Page 114 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

3 COMMON APPARATUS AND BASIC TECHNIQUES

place the fingers of the left hand behind the burette and the thumb in front,
and hold the tap between the thumb and the fore and middle fingers. In this
way, there is no tendency to pull the plug out of the barre1 of the stopcock, and
the operation is under complete control. Any drop adhering to the jet after the
liquid has been discharged is removed by bringing the side of the receiving
vesse1 into contact with the jet. During the delivery of the liquid, the flask may
be gently rotated with the right hand to ensure that the added liquid is well
mixed with any existing contents of the flask.

3.13 WEIGHT BURETTES
For work demanding the highest possible accuracy in transferring various
quantities of liquids, weight burettes are employed. As their name implies, they
are weighed before and after a transfer of liquid. A very useful form is shown
diagrammatically in Fig. 3.3(a). There are two ground-glass caps of which the
lower one is closed, whilst the upper one is provided with a capillary opening;
the loss by evaporation is accordingly negligible. For hygroscopic liquids, a
small ground-glass cap is fitted to the top of the capillary tube. The burette is
roughly graduated in 5 mL intervals. The titre thus obtained is in terms of
weight loss of the burette, and for this reason the titrants are prepared on a
weightlweight basis rather than a weight/volume basis. The errors associated
with the use of a volumetric burette, such as those of drainage, reading, and
change in temperature, are obviated, and weight burettes are especially useful
when dealing with non-aqueous solutions or with viscous liquids.

fLll
Fig. 3.3

An alternative form of weight burette due to Redman6* consists of a
glass bulb, flattened on one side so that it will stand on a balance pan. Above
the flattened side is the stopcock-controlled discharge jet, and a filling orifice
which is closed with a glass stopper. The stopper and short neck into which it
fits are pierced with holes, by alignment of which air can be admitted, thus
permitting discharge of the contents of the burette through the delivery jet.