Page 109 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

PIPETTES 3.1 1

of the glass. For dilute (e.g. 0.1 M) aqueous solutions, the corrections can be
regarded as approximately the same as for water, but with more concentrated
solutions the correction increases, and for non-aqueous solutions the corrections
can be quite large.6b

3.1 0 GRADUATED FLASKS

A graduated flask (known alternatively as a volumetric flask or a measuring
flask), is a flat-bottomed, pear-shaped vesse1 with a long narrow neck. A thin
line etched around the neck indicates the volume that it holds at a certain
definite temperature, usually 20 OC (both the capacity and temperature are
clearly marked on the flask); the flask is then said to be graduated to contain.
Flasks with one mark are always taken to contain the volume specified. A flask
may also be marked to deliver a specified volume of liquid under certain definite
conditions; these are, however, not suitable for exact work and are not widely
used. Vessels intended to contain definite volumes of liquid are marked C or
TC or In, while those intended to deliver definite volumes are marked D or
TD or Ex.
The mark extends completely around the neck in order to avoid errors due
to parallax when making the final adjustment; the lower edge of the meniscus
of the liquid should be tangential to the graduation mark, and both the front
and the back of the mark should be seen as a single line. The neck is made
narrow so that a small change in volume will have a large effect upon the height
of the meniscus: the error in adjustment of the meniscus is accordingly small.
The flasks should be fabricated in accordance with BS 5898 (1980)* and the
opening should be ground to standard (interchangeable) specifications and fitted
with an interchangeable glass or plastic (commonly polypropylene) stopper.
They should conform to either Class A or Class B specification BS 1792 (1982);
examples of permitted tolerances for Class B grade are as follows:

Flask size 5 25 1 O0 250 lOOOmL
Tolerance 0.04 0.06 0.15 0.30 0.80 mL

For Class A flasks the tolerances are approximately halved: such flasks may be
purchased with a works calibration certificate, or with a British Standard Test
(BST) Certificate.
Graduated flasks are available in the following capacities: 1, 2, 5, 10, 20, 50,
100, 200, 250, 500, 1000,2000 and 5000 mL. They are employed in making up
standard solutions to a given volume; they can also be used for obtaining, with
the aid of pipettes, aliquot portions of a solution of the substance to be analysed.

3.1 1 PIPETTES
Pipettes are of two kinds: (i) those which have one mark and delioer a small,
constant volume of liquid under certain specified conditions (transfer pipettes);

* Many modern Eitish Bandards are closely linked to the specifications laid down by the
International Standardisation organisation based in Geneva; in the above example the relevant
-
reference is to ISO 384-1978.'