The Theory behind the Experiment
NaClO, or bleach is a powerful oxidizing agent. We are doing an oxidation-reduction titration to determine the amount of hypochlorite ion present in a sample of commercial bleach. We add iodine ions to the bleach which oxidizes into elemental iodine. Iodine, in the presence of starch, forms a colored triiodide ion which we can see. We titrate the ion with thiosulfate ions back into iodide ions which are colorless. By knowning how much thiosulfate we used for the titration, we can calculate how much of the triiodide ions were present. From that we can calculate how much iodine was formed, how much iodide was oxidized, and, finally, how much hypochlorite we originally had in our bleach.
Sources of Error
Any volumetric transfers of our original solutions may contain errors.
Any errors in buret readings.
Adding too much iodine to the starch may form a complex that may not reversibly ionize.
Any errors in the preparation of the original reagent solutions.