Page 133 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

PURIFICATION OF SUBSTANCES 3.27

correspondingly improved specification, and some manufacturers are now
offering a range of specially purified reagents such as the BDH Ltd 'Aristar'
chemicals, specially purified solvents for spectroscopy (e.g. BDH Ltd 'Spectrosol')
and specially prepared reagents for chromatography.
In some instances, where a reagent of the requisite purity is not available, it
rnay be advisable to weigh out a suitable portion of the appropriate pure metal
(e.g. the Johnson, Matthey 'Specpure' range), and to dissolve this in the
appropriate acid.
It must be remembered that the label on a bottle is not an infallible guarantee
of the purity of a chemical, for the following reasons:
(a) Some impurities rnay not have been tested for by the manufacturer.
(b) The reagent rnay have been contaminated after its receipt from the
manufacturers either by the stopper having been left open for some time,
with the consequent exposure of the contents to the laboratory atmosphere
or by the accidental return of an unused portion of the reagent to the bottle.
(c) In the case of a solid reagent, it rnay not be sufficiently dry. This rnay be
due either to insufficient drying by the manufacturers or to leakage through
the stoppers during storage, or to both of these causes.
However, if the analytical reagents are purchased from a manufacturing firm
of repute, the instructions given (a) that no bottle is to be opened for a longer
time than is absolutely necessary, and (b) that no reagent is to be returned to the
bottle after it has been removed, the likelihood of any errors arising from some
of the above possible causes is considerably reduced. Liquid reagents should be
poured from the bottle; a pipette should never be inserted into the reagent
bottle. Particular care should be taken to avoid contamination of the stopper
of the reagent bottle. When a liquid is poured from a bottle, the stopper should
never be placed on the shelf or on the working bench; it rnay be placed upon
a clean watchglass, and many chemists cultivate the habit of holding the stopper
between the thumb and fingers of one hand. The stopper should-be returned
to the bottle immediately after the reagent has been removed, and al1 reagent
bottles should be kept scrupulously clean, particularly round the neck or mouth
of the bottle.
If there is any doubt as to the purity of the reagents used, they should be
tested by standard methods for the impurities that might cause errors in the
determinations. It rnay be mentioned that not al1 chemicals employed in
quantitative analysis are available in the form of analytical reagents; the purest
commercially available products should, if necessary, be purified by known
methods: see below. The exact mode of drying, if required, will Vary with the
reagent; details are given for specific reagents in the text.

3.27 PURIFICATION OF SUBSTANCES

If a reagent of adequate purity for a particular determination is not available,
then the purest available product must be purified: this is most commonly done
by recrystallisation from water. A known weight of the solid is dissolved in a
volume of water sufficient to give a saturated or nearly saturated solution at
the boiling point: a beaker, conical flask or porcelain dish rnay be used. The
hot solution is filtered through a fluted filter paper placed in a short-stemmed
funnel, and the filtrate collected in a beaker: this process will remove insoluble