I want to test a supplement for the presence of iodine. Iodine should turn starch blue-black. So, can I powderize the supplement tablet, add water to it, drop it on a piece of bread, and if it turns blue-black, that would be an accurate way to test?
Just keep in mind that only elemental iodine will cause a blue-black coloration. Nobody add elemental iodine to a supplement just like nobody can eat chlorine out of table salt (NaCl). The typical compound they add is KIO3, which will not color starch right away, until and unless you reduce it to KI and then oxidize KI to free I2. In any case, search natural foods which contain iodine.