Starch solution

Iodine solution turns blue-black on addition of starch indicator

This is commonly used as a test for iodine. Iodine is only sparingly soluble in aqueous solution, but dissolves readily in potassium iodide solution. Here it has a distinctive brown colour. However, in very dilute solution it appears pale yellow and can be difficult to see. Starch forms an intense blue-black colouration with low concentrations of iodine solution.

It is the indicator used for iodine-sodium thiosulfate titrations. It is much easier to see the blue-black to colourless end-point rather than the very pale yellow to colourless endpoint seen without the indicator. A few drops of starch indicator are added when the iodine solution becomes very pale yellow.