Step 1: Understanding the Concept:
Titration indicators are grouped by the type of chemical event they respond to: metal-ion (complexometric) indicators respond to chelation with a metal ion, while redox indicators respond to a change in oxidation state of the species in solution.
Step 2: Key Formula or Approach:
Sort each option by which category it belongs to. An indicator that changes color because it loses or gains a metal ion belongs to complexometric titrations; one that changes color because it is oxidized or reduced belongs to redox titrations.
Step 3: Detailed Explanation:
Mordant Black-II, Catechol Violet, and Xylenol Orange all work by binding a metal ion to form a colored chelate, then releasing that metal ion to EDTA at the endpoint, causing a visible color shift. That places all three firmly in the complexometric group. Ferroin, in contrast, is the iron(II)-phenanthroline complex classically paired with cerium(IV) or permanganate titrations, where its color changes because the iron center itself is oxidized to iron(III). Since this color change is driven by electron transfer and not by metal-EDTA competition, ferroin does not belong with the other three.
Step 4: Final Answer:
Ferroin is the odd one out, since it is a redox indicator rather than a complexometric indicator.