Question

Which of the following reagents are used as primary standard?
I: $KMn{{O}_{4}}$; II: $NaOH$; III: ${{K}_{2}}C{{r}_{2}}{{O}_{7}}$; IV: $FeS{{O}_{4}}{{(N{{H}_{4}})}_{2}}S{{O}_{4}}.6{{H}_{2}}O$; V: ${{H}_{2}}{{C}_{2}}{{O}_{4}}.2{{H}_{2}}O$
A.All except II, IV
B.All except I, II
C.All except I, II, III
D.Only IV

Answer 1

The primary standard has been defined as the chemical reagent which is considered to be highly pure, stable and should not contain the hydration of water. The weight of the molecule is high. The primary standard solution does not change its concentration and the potential of hydrogen or pH with time, environmental factors and also with temperature.

Complete step-by-step answer: The primary standard solution are chemical compounds which are available in the purest forms. They are not hygroscopic in nature and also the presence of water in them is not at all. They do not change their concentration and are very pure and highly stable.
The $KMn{{O}_{4}}$ is not used as the primary standard because being a good oxidising agent it has the tendency to be reduced by other reagents. It tends to change its concentration upon free leaving. So it is not used as the primary standard.
The sodium hydroxide can be used as the secondary standard but not as primary because it tends to change its concentration with time and also it is not very pure.
Potassium dichromate is used as the primary standard solution because it is pure in nature and is stable too and it does not change its concentration with time and environmental factors.
Mohr’s salt $FeS{{O}_{4}}{{(N{{H}_{4}})}_{2}}S{{O}_{4}}.6{{H}_{2}}O$ is used as primary standard solution because it is stable and pure. It does not change its concentration with change in environmental factors.
Oxalic acid is a primary standard reagent because it is pure and stable also. It does not tend to change its concentration with the environmental factors.

So the correct answer is option B as all are primary standard reagent except $KMn{{O}_{4}}$ and sodium hydroxide.

Note: The use of primary standard solution is done for calculating the concentration of the unknown solutions or the secondary standard solutions. The change in concentration of the secondary standard solution is seen. This is due to the presence of impurities in them. For example EDTA.