Page 113 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

BURETTES 3.12

laboratory clamp is not recommended: the ideal type of holder permits the
burette to be read without the need of removing it from the stand.
Lubricants for glass stopcocks. The object of lubricating the stopcock of a
burette is to prevent sticking or 'freezing' and to ensure smoothness in action.
The simplest lubricant is pure Vaseline, but this is rather soft, and, unless used
sparingly, portions of the grease may readily become trapped at the point where
the jet is joined to the barrel of the stopcock, and lead to blocking of the jet.
Various products are available commercially which are better suited to the
lubrication of burette stopcocks. Silicone-containing lubricants must be avoided
since they tend to 'creep' with consequent contamination of the walls of the
burette.
To lubricate the stopcock, the plug is removed from the barrel and two thin
streaks of lubricant are applied to the length of the plug on lines roughly midway
between the ends of the bore of the plug. Upon replacing in the barrel and
turning the tap a few times, a uniform thin film of grease is distributed round
the ground joint. A spring or some other form of retainer may be subsequently
attached to the key to lessen the chance of it becoming dislodged when in use.
Reference is again made to the Teflon stopcocks and to the diaphragm type
of burette tap which do not require lubrication.
Mode of use of a burette. If necessary, the burette is thoroughly cleaned using
one of the cleaning agents described in Section 3.8 and is then well rinsed with
distilled water. The plug of the stopcock is removed from the barrel, and after
wiping the plug and the inside of the barrel dry, the stopcock is lubricated as
described in the preceding paragraph. Using a small funnel, about 10 mL of the
solution to be used are introduced into the burette, and then after removing
the funnel, the burette is tilted and rotated so that the solution flows over the
whole of the interna1 surface; the liquid is then discharged through the stopcock.
After repeating the rinsing process, the burette is clamped oertically in the burette
holder and then filled with the solution to a little above the zero mark. The
funnel is removed, and the liquid discharged through the stopcock until the
lowest point of the liquid meniscus is just below the zero mark; the jet is inspected
to ensure that al1 air bubbles have been removed and that it is completely full
of liquid. To read the position of the meniscus, the eye must be at the same
level as the meniscus, in order to avoid errors due to parallax. In the best type
of burette, the graduations are carried completely round the tube for each
millilitre (mL) and half-way round for the other graduation marks: parallax is
thus easily avoided. To aid the eye in reading the position of the meniscus a
piece of white paper or cardboard, the lower half of which is blackened either
by painting with du11 black paint or by pasting a piece of du11 black paper upon
it, is employed. When this is placed so that the sharp dividing line is 1-2 mm
below the meniscus, the bottom of the meniscus appears to be darkened and is
sharply outlined against the white background; the level of the liquid can then
be accurately read. A variety of 'burette readers' are available from laboratory
supply houses, and a home-made device which is claimed to be particularly
effective has been described by Woodward and Redman.6c For al1 ordinary
purposes readings are made to 0.05mL, but for precision work, readings
should be made to 0.01-0.02 mL, using a lens to assist the estimation of the
subdivisions.
To deliver liquid from a burette into a conical flask or other similar receptacle,