Page 112 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

3 COMMON APPARATUS AND BASIC TECHNIQUES

through the overfiow chamber. The pipette now contains a definite volume of
solution which is delivered to the receiver by appropriate manipulation of the
tap. These pipettes, are available in a range of sizes from 5-100 mL and are
useful in routine work.
Autodispensers are also useful for measuring definite volumes of solutions
on a routine basis. Solution is forced out of a container by depressing a syringe
plunger: the movement of the plunger and hence the volume of liquid dispensed
are controlled by means of a moveable clamp. The plunger is spring-loaded so
that, when released, it returns to its original position and is immediately ready
for operation again.
Tilting pipettes, which are attached to a reagent bottle, are only suitable for
delivering approximate volumes of solution.

3.12 BURETTES

Burettes are long cylindrical tubes of uniform bore throughout the graduated
length, terminating at the lower end in a glass stopcock and'a jet; in cheaper
varieties, the stopcock may be replaced by a rubber pinch valve incorporating
a glass sphere. A diaphragm-type plastic burette tap is marketed: this can be
fitted to an ordinary burette and provides a delicate control of the outflow of
liquid. The merits claimed include: (a) the tap cannot stick, because the liquid
in the burette cannot come into contact with the threaded part of the tap; (b)
no lubricant is generally required; (c) there is no contact between ground glass
surfaces; and (d) burettes and taps can be readily replaced. Burette taps made
of polytetrafluoroethylene (PTFE or Teflon) are also available; these have the
great advantage that no lubricant is required.
It is sometimes advantageous to employ a burette with an extended jet which
is bent twice at right angles so that the tip of the jet is displaced by some
7.5-10 cm from the body of the burette. Insertion of the tip of the burette into
complicated assemblies of apparatus is thus facilitated, and there is a further
advantage, that if heated solutions have to be titrated the body of the burette
is kept away from the source of heat. Burettes fitted with two-way stopcocks
are useful for attachment to reservoirs of stock solutions.
As with other graduated glassware, burettes are produced to both Class A
and Class B specifications in accordance with the appropriaté standard
[BS 846 (1985); ISO 385 (1984)], and Class A burettes may be purchased with
BST Certificates. Al1 Class A and some Class B burettes have graduation marks
which completely encircle the burette; this is a very important feature for the
avoidance of parallax errors in reading the burette. Typical values for the
tolerances permitted for Class A burettes are:

Total capacity 5 10 50 100mL
Tolerance 0.02 0.02 0.05 0.10 mL

For Class B, these values are approximately doubled. In addition to the volume
requirements, limits are also imposed on the length of the graduated part of
the burette and on the drainage time.
When in use, a burette must be firmly supported on a stand, and various
types of burette holders are available for this purpose. The use of an ordinary