When the distillation is finished, the receiver is
disconnected, and its contents washed out into a beaker and titrated
with "hypo." The standard solution of "hypo" is made by dissolving 19.58
grams of hyposulphite of soda (Na_{2}S_{2}O_{3}.5H_{2}O) in water and
diluting to 1 litre; 100 c.c. are equal to 1 gram of iodine. To
standardise the solution, weigh up 0.25 gram of pure iodine in a small
beaker. Add 2 or 3 crystals of potassium iodide; cover with water; and,
when dissolved, dilute to 50 or 100 c.c. Titrate, and calculate the
standard.
FLUORINE AND FLUORIDES.
Fluorine is frequently met with as calcium fluoride or fluor-spar
(CaF_{2}). It occurs less abundantly as cryolite (Na_{3}AlF_{6}), a
fluoride of aluminium and sodium, which is used in glass-making. Certain
other rarer fluorides are occasionally met with. Fluorine is also found
in apatite, and in some silicates, such as topaz, tourmaline, micas, &c.
Hydrofluoric acid is used for etching glass and opening up silicates. It
attacks silica, forming fluoride of silicon (SiF_{4}), which is
volatile. Silica is by this means eliminated from other oxides, which,
in the presence of sulphuric acid, are fixed. The commercial acid is
seldom pure, and generally weak; and the acid itself is dangerously
obnoxious. The use of ammonium fluoride (or sodium fluoride) and a
mineral acid is more convenient.
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