Question

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 :

• A. B and D Only
• B. D and E Only
• C. C, D and E Only
• D. A, C and D Only

Hint:

Remember the key characteristics of a good primary standard: high purity, stability (non-hygroscopic, not easily oxidized or decomposed), known stoichiometry, and preferably a high molar mass. Evaluate each given compound against these criteria to determine if it's suitable as a primary standard.

Answer 1

The Correct Option is D

In titrimetric analysis, a primary standard is a highly pure reagent that serves as a reference for preparing solutions and calibrating analytical instruments. To qualify as a primary standard, a compound should meet several criteria:

1. High purity: The compound should be of definite and known purity.
2. Stability: It should not absorb moisture or react with air.
3. Non-hygroscopic: It should not attract moisture from the environment.
4. High molar mass: This minimizes the weighing errors.
5. Readily available and inexpensive.

Let's analyze the given options:

1. Na\(_2\)Cr\(_2\)O\(_7\) (Sodium dichromate): This compound is not hygroscopic but is highly toxic and can be hazardous to handle. Thus, it's not preferred as a primary standard.
2. Oxalic acid: It is often used as a primary standard because it is stable and non-hygroscopic to an extent, although it must be dried before use.
3. NaOH (Sodium hydroxide): NaOH is hygroscopic and absorbs water from the air, leading to changes in its concentration. Thus, it is unsuitable as a primary standard.
4. FeSO\(_4\).6H\(_2\)O (Ferrous sulfate heptahydrate): This compound is not used as a primary standard because it is unstable and oxidizes in air.
5. Sodium tetraborate (Borax): It is a good primary standard because it meets many criteria for a primary standard.

Based on the above analysis, the compounds that should not be used as primary standards in titrimetric analysis are:

1. Na\(_2\)Cr\(_2\)O\(_7\)
2. NaOH
3. FeSO\(_4\).6H\(_2\)O

Thus, the correct answer is: A, C and D Only

Answer 2

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.