M 1687 (OTTO) Mercuric iodide, red, GR 99%+ Cas 7774-29-0 - used in some x-ray and gamma ray detection and imaging devices operating at room temperatures. M 1687 (OTTO) Mercuric iodide, red, GR 99%+ Cas 7774-29-0 - used for preparation of Nessler's reagent, used for detection of presence of ammonia.