New redox indicators

A compound of the formula ##STR8## wherein X is oxygen or sulphur;

• R.sub.1 is julolidine, tetrahydroquinoline or tetrahydroquinoline with substituted or unsubstituted C.sub.1 -C.sub.6 alkyl on the nitrogen atom, the substituents being a sulfo phosphonic acid or carboxylic acid residue, or R.sub.1 is a group of the formula ##STR9## wherein R.sub.4 is hydroxyl or mono-or dialkylated amino wherein each alkyl contains up to 6 carbon atoms and is unsubstituted or substituted one or more times by a hydroxyl, alkoxy, halogen, morpholine, or a sulfo, carboxylic acid or phosphonic acid residue, which acid residue is optionally esterified,
• R.sub.5 and R.sub.6, which can be the same or different, are hydrogen, C.sub.1 -C.sub.6 alkyl, C.sub.1 -C.sub.6 alkoxy or C.sub.1 -C.sub.6 alkyl substituted one or more times by halogen, hydroxyl, methoxy, carboxy, a morpholino radical, a sulfo or a phosphonic acid residue, which can also be esterified by methanol or ethanol, or C.sub.1 -C.sub.6 alkoxy substituted by a carboxyl group,
• R.sub.2 is hydrogen, C.sub.1 -C.sub.6 alkyl, or C.sub.1 -C.sub.6 alkyl, substituted one or more times by halogen, hydroxyl, methoxy, carboxy, a morpholino radical, a sulfo acid or a phosphonic acid residue, which can also be esterified by methanol or ethanol, or C.sub.1 -C.sub.6 alkoxy substituted by a carboxyl group, or is julolidine, tetrahydroquinoline or tetrahydroquinoline substituted with substituted or unsubstituted C.sub.1 -C.sub.6 alkyl on the nitrogen atom, the substituents being a sulfo, phosphonic acid or carboxylic acid residue, or is a group of the formula:
• ##STR10## wherein R.sub.5 and R.sub.6 have the same meanings as above and
• R'.sub.4 is hydroxyl, amino, or mono- or dialkylated amino wherein the alkyl groups contain up to 6 carbon atoms and are unsubstituted or substituted one or more times by a hydroxyl, alkoxy, halogen, morpholine, or a sulfo, carboxylic acid or phosphonic acid residue, which acid residue is optionally esterified;
• and
• R.sub.3 has the same meaning as R.sub.2 or is C.sub.1 -C.sub.6 alkoxy, dialkylamino with up to 6 carbon atoms in each alkyl or phenyl, or is cycloalkyl with 3 to 7 carbon atoms, phenyl or pyridyl, and wherein
• at least one of the residues R.sup.2 and R.sup.3 is julolidine, tetrahydroquinoline or tetrahydroquinoline substituted with substituted or unsubstituted C.sub.1 -C.sub.6 alkyl on the nitrogen atom, the substituents being a sulfo, phosphonic acid or carboxylic acid residue, or is a group of the formula:
• ##STR11## wherein R'.sub.4, R.sub.5, R.sub.6 have the same meanings as above and wherein at least one of the substituents R.sub.4 and R'.sub.4 is hydroxyl and the salts thereof.