| http://www.w3.org/ns/prov#value | - Within the scope of the present invention, tetrazolium salts which have proved to be especially useful are those of the general formula: ##STR2## wherein R1 is a hydrogen atom, a carboxyl group or an alkyl, phenyl, nitrophenyl, dinitrophenyl, carboxyl-substituted phenyl or trialkylammonium-phenyl radical, R2, nitrophenyl, biphenylyl or naphthyl radical, R3 is a phenyl, carboxyl-substituted phenyl,
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