Page 111 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

PIPETTES 3.1 1

they are likewise colour-coded in accordance with ISO 1769. Three different
types are available:
Type 1 delivers a measured volume from a top zero to a selected graduation
mark;
Type 2 delivers a measured volume from a selected graduation mark to the
jet: i.e. the zero is at the jet;
Type 3 is calibrated to contain a given capacity from the jet to a selected
graduation mark, and thus to remooe a selected volume of solution.
For Type 2 pipettes the final drop of liquid remaining in the tip must be expelled,
which is contrary to the usual procedure. Such pipettes are therefore distinguished
by a white or sand-blasted ring near the top of the pipette.
For dealing with smaller volumes of solution, micropipettes, often referred
to as syringe pipettes, are employed. These can be of a 'push-button' type, in
which the syringe is operated by pressing a button on the top of the pipette:
the plunger travels between two fixed stops and so a remarkably constant volume
of liquid is delivered. Such pipettes are fitted with disposable plastic tips (usually
of polythene or polypropylene) which are not wetted by aqueous solutions, thus
helping to ensure constancy of the volume of liquid delivered. The liquid is
contained entirely within the plastic tip and so, by replacing the tip, the same
pipette can be employed for different solutions. Such pipettes are available to
deliver volumes of 1 to 1000 pL, and the delivery is reproducible to within about
1 per cent.
Microlitre syringe pipettes are available with capacities ranging from 10 to
250 pL and with the body of the pipette calibrated. When fitted with a needle
tip they are particularly useful for introducing liquids into gas chromatographs
(Chapter 9).
Micrometer syringe pipettes are fitted with a micrometer head which operates
the plunger of the syringe, and when fitted with a stainless steel needle tip can
be used for the dropwise addition of liquid; the volume added is recorded by
the micrometer.

Automatic pipettes. The Dafert pipette (Fig. 3.2) is an automatic version of a
transfer pipette. One side of the two-way tap is connected to a reservoir
containing the solution to be dispensed. When the tap is in the appropriate
position, solution fills the pipette completely, excess solution draining away

Fig. 3.2