Reaction of hypochlorite, ammonia, and phenol catalyzed by a manganous salt produces the blue compound indophenol. Reaction of alkaline phenol and hypochlorite with ammonia reaction forms indophenol blue. Calcium and magnesium may interfere by causing precipitation. Color and turbidity also interfere. Direct and following distillation.
Reaction forms a yellow to reddish-brown solution that indicates the ammonia concentration.
Preliminary distillation into boric acid suggested for removal of turbidity, color, and substances precipitated by hydroxyl ion, such as calcium and magnesium.