Page 110 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

3 COMMON APPARATUS AND BASIC TECHNIQUES

(ii) those in which the stems are graduated and are employed to deliver various
small volumes at discretion (graduated or measuring pipettes).
The transfer pipette consists of a cylindrical bulb joined at both ends to
narrower tubing: a calibration mark is etched around the upper (suction) tube,
while the lower (delivery) tube is drawn out to a fine tip. The graduated or
measuring pipette is usually intended for the delivery of pre-determined variable
volumes of liquid: it does not find wide use in accurate work, for which a burette
is generally preferred. Transfer pipettes are constructed with capacities of 1, 2,
5, 10, 20, 25, 50 and 100mL; those of 10, 25 and 50mL capacity are most
frequently employed in macro work. They should conform to BS 1583 (1986);
ISO 648-1984 and should carry a colour code ring at the suction end to identify
the capacity [BS 5898 (1980)l: as a safety measure an additional bulb is often
incorporated above the graduation mark. They may be fabricated from lime-soda
or Pyrex glass, and some high-grade pipettes are manufactured in Corex glass
(Corning Glass Works, USA). This is glass which has been subjected to an
ion exchange process which strengthens the glass and also leads to a greater
surface hardness, thus giving a product which is resistant to scratching and
chipping. Pipettes are available to Class A and Class B specifications: for the
latter grade typical tolerance values are:

Pipette capacity 5 10 25 50 100mL
Tolerance 0.01 0.04 0.06 0.08 0.12 mL
whilst for Class A, the tolerances are approximately halved.
To use such pipettes, a suitable pipette filler is first attached to the upper or
suction tube. These devices are obtainable in various forms, a simple version
consisting of a rubber or plastic bulb fitted with glass bal1 valves which can be
operated between finger and thumb: these control the entry and expulsion of
air from the bulb and thus the flow of liquid into and out of the pipette. Suction
by mouth must never be used to fil1 a pipette with liquid chemicals or with a
solution containing chemicals.
The pipette is then rinsed with a little of the liquid to be transferred, and
then filled with the liquid to about 1-2cm above the graduation mark. Any
adhering liquid is removed from the outside of the lower stem by wiping with
a piece of filter paper, and then by careful manipulation of the filler, the liquid
is allowed to run out slowly until the bottom of the meniscus just reaches the
graduation mark: the pipette must be held vertically and with the graduation
mark at eye-level. Any drops adhering to the tip are removed by stroking against
a glass surface. The liquid is then allowed to run into the receiving vessel, the
tip of the pipette touching the wall of the vessel. When the continuous discharge
has ceased, the jet is held in contact with the side of the vessel for 15 seconds
(draining time). At the end of the draining time, the tip of the pipette is removed
from contact with the wall of the receptacle; the liquid remaining in the jet of
the pipette must not be removed either by blowing or by other means.
A pipette will not deliver constant volumes of liquid if discharged too rapidly.
The orifice must be of such size that the time of outflow is about 20 seconds
for a 10mL pipette, 30 seconds for a 25 mL pipette, and 35 seconds for a
50 mL pipette.
Graduated pipettes consist of straight, fairly narrow tubes with no central
bulb, and are also constructed to a standard specification [BS 6696 (1986)l;