Page 138 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

3 COMNON APPARATUS AND BASIC TECHNIQUES
for the analytical investigations. It may well be that the material is in such large
pieces that comminution is necessary in order to produce a specimen suitable
for handling in the laboratory. These important factors are considered in
Chapter 5 (Sections 5.2; 5.4), and as explained therein, the material is usually
dried at 105- 110 OC before analysis.

3.30 WElGHlNG THE SAMPLE
If necessary refer to Section 3.5 dealing with the operation of a chemical balance,
and to Sections 3.25 and 3.22 which are concerned with the use of weighing
bottles and desiccators respectively.
The material, prepared as above, is usually transferred to a weighing bottle
which is stoppered and stored in a desiccator. Samples of appropriate size are
withdrawn from the weighing bottle as required, the bottle being weighed before
and after the withdrawal, so that the weight of substance is obtained by difference.
Attention is drawn to vibro-spatulas which are useful adjuncts to the
weighing-out of powders. The spatula is connected to the electric mains, and
the powder is placed on the blade of the spatula. When the current is switched
on, the blade is caused to vibrate and to deposit solid gradually into the beaker
or other container over which it is held: the intensity of the vibration may be
adjusted.

3.31 SOLUTION OF THE SAMPLE
Most organic substances can be dissolved readily in a suitable organic solvent
and some are directly soluble in water or can be dissolved in aqueous solutions
of acids (basic materials) or of alkalis (acidic materials). Many inorganic
substances can be dissolved directly in water or in dilute acids, but materials
such as minerals, refractories, and alloys must usually be treated with a variety
of reagents in order to discover a suitable solvent: in such cases the preliminary
qualitative analysis will have revealed the best procedure to adopt. Each case
must be considered on its merits; no attempt at generalisation will therefore be
made. It is however of value to discuss the experimental technique of the simple
process of solution of a sample in water or in acids, and also the method of
treatment of insoluble substances.
For a substance which dissolves readily, the sample is weighed out into a
beaker, and the beaker immediately covered with a clockglass of suitable size
(its diameter should not be more than about 1 cm larger than that of the beaker)
with its convex side facing downwards. The beaker should have a spout in order
to provide an outlet for the escape of steam or gas. The solvent is then added
by pouring it carefully down a glass rod, the lower end of which rests against
the wall of the beaker; the clockglass is displaced somewhat during this process.
If a gas is evolved during the addition of the solvent (e.g. acids with carbonates,
metals, alloys, etc.), the beaker must be kept covered as far as possible during
the addition. The reagent is then best added by means of a pipette or by means
of a funnel with a bent stem inserted beneath the clockglass at the spout of the
beaker; loss by spirting or as spray is thus prevented. When the evolution of
gas has ceased and the substance has completely dissolved, the underside of the
clockglass is well rinsed with a Stream of water from a wash bottle, care being
taken that the washings fa11 on to the side of the beaker and not directly into