4-(4-Nitrophenylazo)resorcinol is an azo purple dye used experimentally as a pH indicator, showing yellow when the pH value is lower than 11 and purple when the pH value is higher than 13. In slightly alkaline or alkaline environments, it also turns dark blue in the presence of magnesium salts. Azo Violet can also be used to test for the presence of ammonium ions. The color of the ammonium chloride or ammonium hydroxide solution will change depending on the concentration of azo violet used.