FUMING SULPHURIC ACID
is poured on to the undiluted acid and the surface of contact of the two
liquids examined for a brown ring. Very small quantities of nitrous com-
pounds or of nitric acid may be detected respectively by Griess's reagent
(see Waters, p. 6) or diphenylamine used as follows : on to a few crystals of
pure diphenylamine in a dry test-tube arc poured a few c.c. of the acid to
be tested and on this as much distilled water so that two layers are
formed : the zone of contact of the two layers should show no blue colora-
tion, even after standing.
8. Hydrochloric Acid.—The diluted acid is treated with silver nitrate.
9. Hydrofluoric Acid.—The acid is warmed in a platinum dish covered
with a glass coated with wax which lias been scratched so as to ur cover
the glass in places : if hydrofluoric acid is present, the. naked glass is
10. Reducing Substances (Organic. Matter, Sulphurous and Nitrons
Acids).—1$ c.c. of the acid are diluted with 45 c.c, of water and a drop of
decinormal permanganate solution added: the pink colour should persist
for 5 minutes.
11. Quantitative Determination. -In absence of other aeicls, the
sulphuric acid may be titrated with N-alkali ; i c.c. : = 0-04904 gram H2SO«.
When other acids are present, precipitation as barium sulphate must be
Crude commercial sulphuric acid always contains arsenic and ns.nal.ly from
0-8 to 4-4 grams per xoo kilos, although as much as 0-5% has boon found.
FUMING SULPHURIC ACID
This is a mixture of the nionohydrate (HaS()<i) with sulphuric anhydride
and is an oily liquid, rarely colourless but more often brownish ; it fumes
in the air and is more or less turbid and often partly or completely crys-
tallised. Analysis of this acid includes the determinations described for
ordinary sulphuric acid; further, in order to know exactly its value1, its
content of sulphuric anhydride, inonohydrate .acid and sulphurous acid,
where this is present, must be known. Lunge's method, which is as follows,
may be used.
From 3 to 4 grains of the fuming acid, weighed by moans of Lunge and
Rey's special bulb pipe-tie or any similar instrument and with all the necessary
precautions, is dissolved in about half a, litre of very cold water and, when
the liquid has assumed the air-temperature, the volume made, up to 5()l)
c.c. In loo c.c. of this solution the total acidity is determined by means
of N/2-caustic soda solution and methyl orange. In another 100 c.c.,
any sulphurous acid is determined by N/io—iodine and starch paste. The.
sulphuric anhydride and monohydrated sulphuric, acid are then calculated
as follows :
(a) In absence of SOa: Ihe total acidity is calculated as percentage, of
S03; i c.c. N/2-NaOH — 0-02003 gram S03. The sulphuric anhydride
thus calculated is then subtracted from 100, the difference representing
the water in 100 parts of the substance. From the quantity of water thustion