Page 116 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

3 COMMON APPARATUS AND BASIC TECHNIQUES

In al1 calibration operations, the apparatus to be calibrated must be carefully
cleaned and allowed to stand adjacent to the balance which is to be employed,
together with a supply of distilled or de-ionised water, so that they assume the
ternperature of the room. Flasks will also need to be dried, and this can be
accomplished by rinsing twice with a little acetone and then blowing a current
of air through the flask to remove the acetone.
Graduated flask. After allowing the clean dry flask to stand in the balance
room for an hour it is stoppered and weighed. A small filter funnel, the stem
of which has been drawn out so that it reaches below the graduation mark of
the flask, is then inserted into the neck and de-ionised (distilled) water, which
has also been standing in the balance room for an hour, is added slowly until
the mark is reached. The funnel is then carefully removed, taking care not to
wet the neck of the flask above the mark, and then, using a dropping tube,
water is added dropwise until the meniscus stands on the graduation mark. The
stopper is replaced, the flask re-weighed, and the temperature of the water noted.
The true volume of the water filling the flask to the graduation mark can be
calculated with the aid of Table 3.2.
Pipette. The pipette is filled with the distilled water which has been standing
in the balance room for at least an hour, to a short distance above the mark.
Water is run out until the meniscus is exactly on the mark, and the out-flow is
then stopped. The drop adhering to the jet is removed by bringing the surface
of some water contained in a beaker in contact with the jet, and then removing
it without jerking. The pipette is then allowed to discharge into a clean, weighed
stoppered flask (or a large weighing bottle) and held so that the jet of the pipette
is in contact with the side of the vessel (it will be necessary to incline slightly
either the pipette or the vessel). The pipette is allowed to drain for 15 seconds
after the outflow has ceased, the jet still being in contact with the side of the
vessel. At the end of the draining time the receiving vessel is removed from
contact with the tip of the pipette, thus removing any drop adhering to the
outside of the pipette and ensuring that the drop remaining in the end is always
of the same size. To determine the instant at which the outflow ceases, the
motion of the water surface down the delivery tube of the pipette is observed,
and the delivery time is considered to be complete when the meniscus comes
to rest slightly above the end of the delivery tube. The draining time of
15 seconds is counted from this moment. The receiving vessel is weighed, and
the temperature of the water noted. The capacity of the pipette is then calculated
with the aid of Table 3.2. At least two determinations should be made.
Burette. If it is necessary to calibrate a burette, it is essential to establish that
it is satisfactory with regard to (a) leakage, and (b) delivery time, before
undertaking the actual calibration process. To test for leakage, the plug is
removed from the barre1 of the stopcock and both parts of the stopcock are
carefully cleaned of al1 grease; after wetting well with de-ionised water, the
stopcock is reassembled. The burette is placed in the holder, filled with distilled
(de-ionised) water, adjusted to the zero mark, and any drop of water adhering
to the jet removed with a piece of filter paper. The burette is then allowed to
stand for 20 minutes, and if the meniscus has not fallen by more than one scale
division, the burette may be regarded as satisfactory as far as leakage is
concerned.