Question

Which of the following is/are primary standard?
A) Oxalic acid
B) Sodium thiosulphate
C) Sodium hydroxide
D) Potassium dichromate

Answer 1

Hint: A primary standard is a chemical reagent that is highly pure, stable and should not have water of hydration and it has high molecular weight.
Primary standard solution which does not change its concentration and pH with time, temperature and with environment factors.

Complete answer:
> Primary standard chemical compounds are available in purest form, they are not hygroscopic and there is no water content in their pure form. We know that the primary standard solution won’t change concentration and it is extremely pure, stable.
> Coming to given options, option A, Oxalic acid. Oxalic acid solution is a primary standard because it is highly pure, stable and does not change its concentration with environmental factors.
> Coming to option B, Sodium thiosulphate, it is a secondary standard reagent. Sodium thiosulphate is also called hypo. Its concentration changes with time and it is not highly pure. So, it is not correct.
> Coming to option C, Sodium hydroxide. It is a secondary standard reagent. Sodium hydroxide changes its concentration with time and it is not highly pure. So, it is not correct.
> Coming to option D, Potassium dichromate. Potassium dichromate solution is a primary standard solution because it is highly pure, stable and does not change its concentration with environmental factors.
Therefore Oxalic acid and Potassium dichromate are primary standard reagents and don’t contain any hydration. (means no water molecules present in them in pure form)

So, the correct options are A, Oxalic acid and D, Potassium dichromate.

Note: By using primary standard solution we can find the concentration of unknown solutions or secondary standard solutions.
Secondary solutions change its concentration with time due to the impurities (means hydrated) present in it. Ex: Sodium Hydroxide, EDTA etc.