Chemistry Reference
In-Depth Information
The first solution, above, is slightly stronger than 1
M
, since the factor is
greater than 1.000. The second solution is slightly weaker than half molar,
as the factor is less than 1.000. It follows that a solution with a factor of
1.000 is of precisely the stated molarity.
If the absolute molarity of the solution is required, it can easily be
found by multiplying the factor and the nominal molarity. For instance, in
the examples above, the first solution has an absolute molarity of 1
M
1.026
1.026
M
, which as predicted above is slightly stronger than 1
M
.
Similarly, the second solution has an absolute molarity of 0.499
M
(i.e. 0.5
M
0.998). It follows from this that the factor of a solution is simply the ratio
Actual concentration
——————————————
Desired or nominal concentration
Factors are used in volumetric analysis because they simplify calculations (a
laudable aim, in any subject). Consider the first solution above: the strength
of the solution is 1
M
(1.026). If 10 mL of this solution were removed, by
pipette, transferred to a 100 mL volumetric flask, and made up to volume
with water, the resulting solution would have a concentration of 0.1
M
(1.026). The original solution has been diluted tenfold, but the factor of the
new solution remains as 1.026. This illustrates an important principle,
namely, that once a factor has been determined for a volumetric solution,
subsequent dilution or reaction will not affect it (although see later for an
exception to this).
Once the factor for a solution is known (i.e. once the solution has
been
standardised
), multiplication of the experimentally determined
volume by the factor will yield what the volume would have been if the solu-
tion had been precisely the nominal molarity (i.e. if the factor had been
1.000). In practice, very few volumetric solutions are factor 1.000; this is
due, in the main, to the time that would be taken to weigh out a sample to
four decimal places. Volumetric solutions are usually prepared by weighing
out approximately the desired weight of sample, then standardising the
resulting solution against a solution of known concentration.
All volumetric solutions used in pharmaceutical analysis are prepared
from a
primary standard
. This is a compound that can be obtained in a very
high level of purity (
99.9%). Examples of compounds used as primary
standards include sodium carbonate (Na
2
CO
3
) and potassium hydrogen
phthalate (C
8
H
5
O
4
K). Compounds such as these can be weighed accurately,
to four or even six decimal places, and made up to volume in a volumetric
flask to give a solution of known molarity. Solutions that are prepared by
standardisation against a primary standard are referred to as secondary
standards. A solution standardised against a secondary standard is termed
