If they contain sodium salts
of a sulfonic acid group in addition to a phenolic group,
they are referred as an acid azo dye. All such dyes having
phenolic and sulfonic acid moieties, contain hydroxyl (–
OH) and sulfonic (–SO3H) acid groups as auxochromic
group. Such auxochromic (–OH) and chromophoric
(C‚O) groups containing the compound, i.e., 2-hydroxy-4-methoxybenzophenone-5-sulfonic acid has shown wide applications as a polymer additive