A testing elements for the calorimetric determination of oxidizable gas and / or vapor components in gas mixtures is provided. The testing element contains, besides usual oxidizing agents, at least one
redox indicator, which is in the oxidized form. The
redox indicator is preferably a
benzidine derivative according to the general formulain which the radicals R are identical or different and denote a
hydrogen,
alkyl,
aryl,
halogen,
trifluoromethyl, cyano, nitro, dialkylamino, ester, sulfoxyl ester, alkyloxy or aryloxy, X denotes a
halogen, Y denotes a
trifluoromethyl, cyano, nitro, dialkylamino, ester, sulfoxyl ester, alkyloxy or aryloxy, n is an integer from zero to three, and m equals 5-n. The testing elements are preferably designed as
detector tubes that contain carrier materials, to which the oxidizing reagents and
redox indicators are applied.