The Iodine Starch test was developed in 1814. The test involves using iodine and an iodide (usually potassium iodide) in solution. It can detect extremely low levels of starch - the starch granules turning an intense dark blue-black colour.

It's a highly standardized test, and although in routine use for more than 200 years, the chemical mechanism of the reaction is still not known.

A 2016 research project suggested that the test might have similarities with a compound known as the pyrroloperylene–iodine complex.

The crystal structure of the pyrroloperylene–iodine complex (with a formula close to pyrroloperylene–I_2.2) has been solved. The structure contains infinite polyiodide chains, the Raman spectrum of which resembles that of the famous starch–iodine complex, thus casting light on a historical puzzle."

Angewandte Chenie , Volume 55, Issue 28

Note: The test is also used in reverse, to detect low levels of iodine