Final answer:
The presence of excess MnO4- ions indicates the end-point in redox titration because they cause the solution to turn pink when there are no more reducing agents to react with.
Step-by-step explanation:
The presence of excess MnO4- ions acts as an indicator in redox titration because they turn the solution pink at the end-point. This occurs in the titration of strong reducing agents such as Fe2+ with a potassium permanganate (KMnO4) solution. During the titration, MnO4- ions react with Fe2+ to form Mn2+, and the solution remains colorless while Fe2+ ions are present. When all the Fe2+ has reacted, the next small excess amount of MnO4- will not be reduced, resulting in a permanent light pink color in the solution.