Thiazolyl tetrazolium salts with increased solubility in aqueous solutions have alkylammonium alkoxy substituents, preferably trimethyl ammonium propoxy groups substituents, on phenyl rings attached to the tetrazolium ring. Thiazolyl tetrazolium salts with increased solubility in aqueous solutions have alkyl ammonium alkoxy substituents, preferably trimethyl ammonium propoxy groups substituents, on phenyl rings attached to the tetrazolium ring, e.g. the compounds of the general formula shown below. Formula (A-)a.