Question

Choose the correct statement(s) with regard to the complexometric titration of [Ni(H$_2$O)$_6$]$^{2+}$ ion with a complexing agent Na$_2$H$_2$EDTA, using a suitable indicator under basic conditions.

• A. The correct choice of indicator is phenolphthalein
• B. The stoichiometry of nickel and EDTA ions in the metal complex formed between metal and complexing agent is 1:1
• C. The binding constant for the complex formed between metal ion and complexing agent is higher than that for the complex formed between metal ion and indicator
• D. The stoichiometry of nickel and EDTA ions in the metal complex formed between metal and complexing agent is 1:3

Hint:

EDTA generally forms 1:1 complexes with metal ions, and the metal-EDTA complex must be more stable than the metal-indicator complex for successful titration

Answer 1

The Correct Option is B, C

Step 1: Understand complexometric titration.
Complexometric titration involves formation of a stable complex between a metal ion and a complexing agentHere, Ni$^{2+}$ reacts with EDTA under basic conditions

Step 2: Role of EDTA.
EDTA is a hexadentate ligandIt binds metal ions strongly through nitrogen and oxygen donor atoms

Step 3: Stoichiometry of Ni$^{2+}$ and EDTA.
Most metal ions form 1:1 complexes with EDTA, irrespective of metal chargeFor nickel:
\[ \text{Ni}^{2+} + \text{EDTA}^{4-} \rightarrow [\text{Ni(EDTA)}]^{2-} \]

Step 4: Check statement (B).
Since nickel and EDTA form a 1:1 complex, statement (B) is correct

Step 5: Role of indicator.
In complexometric titration, the indicator first forms a weak coloured complex with metal ionAt the end point, EDTA displaces the indicator from this complex

Step 6: Compare binding constants.
For a sharp end point, the metal-EDTA complex must be more stable than the metal-indicator complexTherefore, the binding constant of Ni-EDTA complex must be higher

Step 7: Conclusion.
Phenolphthalein is an acid-base indicator, not the suitable metallochromic indicator for this titrationAlso, EDTA does not form a 1:3 complex with Ni$^{2+}$Hence, correct statements are (B) and (C)
\[ \boxed{\text{(B) and (C)}} \]