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: Stoichiometry.
EDTA is hexadentate and binds nickel in a one to one ratio, so statement B is correct.

Step 2: Binding strength.
For a sharp end point the metal to EDTA complex must be more stable than the metal to indicator complex, so statement C is correct.

Step 3: Reject the rest.
Phenolphthalein is not the right metal indicator, and the one to three ratio is wrong.

Step 4: Answer.
\[ \boxed{\text{B and C}} \]