2.3.1. Tetrazotization of 4,40-diamino diphenylmethane-2-
sulfonic acid (FC-acid) and coupling with H-acid
FC-acid (2) (3.36 g, 0.01 mol) was suspended in H2O (80 ml).
Hydrochloric acid (15 ml) was added drop wise to this well
stirred suspension. The mixture was gradually heated up to
70 C till clear solution obtained. The solution was cooled to
0–5 C in an ice bath. A solution of NaNO2 (1 g, 0.014 mol)
in H2O (10 ml) previously cooled to 0 C, was then added over
a period of 5 min with stirring. Stirring was continued for an
hour maintaining the same temperature, with positive test
for nitrous acid with required amount of a solution of a sulfamic
acid. The clear tetrazo solution at 0–5 C was used for subsequent
coupling reaction.