Laboratory Methods of Organic Chemistry - Sciencemadness Dot Org

36 WOKKING WITH COMPKESSED GASES
The elements under (1), with very low critical temperatures, are
in the gaseous state in the cylinders, but the substances under (2) are
present in liquid form. Oxygen, hydrogen, and nitrogen are usually
under a pressure of 150 atmospheres in cylinders having a capacity
of 10 litres; after filling, therefore, the cylinders contain gas sufficient
to occupy 1-5 cubic metres at atmospheric pressure. The
taps of the hydrogen cylinders have reversed screw threads so as to
prevent accidental charging with oxygen.
In the laboratory all cylinders should be fitted with reducing valves,
for the maintenance of which an assistant should be responsible.
The use by itself of the simple valve in the head of the cylinder makes
regulation of the gas stream difficult and invariably leads to waste.
For all gases (chlorine included) the so-called conical valve of
aluminium bronze can be used. It can be
made cheaply by any skilled mechanic
(Fig. 28).
In all work with gases—whether from
cylinders or from Kipp's apparatus — a
means of gauging the velocity of the current
must be used. Except with ammonia, a
small bubbler containing concentrated
sulphuric acid serves. It can be attached
to the cylinder itself. As a rule, in order to
dry at the same time, a wash-bottle is attached,
preferably not one in two parts
with a ground glass joint, because such
bottles are often forced open by the slightest
excess pressure. 1 If the gases have to be
dried very thoroughly a wash-bottle with
FIG. 28
concentrated sulphuric acid is not sufficient,
and one or two u-tubes containing phos-
phorous pentoxide spread on glass wool are added. Ammonia is
passed through potassium hydroxide solution 1 : 1 and is then dried
by passage through a tower containing potassium hydroxide and
calcium oxide.
Ordinary laboratory apparatus cannot be used with, gas from a
cylinder for carrying out reactions under pressure in a closed system.
For example, if it is desired to leave a reaction solution under pressure
1 After use the connection between steel cylinder and wash-bottle should always
be broken so as to prevent sucking back of the sulphuric acid.