organic reagents used in analytical chemistry. There are three different reagents: arsenazo-I,-II, and -III. Arsenazo is a fine crystalline powder, dark brown or black (arsenazo-lll) in color, easily soluble in water. All arsenazo reagents are derivatives of chromotropic acid (1) and various substitutes of phenolarsenic acid (2). Arsenazo-II and arsenazo-III differ from arsenazo-I (3) by their more complicated structure. Arsenazo forms colored solutions with
many elements; these solutions can be used for the photometric determination (arsenazo -I) of the elements. They form strong complexes with such elements as thorium, zirconium, and hafnium.