Oxidation-Reduction Methods
Two types of indicators are commonly used: adsorption and that change with oxidation – reduction potential. The former is illustrated by the use of starch solution to indicate the end point when solution of iodine are titrated with sodium thiosulfate titration is “by eye” until the iodine concentration is near extinction, as shown by a pale yellow color. Upon the addition of a good starch indicator a blue solution results. This is due to adsorption of iodine upon the surface of the colloidal starch particles. As the titration proceeds, iodine is released from the starch, and disappearance of the blue color is taken as the end point. Other adsorption indicators show when an excess of titrant has been added. Starch acts in this manner when iodine solutions are used as the titrant.