Diphenylamine is manufactured by the thermal deamination of aniline over oxide catalysts:
: 2 CHNH   →    (CH)NH  +  NH
It is a weak base, with a K of 10.  With strong acids, it forms salts.  For example, treatment with sulfuric acid gives the bisulfate [(CH)NH][HSO] as a white or yellowish powder with m.p. 123-125 °C.
Diphenylamine undergoes various cyclisation reactions.  With sulfur, it gives phenothiazine, a precursor to pharmaceuticals.
: (CH)NH  +  2 S   →   S(CH)NH  +  HS
With iodine, it undergoes dehydrogenation to give carbazole, with release of hydrogen iodide:
: (CH)NH  +  I  →   (CH)NH  +  2 HI
Arylation with iodobenzene gives triphenylamine.            it is also used as a test reagent in the dische's test .