For standardizing NaOH solution, which of the following is used as a primary standard?

First, recall what a primary standard is. A primary standard is a substance that possesses a very high purity, is stable in air, has a known and fixed composition, has a reasonably high molar mass (to minimise weighing errors), and reacts completely and stoichiometrically with the reagent being standardised. Such a substance is weighed accurately and then used to determine the exact concentration of a titrant.

We have to select a compound that can accurately standardise an aqueous $$\text{NaOH}$$ solution. Sodium hydroxide absorbs both $$\text{CO}_2$$ and moisture from air, so its concentration changes with time. Hence it cannot be weighed directly to prepare an exactly known solution; instead, it must be titrated against a primary standard acid.

Now, let us examine the given options in the light of the required properties:

$$\textbf{(A) Sodium tetraborate (Borax, } \text{Na}_2\text{B}_4\text{O}_7\cdot10\text{H}_2\text{O}\text{)}$$ Although fairly pure and stable, in basic titrations borax is generally used to standardise strong acids, not strong bases. Moreover, borax itself produces a buffered, weakly basic solution, making end-point detection with phenolphthalein less sharp for direct standardisation of $$\text{NaOH}$$.

$$\textbf{(B) Ferrous ammonium sulphate (Mohr’s salt, } \text{FeSO}_4\cdot(\text{NH}_4)_2\text{SO}_4\cdot6\text{H}_2\text{O}\text{)}$$ Mohr’s salt is mainly employed in redox titrations (e.g., with $$\text{KMnO}_4$$ or $$\text{K}_2\text{Cr}_2\text{O}_7$$). It is not an acid compound suitable for a simple acid-base titration with $$\text{NaOH}$$.

**(C)** Oxalic acid (usually the dihydrate, $$\text{(COOH)}_2\cdot2\text{H}_2\text{O}$$) Oxalic acid dihydrate is highly pure, has a fixed and known stoichiometry, is stable on storage, and possesses a reasonably high molar mass of $$126\ \text{g mol}^{-1}$$. It behaves as a diprotic acid, giving a clear stoichiometric reaction with $$\text{NaOH}$$: $$\text{(COOH)}_2 + 2\,\text{NaOH} \;\longrightarrow\; \text{Na}_2\text{C}_2\text{O}_4 + 2\,\text{H}_2\text{O}$$ Because the stoichiometry is exact and the end point with phenolphthalein is sharp, oxalic acid is ideally suited for the standardisation of sodium hydroxide solutions.

$$\textbf{(D) Dilute HCl}$$ Dilute hydrochloric acid is itself a solution of unknown exact concentration and cannot be weighed directly as a solid; hence it cannot serve as a primary standard.

From the above discussion it is evident that only oxalic acid satisfies all the criteria of a primary standard for the titration of $$\text{NaOH}$$.

Hence, the correct answer is Option C.