Assertion: \[{\rm{N}}{{\rm{a}}_{\rm{2}}}{\rm{C}}{{\rm{r}}_{\rm{2}}}{{\rm{O}}_{\rm{4}}}\]
is not a primary standard in volumetric analysis.
Reason: \[{\rm{N}}{{\rm{a}}_{\rm{2}}}{\rm{C}}{{\rm{r}}_{\rm{2}}}{{\rm{O}}_{\rm{4}}}\]
is hygroscopic.
A. Assertion is true, Reason is true and Reason is a correct explanation for Assertion
B. Assertion is true, Reason is true and Reason is a correct explanation for Assertion
C. Assertion is true but Reason is false
D. Assertion is false but Reason is true

Hint: We know that volumetric analysis is a technique of quantitative analysis in which the quantity of a substance is known by measuring the volume that it occupies in the volume of a second substance whose combination with the first is in known proportions.

Complete step by step answer:
Let’s understand the volumetric process in detail. The name itself indicates that, in this method, for a solution whose concentration is known, the volume is measured and applied this volume in calculating the concentration of the analyte.

Let’s understand the primary standard in volumetric analysis. The primary standard is a pure compound that is used to prepare a standard solution measuring a certain amount of it and then diluting it to a definite volume of solution in the volumetric flask.

Now, we discuss the hygroscopic substance. It is that type of substance that attracts water readily from the surroundings either by absorption or adsorption. Some examples of hygroscopic substances are sulphuric acid (conc.), sodium dichromate etc.

Here, given that sodium dichromate \[\left( {{\rm{N}}{{\rm{a}}_{\rm{2}}}{\rm{C}}{{\rm{r}}_{\rm{2}}}{{\rm{O}}_{\rm{4}}}} \right)\]is not used in volumetric analysis. This statement is correct because the nature of sodium dichromate is hygroscopic (reacts readily with water) and that’s why measuring its accurate weight is impossible in normal atmospheric conditions. The reaction of a hygroscopic substance with moisture (water) results in inaccuracy in weight.

Therefore, we can say that due to the hygroscopic nature of \[{\rm{N}}{{\rm{a}}_{\rm{2}}}{\rm{C}}{{\rm{r}}_{\rm{2}}}{{\rm{O}}_{\rm{4}}}\], it is not used in the volumetric process.

Hence, the correct answer is option A.

Note:
A good primary standard is the one that possesses high purity level, low reactivity, and equivalent mass. Sodium carbonate can be used as a primary standard as its molarity remains constant for a long period of time.