Final answer:
In titration, indicators help establish the equivalence point by changing color, signaling that the reaction between the titrant and the analyte is complete, and allowing for the determination of the unknown species' concentration.
Step-by-step explanation:
In a titration experiment to determine the concentration of an unknown species, the role of an indicator is crucial. Option 1 expresses the correct role of an indicator: Indicators help establish the equivalence point, marking the precise moment when the reaction is complete. An indicator is a substance that undergoes a distinct color change when the solution changes from acidic to basic, or vice versa, which typically occurs at the equivalence point in acid-base titrations. By having different indicators that change color at different pH levels, one must select the correct indicator to match the expected pH range at the equivalence point. The moment when the indicator changes color indicates that the quantities of acid and base in the solution are stoichiometrically equivalent, meaning the reaction has reached its equivalence point.
This visual cue is what tells the experimenter to stop the addition of the titrant, allowing the concentration of the unknown species to be calculated based on the volume and concentration of the known species (the titrant) and the balanced chemical equation.
Indicators do not provide the initial concentration of the unknown species (Option 2), speed up the reaction (Option 3), or increase the accuracy of volume measurements (Option 4).