Jul 132013
 

Primary Standard Substance

The solution having known concentration is called standard solution. The substance whose standard solution can be prepared directly by dissolving required amount of the substance in definite volume of solution is called primary standard substance and the resulting solution is called primary standard solution.

Oxalic acid, sodium carbonate, sodium thiosulphate etc. are some examples of primary standard substance.
There are some requirements for a substance to be primary standard substance. They are:

  •  The substance should be easily available in pure state.
  • It should not be hygroscopic in nature.
  • It should have good storage property. It means that its strength should last long.
  • IT should have fairly high molecular weight to minimize error during weight measuring.

Normality Factor (f)

It is difficult to measure exact weight of a substance. So, in order to overcome this problem, weight of substance can be measured which is near to the theoretical weight to be measured. When the weight measured is different from the weight to be measured, normality factor comes into action.
Normality factor of a solution may be defined as the ratio of weight measured to the theoretical weight to be measured. Mathematically,

Normality factor (f) = Weight measuredWeight to be measured

The value of normality factor must always be nearly equal to one. The exact normality of a solution is then calculated as:

Exact normality = Normality to be prepared × Normality factor (f)

For example, 0.63 gram of oxalic acid is required to prepare 100ml of 0.1N oxalic acid solution. If 0.65 gram oxalic acid is taken,

Normality factor (f) = 0.650.63 = 1.03
∴ Exact normality = N10 (f = 1.03)

Note: The solution which is standardized with the help of another primary standard solution is called secondary standard solution. In other words, secondary standard solution is the solution whose standard solution cannot be obtained by directly dissolving the required amount of substance in definite volume of solution.