Volumetric Analysis
Volumetric Analysis is a method that has changed very little over many years.
It involves the reacting a measured volume of a standard solution with a measured
volume of the solution of unknown concentration. These are normally acid / base
reactions and the process of is called performing a titration
Standard Solutions
When we select a suitable standard for volumetric analysis we need to ensure
that it has a precisely known concentration since the accuracy of our calculated
concentration depends on it. However all strong acids and bases as well as some
weak acids and bases present some difficulty in this regard.
Strong Acids
These are supplied by the manufacturer in concentrated form and are by their
nature of variable concentration, which is provided only as an approximate value.
As a result we cannot know the precise concentration after diluting them.
Strong Bases
NaOH can absorb CO2 from the atmosphere, which means that as you attempt to
weigh the commercially available pellets, the balance will show that the pellets
arte continually increasing in weight. Also some of the base will be consumed
in the acid / base reaction with the CO2. Also NaOH is hygroscopic, which means
it will absorbs the moisture from the atmosphere eventually dissolving it. This
process will also cause the mass of the NaOH to increase during the weighing.
As a result we cannot directly determine their concentration from their mass
and volume
Such substances are called secondary standards because they must always be
standardised before they are used to determine the concentration of any other
substance.
A substance that can be weighed out to provide a solution of known composition
and concentration is called a primary standard.
Criteria for a Primary Standard
Obtainable in very pure form
Has a known formulae (including a known degree of hydration)
Does not absorb any moisture of other chemicals from the air
Does not decompose or lose water of hydration on storage or when exposed to
air
Has a reasonable high Molar Mass
Is completely soluble in water at Room Temperature
Example of primary standards for acid bases reactions are the weak base anhydrous sodium carbonate Na2CO3 and the weak monoprotic acid, potassium hydrogen phthalate KH(C8H4O4)
Uncertainties
You need to be aware of the limitations of the equipment you are using in the
analysis. Each instrument has a particular uncertainty, which tells us the real
range in which the real range probably occurs.
These uncertainties have a cumulative effect on the concentration we calculate
from the readings of these instruments. An analyst needs to consider this uncertainty
when determining whether to accept a given result as being satisfactory. Take
for an example a tablet where it has been determined that there is 296 ±
5 mg of Asprin in it. If the tablet is meant to have 300 mg of Asprin in it,
then this result is quite satisfactory
Anhydrous contains no water hydration , ie zero degrees of hydration
Degree of Hydration means the number of water molecules per formula unit in
a molecule, ie CuSO4.5H2O, has 5 degrees of hydration