Salicylaldehyde normally just reacts with primary amines to form a stable imine that does not titrate with HCI. The remaining secondary and tertiary amines do titrate. This imidazolidine in Eq. (6) now does not titrate with HCI, giving a false low reading for secondary amine. The 1, 3-propanediamines undergo a similar reaction, forming hexahydropyrimidines.