Phenyl substituted 2-thiazolyl tetrazolium salt indicators

Phenyl substituted 2-thiazolyl tetrazolium salt indicators characterized by the formula:

• ##STR11## wherein R.sup.1 and R.sup.2 are such that the 2-thiazolyl group is 4,5-bis(4-methoxyphenyl)thiozol-2-yl;
• 4,5-diphenylthiazol-2-yl;
• 4-phenylthiazol-2-yl;
• 4-phenyl-5-methylthiazol-2-yl or 4-naphthyl-5-phenylthiazol-2-yl;
• wherein R.sup.4 is 4-carboxyphenyl;
• phenyl;
• 3-pyridyl;
• 4-nitrophenyl;
• 3,5-dicarboxyphenyl;
• 4-nitrophenyl;
• 8-quinolyl;
• or 3,4,5-trimethoxyphenyl;
• wherein R.sup.3 is 3-4-methylenedioxyphenyl;
• 4-methoxyphenyl;
• 2-thienyl, 4-fluorophenyl;
• or 3-thienyl and X.sup..crclbar. is a counteranion and wherein the compounds are further characterized in that the reflectance spectra exhibited by each of said compounds when reduced to its colored formazan state varies by less than about 17% over a wavelength range of 50 nm when the midpoint of the 50 nm range is 640 nm or greater.