prov:value
| - and R2 independently of one another are hydrogen, a sulpho group and salts thereof, a carboxyl group and salts thereof or a hydroxyl group and at least one of the radicals R1 and R2 is a sulpho or carboxyl group or a salt thereof, and R3 and R4 independently of one another are hydrogen, alkyl, hydroxyalkyl, cyanoalkyl, sulphoalkyl, carboxyalkyl or halogenoalkyl, having, in each case, 1 to 6 carbo
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