Question

Why NaOH is not a primary standard?

Answer 1

Hint: Primary standard is a reagent that is very pure, representative of the number of moles the substance contains, and easily weighed. A reagent is a chemical used to cause a chemical reaction with another substance. Often, reagents are used to test for the presence or quantity of specific chemicals in solution. Primary standards are typically used in titration to determine an unknown concentration and in other analytical chemistry techniques.

Complete step by step answer:
A good primary standard has criteria which is given below:
a)High level of purity
b)High equivalent weight
c)Nontoxic
c)Inexpensive and readily available
d)Is not likely to absorb moisture from the air to reduce changes in mass in humid versus dry environments.
NaOH has no criteria given above. It is not suitable for the primary standards because they absorb the moisture from the atmosphere. It also absorbs carbon dioxide from the atmosphere.
Hence, NaOH is not a primary standard because it has no quality of primary standard.

Additional information:
Analytical standards are divided into two categories: a primary standard is a reagent for which we can dispense an accurately known amount of analyte. Reagents that do not meet these criteria are secondary standards.

Note: NaOH is secondary standard because NaOH contains impurities of NaCl, $N{a_2}S{O_4}$ etc. It absorbs water from the atmosphere and determines the concentration of NaOH in a solution, which is titrated against a primary standard weak acid. That’s why we remember that NaOH is not the primary standard, it is a secondary standard.