NEW MATERIAL:Compounds of formula I [R1 is formula II, formula III (R3 is alkyl, aralkyl, allyl, 3-phenylallyl; X is alkyl, alkoxy, aryl, aryloxy, halogen, hydroxyl, H); Y is anionic functional group] or formula IV (R2 is formula V or formula VI).

EXAMPLE: 4-[Bis(2,4-dinitrophenyl)methyl]-1-ethylpyridinium chloride.

USE: Reagent for detecting ammonia: it has high sensitivity and keeps high sensitivity even in a dry or hydrophobic medium. Further, it shows high stability to light and heat.

PREPARATION: The reaction of a quaternary salt of methyl-substituted pyridine or quinoline with dinitrochlorobenzene is carried out in acetonitrile in the presence of triethylamine and the resultant product is isolated and made to react with an appropriate acid to give the compound of formula I.