Final answer:
The Sandell-Koltthoff reaction measures iodide concentration through an indicator color change, with the endpoint visually detected by a change in color due to the formation of a complex with iodide.
Step-by-step explanation:
The response measured as a function of iodide concentration in the Sandell-Koltthoff reaction is indicator color change.
The reaction is typically used to determine the concentration of iodide in a solution.
An indicator, such as starch, is used to form a complex with the oxidized form of iodide (I3-). As the reaction progresses, the color change that occurs from the presence of iodide and the indicator can be visually detected, signaling the endpoint of the reaction.