Step 1: Nitrous acid is diazotized with p-aminobenzene sulfonic acid, but usually sodium nitrite and hydrochloric acid are used, not nitrous acid directly. This step reacts at low temperature to produce the diazonium salt. HCl + HNO2 + H2N-Ph-SO3H ==ClN2-Ph-SO3H +2H2O Ph is the benzene ring. In the second step, the diazonium salt couples with a(alpha)-naphthylamine Acidic conditions produce an azo compound with the coupling site adjacent to the amino group of a-naphthylamine. clN2-Ph-SO3H + H2N-Np == H2N-Np-N=N-Ph-SO3H +HCl