CHAPTER XIX. SULPHURIC ACID

90. Preparation.

Experiment 55.--Having fitted a cork with four or five
perforations to a large t.t., pass a d.t. from three of these to
three smaller t.t., leaving the others open to the air, as in
Figure 28. Into one t.t. put 5 cc. H2O, into another 5 g. Cu
turnings and 10 cc. H2SO4, into the third 5 g. Cu turnings and 10
cc. dilute HNO3, half water. Hang on a ring stand, and slowly
heat the tubes containing H2O and H2SO4. Notice the fumes that
pass into the large t.t.

Trace out and apply to Figure 28 these reactions:--

(1) Cu + 2 H2SO4 = CuSO4 + 2 H2O + SO2.

(2) 3 Cu + 8 HNO3 = 3 Cu(NO3)2+ 4 H2O + 2 NO.

(3) NO + O = NO2.

(4) SO2 + H2O + NO2 =H2SO4 + NO.

(4) comes from combining the gaseous products in (1), (2), (3).
In (3), NO takes an atom of O from the air, becoming NO2, and at
once gives it up, to the H2SO3 (H2O + SO2), making H2SO4, and
again goes through the same operation of taking up O and passing
it along. NO is thus called a carrier of O. It is a reducing
agent, while NO2 is an oxidizing agent. This is a continuous
process, and very important, since it changes useless H2SO3 into
valuable H2SO4. If exposed to the air, H2SO3 would very slowly
take up O and become H2SO4.

Instead of the last experiment, this may be employed if
preferred: Burn a little S in a receiver. Put into an
evaporating-dish, 5 cc. HNO3, and dip a paper or piece of cloth
into it. Hang the paper in the receiver of SO2, letting no HNO3
drop from it. Continue this operation till a small quantity of
liquid is found in the bottle. The fumes show that HNO3 has lost
O. 2 HNO3 + SO2 = H2SO4 + 2 NO2.

91. Tests for H2SO4.

Experiment 56.--(1) Test the liquid with litmus. (2) Transfer it
to a t.t., and add an equal volume of BaCl2 solution. H2SO4 +
BaCl2 = ? Is BaSO4 soluble? (3) Put one drop H2SO4 from the
reagent bottle in 10 cc. H2O in a clean t.t., and add 1 cc. BaCl2
solution. Look for any cloudiness. This is the characteristic
test for H2SO4 and soluble sulphates, and so delicate that one
drop in a liter of H2O can be detected. (4) Instead of H2SO4, try
a little Na2SO4 solution. (5) Put two or three drops of strong
H2SO4 on writing-paper, and evaporate, high over a flame, so as
not to burn the paper. Examine it when dry. (6) Put a stick into
a t.t. containing 2 cc. H2SO4, and note the effect. (7) Review
Experiment 5. (8) Into an e.d. pour 5 cc. H2O, and then 15 cc.
H2SO4. Stir it meantime with a small t.t. containing 2 or 3 cc.
NH4OH, and notice what takes place in the latter; also note the
heat of the e.d.

The effects of (5), (6), (7), and (8) are due to the intense
affinity which H2SO4 has for H2O. So thirsty is it that it even
abstracts H and O from oxalic acid in the right proportion to
form H2O, combines them, and then absorbs the water.

92. Affinity for Water.--This acid is a desiccator or dryer, and
is used to take moisture from the air and prevent metallic
substances from rusting. In this way it dilutes itself, and may
increase its weight threefold. In diluting, the acid must always
be poured into the water slowly and with stirring, not water into
the acid, since, as H2O is lighter than H2SO4, heat enough may be
set free at the surface of contact to cause an explosion.
Contraction also takes place, as may be shown by accurately
measuring each liquid in a graduate, before mixing, and again
when cold. The mixture occupies less volume than the sum of the
two volumes. For the best results the volume of the acid should
be about three times that of the water.

93. Sulphuric Acid made on a Large Scale involves the same
principles as shown in Experiment 55, excepting that S02 is
obtained by burning S or roasting FeS2 (pyrite),

[Fig. 29.]

and HNO3 is made on the spot from NaNO3 and H2SO4. SO2 enters a
large leaden chamber, often 100 to 300 feet long, and jets of
steam and small portions of HNO3 are also forced in. The "chamber
acid" thus formed is very dilute, and must be evaporated first in
leaden pans, and finally in glass or platinum retorts, since
strong H2SO4, especially if hot, dissolves lead. See Experiment
124. Study Figure 29, and write the reactions. 2 HNO3 breaks up
into 2 NO2, H2O, and O. 94. Importance.--Sulphuric acid has been
called, next to human food, the most indispensable article known.
There is hardly a product of modern civilization in the
manufacture of which it is not directly or indirectly used.
Nearly a million tons are made yearly in Great Britain alone. It
is the basis of all acids, as Na2CO3 is of alkalies. It is the
life of chemical industry, and the quantity of it consumed is an
index of a people's civilization. Only a few of its uses can be
stated here. The two leading ones are the reduction of Ca3(PO4)2
for artificial manures and the sodium carbonate manufacture.
Foods depend on the productiveness of soils and on fertilizers,
and thus indirectly our daily bread is supplied by means of this
acid; and from sodium carbonate glass, soap, saleratus, baking-
powders, and most alkalies are made directly or indirectly. H2SO4
is employed in bleaching, dyeing, printing, telegraphy,
electroplating, galvanizing iron and wire, cleaning metals,
refining Au and Ag, making alum, blacking, vitriols, glucose,
mineral waters, ether, indigo, madder, nitroglycerine, gun-
cotton, parchment, celluloid, etc., etc.

FUMING SULPHURIC ACID.

95. Nordhausen or Fuming Sulphuric Acid, H2S207 used in
dissolving indigo and preparing coal-tar pigments, is made by
distilling FeSO4. 4FeSO4 + H2O = H2S207 + 2Fe203 + 2S02. This was
the original sulphuric acid. It is also formed when S03 is
dissolved in H2SO4. When exposed to the air, S03 escapes with
fuming.