Complex sentence: Unreacted iodobromine (or iodine monochloride) is reacted with potassium iodide which converts it to iodine, whose concentration can be determined by titration with sodium thiosulfate.
Split into simpler sentences: Unreacted iodobromine (or iodine monochloride) is reacted with potassium iodide which converts it to iodine. The iodine concentration is then determined by titration with sodium thiosulfate.