Compounds that should not be used as primary standards in titrimetric analysis are: A. Na2Cr2O7 B. Oxalic acid C. NaOH…

🧠 What makes a primary standard? A primary standard must be highly pure, stable in air, non-hygroscopic, and not change with time. If it absorbs moisture, $\ce{CO2}$, or oxidizes in storage, it cannot give a true mass.

🗺️ Check each compound

• A. $\ce{Na2Cr2O7}$ — hygroscopic (sucks moisture). The potassium version $\ce{K2Cr2O7}$ is the proper primary standard. Bad.
• B. Oxalic acid — stable solid, sharp formula $\ce{H2C2O4 \cdot 2H2O}$. Good.
• C. NaOH — absorbs both moisture and $\ce{CO2}$ from air. Bad.
• D. $\ce{FeSO4 \cdot 6H2O}$ — easily oxidizes Fe$^{2+}$ to Fe$^{3+}$ on the shelf. Bad.
• E. Sodium tetraborate (borax) — stable, definite formula. Good.

So the bad list is A, C, D.

⚠️ Common mistake Many students mark NaOH as a primary standard because it is common in titrations. But NaOH is always standardised against a true primary standard like oxalic acid or KHP — it is not one itself.

$\boxed{\text{Answer: (d)}}$