Page 132 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

3 COMMON APPARATUS AND BASIC TECHNIQUES
3.25 WElGHlNG BOTTLES
Most chemicals are weighed by diference by placing the material inside a
stoppered weighing bottle which is then weighed. The requisite amount of
substance is shaken out into a suitable vesse1 (beaker or flask), and the weight
of substance taken is determined by re-weighing the weighing bottle. In this
way, the substance dispensed receives the minimum exposure to the atmosphere
during the actual weighing process: a feature of some importance if the
material is hygroscopic.
The most convenient form of weighing bottle is one fitted with an external
cap and made of glass, polythene or polycarbonate. A weighing bottle with an
internally fitting stopper is not recommended; there is always the danger that
small particles may lodge at the upper end of the bottle and be lost when the
stopper is pressed into place.
If the substance is unaffected by exposure to the air, it may be weighed
on a watch glass, or in a disposable plastic container. The weighing funnel
(Fig. 3.8) is very useful, particularly when the solid is to be transferred to a
flask: having weighed the solid into the scoop-shaped end which is flattened so
that it will stand on the balance pan, the narrow end is inserted into the neck
of the flask and the solid washed into the flask with a Stream of water from a
wash bottle.

Fig. 3.8

Woodward and Redman6' have described a specially designed weighing bottle
which will accommodate a small platinum crucible: when a substance has been
ignited in the crucible, the crucible is transferred to the weighing bottle and
subsequently weighed in this. This device obviates the need for a desiccator.
If the substance to be weighed is a liquid, it is placed in a weighing bottle
fitted with a cap carrying a dropping tube.

REAGENTS AND STANDARD SOLUTIONS

3.26 REAGENTS
The purest reagents available should be used for quantitative analysis; the
analytical reagent quality is generally employed. In Great Britain 'AnalaR'
chemicals from BDH Chemicals conform to the specifications given in the
handbook 'AnalaR' Standards for Laboratory Chemicals." In the USA the
American Chemical Society committee on Analytical Reagents has established
standards for certain reagents, and manufacturers supply reagents which are
labelled 'Conforms to ACS Specifications'. In addition, certain manufacturers
market chemicals of high purity, and each package of these analysed chemicals
has a label giving the manufacturer's limits of certain impurities.
With the increasingly lower limits of detection being achieved in various
types of instrumental analysis, there is an ever growing demand for reagents of