Chemistry Reference
In-Depth Information
Graduated pipettes
Graduated pipettes are calibrated to allow a single piece of glassware to
deliver a range of volumes: common sizes are 1 mL and 10 mL. These
pipettes are considerably less accurate than transfer pipettes, and there is no
place for them in an analytical chemistry laboratory. If very small volumes
need to be transferred, use should be made of accurate glass syringes (e.g.
a 'Hamilton' syringe) or an automatic micropipette.
Burettes
Burettes are used to deliver variable volumes of reagent accurately. The most
useful size is the 50 mL burette. These burettes are calibrated in units of
0.1 mL, but students should be encouraged to read to the nearest 0.05 mL.
Once students have achieved some skill in titration techniques, they will be
able to read the burette to the nearest 0.02 mL. This will involve splitting
each 0.1 mL graduation into five - i.e. 0.02, 0.04, 0.06, 0.08 and 0.1 mL.
All of the volumetric glassware described above is designed for use at
ambient room temperature and should never be used for hot liquids or
placed in hot ovens and the like to dry.
Units of concentration
Before we consider topics such as the design of an assay, calculation of drug
purity, and so on, it is useful to revise the units and terms chemists use for
amount of substance and concentration. The fundamental unit of quantity
or amount of substance used in chemistry is the
mole
. The mole is the
amount of a substance (either elements or compounds) that contains the
same number of atoms or molecules as there are in 12.0000 g of carbon-12.
This number is known as the Avogadro number (after Amedeo Avogadro,
an Italian chemist) or Avogadro's constant, and has the value 6.02
10
23
.
When this amount of substance is dissolved in solvent (usually water) and
made up to 1 litre, a 1 molar (1
M
) solution is produced. In a similar way, if
one mole of substance were made up to 2 litres of solvent, a 0.5
M
solution
would result, and so on. The litre is not the SI unit of volume but, along
with the millilitre (mL), is still used in the
British Pharmacopoeia
.
In pharmaceutical analysis laboratories, concentration is usually
expressed as (for example) 1
M
(1.026) or 0.5
M
(0.998). The nominal
concentration is given as molarity, while the number in brackets refers to the
factor
(
f
) of the solution. The factor of a volumetric solution tells you by
how much the given solution differs from the nominal, or desired strength.
