2.9 SPECIFIC NOTES ON THE USE OF VOLUMETRIC GLASSWARE
2.9.1 The zero mark on a burette is a defined mark. All other graduations are positioned
relative to this mark, usually by establishing the full capacity mark relative to the zero
mark, and then sub-dividing the intervening length to give intermediate graduations.
Thus all graduations, except for the zero mark, are subject to uncertainty.
2.9.2 When using volumetric glassware, there are several recognised rules of good
practice:
(a) wherever possible, burettes or graduated pipettes shall be chosen so that the
intended delivered volume is at least 50% of the total capacity of the glassware.
Similarly, measuring cylinders shall be chosen so that the measured volume is at
least 50% of the total capacity of the glassware;
(b) the calibration of burettes, graduated pipettes and measuring cylinders, where
appropriate, should be performed at several points rather than at just a single point
at the maximum capacity;
(c) when using burettes, optical aids are useful to enable the scale to be read more
easily.
2.10 CARE OF VOLUMETRIC GLASSWARE
2.10.1 Care should be taken to ensure that volumetric glassware is used and maintained in
a way that does not cause damage to it nor alter its calibration. Extremes of
temperature, including high temperature cleaning, and oven or hot air drying, which
might lead to permanent changes in the capacity, shall be avoided. Certain solvents,
strong acids/alkalis or surfactants may attack or alter the wetting characteristics of the
glass, which in turn may affect draining properties. Machine washing should be
avoided wherever possible since this is a common source of mechanical damage.
Any contamination that is not water-soluble should be removed with an appropriate
solvent before the glassware is washed. Any guidance available from the supplier
should be followed. Before use, the tips of pipettes and burettes shall be examined
for mechanical damage and possible obstructions.
2.10.2 Grease is sometimes used as a lubricant for burette stopcocks. As well as being an
obvious source of chemical contamination, it can adversely affect the volumetric
capacity. Lubricating grease, if necessary, should therefore be used carefully.
2.11 MARKING AND SEGREGATION
2.11.1 Individually calibrated items shall be readily identifiable against a record of the
calibration results. Any items calibrated for a specific application should be
segregated from the general stock of glassware.
2.11.2 Where additional identification is required, the methods of marking shall avoid any
risk of damage to the item, and the mark positioned in a non-critical area, eg above
the top graduation mark. Any marking media should be resistant to solvents and
other chemicals. Mechanical etching is the preferred method of marking.
2.12 MEASURING CYLINDERS
2.12.1 These items are not intended for work demanding the highest accuracy. The original
Class A and B categories are no longer officially in use.
2.12.2 For many applications involving the use of cylinders for dispensing approximate
volumes of liquid reagents, no calibration is necessary. Where cylinders are used to
make quantitative measurements of volumes, calibration of individual items may be
necessary. For example, this may be required in the determination of the bulk