Page 107 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

GRhOUATED APPARATUS 3.8

as thus defined and the cubic decimetre was established as
1 litre = 1.000028 dm3 or 1 millilitre = 1.000028 cm3
In 1964 the Conférence Générale des Poids et des Mésures (CGPM) decided
to accept the term litre as a special name for the cubic decimetre, and to discard
the original definition. With this new meaning of the term litre (L), the millilitre
(mL) and the cubic centimetre (cm3) are identical.

3.8 GRADUATED APPARATUS
The most commonly used pieces of apparatus in titrimetric (volumetric) analysis
are graduated flasks, burettes, and pipettes. Graduated cylinders and weight
pipettes are less widely employed. Each of these will be described in turn.
Graduated apparatus for quantitative analysis is generally made to
specification limits, particularly with regard to the accuracy of calibration. In
the United Kingdom there are two grades of apparatus available, designated
Class A and Class B by the British Standards Institution. The tolerance limits
are closer for Class A apparatus, and such apparatus is intended for use in work
of the highest accuracy: Class B apparatus is employed in routine work. In the
United States, specifications for only one grade are available from the National
Bureau of Standards at Washington, and these are equivalent to the British
Class A.
Cleaning of glass apparatus. Before describing graduated apparatus in detail,
reference must be made to the important fact that al1 such glassware must be
perfectly clean and free from grease, otherwise the results will be unreliable.
One test for cleanliness of glass apparatus is that on being filled with distilled
water and the water withdrawn, only an unbroken film of water remains. If the
water collects in drops, the vesse1 is dirty and must be cleaned. Various methods
are available for cleaning glassware.
Many commercially available detergents are suitable for this purpose, and
some manufacturers market special formulations for cleaning laboratory
glassware; some of these, e.g. 'Decon 90' made by Decon Laboratories of
Portslade, are claimed to be specially effective in removing contamination due
to radioactive materials.
'Teepol' is a relatively mild and inexpensive detergent which may be used
for cleaning glassware. The laboratory stock solution may consist of a 10 per
cent solution in distilled water. For cleaning a burette, 2 mL of the stock solution
diluted with 50 mL of distilled water are poured into the burette, allowed to stand
for 4 to 1 minute, the detergent run off, the burette rinsed three times with tap
water, and then several times with distilled water. A 25 mL pipette may be
similarly cleaned using 1 mL of the stock solution diluted with 25-30 mL of
distilled water.
A method which is frequently used consists in filling the apparatus with
'chromic acid cleaning mixture' (CARE), a nearly saturated solution of
powdered sodium dichromate or potassium dichromate in concentrated sulphuric
acid, and allowing it to stand for several hours, preferably overnight; the acid
is then poured off, the apparatus thoroughly rinsed with distilled water, and
allowed to drain until dry. [It may be mentioned that potassium dichromate
is not very soluble in concentrated sulphuric acid (about 5 g per litre), whereas