Page 117 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

PURIFIED WATER 3.17

To test the delivery time, again separate the components of the stopcock,
dry, grease and reassemble, then fil1 the burette to the zero mark with distilled
water, and place in the holder. Adjust the position of the burette so that the jet
comes inside the neck of a conical flask standing on the base of the burette
stand, but does not touch the side of the flask. Open the stopcock fully, and
note the time taken for the meniscus to reach the lowest graduation mark of
the burette: this should agree closely with the time marked on the burette, and
in any case, must fa11 within the limits laid down by BS 846 (1985).
If the burette passes these two tests, the calibration may be proceeded with.
Fil1 the burette with the distilled water which has been allowed to stand in the
balance room to acquire room temperature: ideally, this should be as near to
20 OC as possible. Weigh a clean, dry stoppered flask of about 100 mL capacity,
then, after adjusting the burette to the zero mark and removing any drop
adhering to the jet, place the flask in position under the jet, open the stopcock
fully and allow water to flow into the flask. As the meniscus approaches the
desired calibration point on the burette, reduce the rate of flow until eventually
it is discharging dropwise, and adjust the meniscus exactly to the required mark.
Do not wait for drainage, but remove any drop adhering to the jet by touching
the neck of the flask against the jet, then re-stopper and re-weigh the flask.
Repeat this procedure for each graduation to be tested; for a 50 mL burette,
this will usually be every 5 mL. Note the temperature of the water, and then,
using Table 3.2, the volume delivered at each point is calculated from the weight
of water collected. The results are most conveniently used by plotting a
calibration curve for the burette.

WATER FOR LABORATORY USE

3.17 PURIFIED WATER
From the earliest days of quantitative chemical measurements it has been
recognised that some form of purification is required for water which is to be
employed in analytical operations, and with increasingly lower limits of detection
being attained in instrumental methods of analysis, correspondingly higher
standards of purity are imposed upon the water used for preparing solutions.
Standards have now been laid down for water to be used in laborat~ries.~
which prescribe limits for non-volatile residue, for residue remaining after
ignition, for pH and for conductance. The British Standard 3978 (1987)
(ISO 3696-1987) recognises three different grades of water.
(a) Grade 3 is suitable for ordinary analytical purposes and may be prepared
by single distillation of tap water, by de-ionisation, or by reverse osmosis:
see bel0 W.
(b) Grade 2 is suitable for more sensitive analytical procedures, such as atomic
absorption spectroscopy and the determination of substances present in
trace quantities. Water of this quality can be prepared by redistillation of
Grade 3 distilled water, or by the distillation of de-ionised water, or of the
product of reverse osmosis procedures.
(c) Grade 1 water is suitable for the most stringent requirements including
high-performance liquid chromatography and the determination of substances
present in ultratrace amounts. Such water is obtained by subjecting