Starch solutions are used as indicators in the detection of the endpoint of titrations involving Iodine solutions. Starch reacts with Iodine in the presence of Iodide ion to form an intensely colored blue complex, which is visible at very low concentrations of Iodine, making it a very good indicator in both direct and indirect lodometric titrations.
The Starch-Iodide complex is not very soluble in water, so the starch is added near the endpoint of an Iodine titration, when the Iodine concentration is low. This eliminates errors due to the fact that some Iodine may remain adsorbed on the complex and go undetected.
The blue color produced is also destroyed by alkali and heat, so titrations involving Starch indicator should be conducted at a pH below 8 and at temperatures near room temperature (this will also minimize loss of volatile Iodine from the titration vessel).
Starch should also not be used in a strongly acid medium because hydrolysis of the starch occurs.
Use of Starch indicator in the titration of very dilute solutions of Iodine by Thiosulfate may result in the reappearance of Iodine due to intermediate reactions near the endpoint, resulting in the need to make sure that the endpoint has indeed been reached.
RICCA CHEMICAL COMPANY starch solutions are preserved with Salicylic Acid, rather than the toxic and environmentally harmful Mercury or Chloroform preservatives used in some formulations.