What is the starch indicator, and how does it change?

Starch is a indicator in the iodometric titration and it turns deep dark blue when iodine is present in a solution. The starch under warming condition forms amylose and amyl pectin's which combine with iodine to produce dark blue color. In absence of iodide ion starch indicator is colorless.

Explanation:

Starch is a viable indicator in the titration process because it turns deep dark blue when iodine is present in a solution. When starch is heated in water, decomposition occurs and beta-amylose is produced. Beta-amylose combines with iodine, resulting in a dark blue color change. The iodine-starch test is a chemical reaction that is used to test for the presence of starch or for iodine. The combination of starch and iodine is intensely "blue-black". The interaction between starch and triiodide is the basis for iodometry. In an iodometric titration, a starch solution is used as an indicator since it can absorb the I2 that is released. This absorption will cause the solution to change its color from deep blue to light yellow when titrated with standardized thiosulfate solution. This indicates the end point of the titration. Aside from their basic nutritional uses, starches are used in brewing and as thickening agents in baked goods and confections. Starch is used in paper manufacturing to increase the strength of paper and is also used in the surface sizing of paper. When starch is heated in water, various decomposition products are formed, among which is beta-amylose which forms a deep blue-black complex with iodine. The starch indicator solution must be freshly prepared since it will decompose and its sensitivity is decreased.