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
In titrimetric analysis, a primary standard is a highly pure reagent used for solution preparation and instrument calibration. To be a primary standard, a compound must possess the following characteristics:
- High purity: Defined and known purity level.
- Stability: Resistance to moisture absorption and air reactions.
- Non-hygroscopic: Does not attract atmospheric moisture.
- High molar mass: Minimizes weighing errors.
- Availability and cost: Readily available and inexpensive.
Evaluation of the provided options:
- Na\(_2\)Cr\(_2\)O\(_7\) (Sodium dichromate): While not hygroscopic, its high toxicity makes it hazardous and thus unsuitable as a primary standard.
- Oxalic acid: Frequently employed as a primary standard due to its reasonable stability and non-hygroscopic nature, though drying is necessary prior to use.
- NaOH (Sodium hydroxide): Hygroscopic, it absorbs atmospheric moisture, altering its concentration. Consequently, it is not appropriate as a primary standard.
- FeSO\(_4\).6H\(_2\)O (Ferrous sulfate heptahydrate): Not utilized as a primary standard because it is unstable and prone to oxidation in air.
- Sodium tetraborate (Borax): A suitable primary standard, meeting most required criteria.
Based on this evaluation, the compounds unsuitable for use as primary standards in titrimetric analysis are:
- Na\(_2\)Cr\(_2\)O\(_7\)
- NaOH
- FeSO\(_4\).6H\(_2\)O
Therefore, the correct selection is: A, C and D Only
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Statement I: In the oxalic acid vs KMnO$_4$ (in the presence of dil H$_2$SO$_4$) titration the solution needs to be heated initially to 60°C, but no heating is required in Ferrous ammonium sulphate (FAS) vs KMnO$_4$ titration (in the presence of dil H$_2$SO$_4$).
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