Compounds that should not be used as primary standards in titrimetric analysis are :
A. Na\( _2 \)Cr\( _2 \)O\( _7 \)
B. Oxalic acid
C. NaOH
D. FeSO\( _4 \).6H\( _2 \)O
E. Sodium tetraborate
Choose the most appropriate answer from the options given below :
Show Hint
- B and D Only
- D and E Only
- C, D and E Only
- A, C and D Only
The Correct Option is D
Solution and Explanation
A primary standard is a highly pure, stable compound with a known exact composition that can be accurately weighed and dissolved to create a solution of known concentration.
The properties of a good primary standard include high purity, stability (not hygroscopic, efflorescent, or easily decomposed), high molar mass (to minimize weighing errors), and solubility in a suitable solvent.
Let's analyze each compound:
A. Na\( _2 \)Cr\( _2 \)O\( _7 \) (Sodium dichromate): While it has high purity and molar mass, it can be hygroscopic, meaning it absorbs moisture from the air, which can change its effective mass and thus the concentration of the prepared solution.
Therefore, it is not an ideal primary standard.
B. Oxalic acid (H\( _2 \)C\( _2 \)O\( _4 \).2H\( _2 \)O): It is a stable, easily purified solid with a known molar mass and is non-hygroscopic. It is a good primary standard.
C. NaOH (Sodium hydroxide): It is highly hygroscopic, meaning it readily absorbs moisture and carbon dioxide from the air. This makes it difficult to weigh accurately, and its composition can change over time. Therefore, it cannot be used as a primary standard.
D. FeSO\( _4 \).6H\( _2 \)O (Iron(II) sulfate hexahydrate): It is unstable and easily oxidized by atmospheric oxygen to iron(III) compounds. Its composition is not reliably constant. Therefore, it is not a good primary standard. E. Sodium tetraborate (Na\( _2 \)B\( _4 \)O\( _7 \).10H\( _2 \)O or borax): It has a known composition and can be obtained in pure form. It is reasonably stable but can lose water of crystallization upon heating.
It is often used as a primary standard for acids. Based on this analysis, the compounds that should not be used as primary standards are Na\( _2 \)Cr\( _2 \)O\( _7 \) (A), NaOH (C), and FeSO\( _4 \).6H\( _2 \)O (D).
The correct option is (4) A, C and D Only.
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