iodometry
/ eye-uh-DOM-ih-tree /
Imagine you cannot weigh a crowd directly, so you hand each person one token, then count the tokens that come back. Iodometry uses iodine molecules as those countable tokens to measure oxidisers you cannot easily titrate head-on.
Iodometry is a redox titration in which an oxidising analyte first liberates a measured amount of iodine from iodide ions; that freed iodine is then titrated with a standard thiosulfate solution. Because the iodine released is proportional to the analyte, counting it (via the titration) tells you the analyte amount — an indirect approach.
It matters because it lets one reliable thiosulfate titrant measure a whole range of oxidisers, from chlorine in water to copper in alloys. Its signature touch is a starch indicator that forms an intense blue-black with iodine and vanishes the instant the last iodine is consumed, giving an unusually sharp end point — though the iodine must be handled quickly, as it is volatile and slowly attacked by air.
To measure copper in an alloy, the dissolved copper(II) frees iodine from added iodide; the liberated iodine is titrated with standard sodium thiosulfate, and starch turns the solution blue-black until the end point, where the colour vanishes.
Set free a measured amount of iodine, then titrate it to count the analyte.
Strictly, this indirect 'liberate-then-titrate' route is iodometry; directly titrating a reducing analyte with a standard iodine solution is the related but distinct iodimetry. Add the starch indicator only near the end point, since iodine-starch fades slowly if formed too early.