text: Murexide (NHCHNO, or CHNO·NH), also called ammonium purpurate or MX, is the ammonium salt of purpuric acid. It is a purple solid that is soluble in water.  The compound was once used as an indicator reagent. Aqueous solutions are yellow at low pH, reddish-purple in weakly acidic solutions, and blue-purple in alkaline solutions.