Primary Standard Substance

The reference substance which is used to prepare a primary standard solution is called a primary standard substance. To become a primary standard substance, the substance should fulfill the following requirements.

1. It should be readily avoidable in a pure state.
2. There should not be any difficulty in handling the substance i.e. it should not be extremely hygroscopic or reactive with the atmosphere.
3. The chemical composition of the substance should not alter in a pure state or aqueous form for sufficiently a long time.
4. I should have a high equivalent weight so that error (mistake) during weighing causes the minimum error.

Some examples of primary standard substances are anhydrous (Na2CO3), Oxalic acid crystal.

Deci-normal Solution:-

The solution which contains 1/10th grams equivalent of solute in one liter of a solution is called a decinormal solution.

For example, for the preparation of a decinormal solution of 250ml of sodium carbonate, the weight to be taken can be calculated as:-

W = V * S * E / 1000

where W= weight to be taken 

V= volume in ml

S = strength in normality

E = Equivalent weight

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Thus, 1.325gm of sodium carbonate is accurately measured and is taken in a volumetric flask. Now the water is added to the volumetric flask and makes the solution up to the marked given in the flask.

Normality factor (f)

The normality factor is defined as the ratio of actual weight taken to the theoretical weight. It is denoted by ‘f’.

Normality factor(f) = actual weight taken / theoretical weight

                                = Weight taken / Weight to be taken  

For example, for the preparation of a decinormal solution of sodium carbonate, 1.325gm sodium carbonate should be taken but if we take 1.325gm sodium carbonate for the preparation of the solution then, there will be not the exact decinormal solution of sodium carbonate. The actual concentration of such a solution can be calculated by finding the normality factor.

soln

Normality factor(f) = Weight taken / Weight to be taken  

                             = 1.35 / 1.325

                             = 1.018

Hence, the actual concentration of such solution can be written as  N/10(f=1.018) or 0.1018N

• It helps in determining the exact concentration of a given solution.