Further, it is preferred that the oxidation potential in pH 10 water of p-phenylenediamine derivative, i.e., compound of the general formula (6) wherein R7 is a hydrogen atom do not exceed 5 mV (vs. SCE). [0283] R7 preferably represents R11???O???CO???, R14???SO2??? or R15???W???C(R16)(R17)???, most preferably R11???O???CO???. [0284] R11 preferably represents an alkyl group, or a group containing