes upon exposure to air and it must be kept in a closely
stoppered flask.
Determination of Iron in an Ore.—Prepare a sample of iron ore by grinding
to pass a 100-mesh sieve. Weigh four portions of exactly 0.5 gm each,
using the counterpoised glasses and brushing the ore into porcelain crucibles.
Heat the inclined crucibles for 5 minutes over the desk burner, cool, place
in casseroles and dissolve in hydrochloric acid, with or without the addition
of stannous chloride. Reduce each solution just before titration, following
the directions given for dissolving and reducing by method (6) of the per-
manganate method but do not add the solution of manganous sulphate and
phosphoric and sulphuric acids. Dilute to 100 cc. The titration is carried
out exactly as directed for standardizing potassium dichromate solution.
Calculate the per cent of iron in the ore.
ALUMINIUM
The direct determination of aluminium is made by precipi-
tating the hydroxide, changing this to oxide by ignition, and
weighing the product:
A1C18 + 3NH4OH -> A1(OH)3 + 3NEUC1; (1)
2A1(OH)3 -» A1203 + 3H20. (2)
If iron and aluminium occur together they are precipitated
together and the product of ignition is a mixture of ferric oxide
and aluminium oxide. In such a case the usual procedure is to
weigh the oxide mixture, then dissolve and determine the iron
volumetrically, calculating this to oxide and subtracting from
the weight of mixed oxides to find the weight of aluminium oxide.
Or the aluminium may be determined directly by precipitating
as phosphate, first reducing the iron to the ferrous condition by
sodium thiosulphate; ferrous phosphate is sufficiently soluble to
make a separation possible. This method will be described
later for the determination of aluminium in soils (page 258).
Solubility.—The solubility of aluminium hydroxide in water
is not definite as this substance belongs to the colloid class. The
presence of various salts diminishes the solubility to a low