Compounds that should not be used as primary standards in titrimetric analysis are : Choose the most appropriate answer from the options given below:
- C, D and E Only
- B and D Only
- D and E Only
- A, C and D Only
Answer
(4) A, C and D Only
Solution
A primary standard should be available in pure form, should not be hygroscopic or efflorescent, should not react with air (O2, CO2) and should have a fixed composition.
A. Na2Cr2O7 is hygroscopic (hence K2Cr2O7 is used instead). Not a primary standard.
B. Oxalic acid (H2C2O4 2H2O) is a primary standard.
C. NaOH is hygroscopic and absorbs CO2 from air. Not a primary standard.
D. Hydrated ferrous sulphate loses water of crystallisation and is oxidised by air to Fe(III) (Mohr's salt is used instead). Not a primary standard.
E. Sodium tetraborate (borax, Na2B4O7 10H2O) is a primary standard.
A, C and D only.
Answer: (4)