The azo-dye Y formed in the following reactions is Sulphanilic acid + NaNO 2 + CH 3 COOH → X

Reaction of aniline sulphonic acid with ${\mathrm{NaNO}}_2+{\mathrm{CH}}_3\mathrm{COOH}$ gives diazo ester sulphonic acid. Which on further treatment with naphthalene derivative undergoes diazo coupling reaction to give red azo dye.

This is known as Griess-Ilosvay test.