Question

Complexometric titration indicators Except

• A. Mordant black - II
• B. Ferroin
• C. Catechol violet
• D. Xylene orange

Hint:

Remember, indicators for complexometric titrations form metal complexes; Ferroin is a redox indicator, not suitable for this purpose.

Answer 1

The Correct Option is B

- Complexometric titrations involve the use of indicators that form colored complexes with metal ions, helping to visually detect the endpoint.
- Common complexometric indicators include Mordant Black II, Catechol Violet, and Xylene Orange, all of which change color upon binding with metal ions such as Ca\(^{2+}\) or Mg\(^{2+}\).
- Ferroin, however, is primarily a redox indicator used in redox titrations, especially with iron(II) and iron(III) ions, and does not act as a complexometric indicator.
- Therefore, Ferroin is an exception and is not used as an indicator in complexometric titrations.

Answer 2

The question asks which of the four listed substances is NOT used as a complexometric titration indicator. A true complexometric indicator must form a colored complex with the metal ion being titrated, one that is weaker than the metal-EDTA complex so the indicator releases its color at the endpoint.

1. Mordant Black-II (Eriochrome Black T): This dye forms a wine-red complex with metal ions such as Mg²⁺ and Ca²⁺, which turns blue once EDTA displaces the metal at the endpoint. It fits the definition of a complexometric indicator.
2. Ferroin: Ferroin is a 1,10-phenanthroline-iron(II) complex used because its color changes between the reduced and oxidized iron states during a redox reaction. It works by electron transfer, not by competing for a metal ion in a chelation equilibrium, so it belongs to redox titrations rather than complexometric ones.
3. Catechol Violet: This dye also forms distinctly colored chelates with metal ions like Bi³⁺ and Th⁴⁺ and is used as a metal-ion indicator in complexometric titrations.
4. Xylenol Orange: This indicator forms red or pink complexes with several metal ions and is widely used in complexometric titrations of metals such as Bi³⁺, Pb²⁺ and Zn²⁺.

Three of the four options act by metal-chelation color change, which is the defining behavior of a complexometric indicator. Ferroin instead works through a redox color change, so it stands apart from the group.

Therefore, the correct answer is Ferroin.