QUANTITATIVE DETERMINATIONS	73
pearance of the red color. Add two drops more of stannous chloride solution
then cool quickly by immersing the flask in running water. When cool add,
all at once, 25 cc of a 5-per cent solution of mercuric chloride and mix well
with the solution. The precipitate should be pure white mercurous chloride
without a trace of gray mercury. Dilute to 500 cc with recently boiled
and cooled distilled water and add 50 cc of a solution containing 144
gm of phosphorous pentoxide, 245 gm of sulphuric acid and 67 gm of
crystallized manganous sulphate in each liter of solution. Titrate at once
with standard potassium permanganate solution and calculate the per cent
of iron in the ore.
By Bichromate.—In acid solutions ferrous salts are oxidized
completely by dichromates. Potassium dichrornate, a salt
readily purified by crystallization, is generally used as a standard.
The reaction between this salt and ferrous chloride is expressed
by the equation:
6FeCl_2 + K_2O_20_7 + 14HCl~*6FeCl_3 + 2KC1 + 2CrCl_3 + 7H_20.
As in the oxidation of iron by permanganates the acid actually
takes a part in the reaction and if an insufficient amount is
present, a basic condition will result and a precipitate of basic
salts of iron and of chromium will form.
Potassium dichromate possesses several advantages over
potassium permanganate as a standard oxidizing ag^nt. It is
relatively more stable and therefore may be obtained in a state
of uniform purity. This makes it possible to standardize solu-
tions by direct weighing when the degree of purity of the salt
has been established by analysis. The relative stability is the
same with solutions and the standard solution can be kept almost
Indefinitely without changing its concentration. Potassium
dichromate may also be used for the titration of iron and other
reducing agents in presence of hydrochloric acid or chlorides,
without oxidation of the latter taking place. This is a very
decided advantage in the determination of iron since it makes
possible the use of stannous chloride as a reducing agent without
the addition of manganous sulphate and phosphoric acid. There
is no indicator that can be added directly to the solution which is
being titrated by potassium dichromate and the color of the
standard solution is not sufficiently intense to be of any use for
this purpose. The indicator that is commonly used is a very
dilute solution of potassium ferricyanide, placed in drops on a