The present invention provides a compound having the formula: ##STR1## in which M is hydrogen, sodium, potassium, calcium, magnesium, ammonium, mono-, di-, tri- or tetra-C.sub.1 -C.sub.4 alkylammonium, mono-, di- or tri-C.sub.1 -C.sub.4 hydroxyalkylammonium or ammonium that is di- or tri-substituted by a mixture of C.sub.1 -C.sub.4 alkyl and C.sub.1 -C.sub.4 hydroxyalkyl groups; R.sub.1 is a group having the formula: ##STR2## in which M has its previous significance and n is 1 or 2; and R.sub.2 is O--C.sub.1 -C.sub.12 alkyl or N(R.sub.3)(R.sub.4) in which R.sub.3 and R.sub.4, independently, are hydrogen, C.sub.1 -C.sub.2 alkyl or C.sub.1 -C.sub.4 hydroxyalkyl, or R.sub.3 and R.sub.4, together with the nitrogen atom, are the atoms required to form a pyrrolidine, piperidine or morpholine group. The compounds of formula (1) are particularly useful as fluorescent whitening agents for cotton in discontinuous processes, in the course of which an acidic treatment, especially a finishing treatment using a polymer resin, occurs.