Page 120 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

3 COMMON APPARATUS AN0 BASIC TECHNIQUES

A polythene wash bottle is available commercially and is inexpensive. It is
fitted with a plastic cap carrying a plastic jet, and has flexible sides. The bottle
can be held in the hand; application of slight pressure by squeezing gives an
easily controllable jet of water. It is more or less unbreakable and is inert to
many wash liquids. A polythene wash bottle should be used only for cool liquids.
Polythene wash bottles are sometimes charged with wash liquids other than
water. Attention must be drawn to the fact that the components of some wash
solutions may pass into the polythene and may be released into the space in
the bottle when it is set aside: repeated fillings and rinsings may be required to
remove the chemicals from the bottle. It is safer to label the wash bottle and
to reserve it for the special wash liquid. Such wash solutions include a weakly
acid solution saturated with hydrogen sulphide, dilute aqueous ammonia,
saturated bromine water, and dilute nitric acid.

GENERAL APPARATUS

3.1 9 GLASSWARE, CERAMICS, PLASTIC WARE
In the following sections, a brief account of general laboratory apparatus relevant
to quantitative analysis will be given. The commonest materials of construction
of such apparatus are glass, porcelain, fused silica, and various plastics; the
merits and disadvantages of these are considered below.
Glassware. In order to avoid the introduction of impurities during analysis,
apparatus of resistance glass should be employed. For most purposes Pyrex
glass (a borosilicate glass) is to be preferred. Resistance glass is very slightly
affected by al1 solutions, but, in general, attack by acid solutions is less than
that by pure water or by alkaline solutions; for this reason the 1atter.should be
acidified whenever possible, if they must be kept in glass for any length of time.
Attention should also be given to watch, clock, and cover glasses; these should
also be of resistance glass. As a rule, glassware should not be heated with a
naked flame; a wire gauze should be interposed between the flame and the glass
vessel.
For special purposes, Corning Vycor glass (96 per cent silica) may be used.
It has great resistance to heat and equally great resistance to thermal shock,
and is unusually stable to acids (except hydrofluoric acid), water, and various
solutions.
The most satisfactory beakers for general use are those provided with a
spout. The advantages of this form are: (a) convenience of pouring, (b) the
spout forms a convenient place at which a stirring rod may protrude from a
covered beaker, and (c) the spout forms an outlet for steam or escaping gas
when the beaker is covered with an ordinary clock glass. The size of a beaker
must be selected with due regard to the volume of the liquid which it is to
contain. The most useful sizes are from 250 to 600 mL.
Conical (or Erlenmeyer's) flasks of 200-500mL capacity find many
applications, for example, in titrations.
Funnels should enclose an angle of 60". The most useful sizes for quantitative
analysis are those with diameters of 5.5, 7 and 9 cm. The stem should have an
interna1 diameter of about 4 mm and should not be more than 15 cm long. For