4-(4-Nitrophenylazo)resorcinol, also known as Azo Violet, is an azo purple dye used experimentally as a pH indicator, turning yellow below pH 11 and purple above pH 13. It becomes dark blue in slightly alkaline or alkaline environments with magnesium salts and can also test for ammonium ions, with the color changing based on the concentration of Azo Violet in ammonium chloride or ammonium hydroxide solutions.