xy-4,4'-biphenylene]ditetrazolium chloride, o-Tolyl Tetrazolium Red, p-Tolyl Tetrazolium Red, Piperonyl Tetrazolium Blue, p-Anisyl-p-Nitro Blue Tetrazolium Chloride, or Veratryl Tetrazolium Blue; and said ferricyanide/ferrocyanide salt is a mixture of an alkali metal ferricyanide and an alkali metal ferrocyanide.