Patent ID: 8821688

Claim:
A compound of formula (1), wherein R 1 is hydrogen or SO 3 − , R 2 is hydrogen or SO 3 − , R 3 is hydrogen, C 1-4 alkyl, C 2-3 hydroxyalkyl, CH 2 CO 2 − , CH 2 CH 2 CONH 2 or CH 2 CH 2 CN, R 4 is C 1-4 alkyl, C 2-3 hydroxyalkyl, CH 2 CO 2 − , CH(CO 2 − )CH 2 CO 2 − or CH(CO 2 − )CH 2 CH 2 CO 2 − , benzyl, or R 3 and R 4 together with the neighboring nitrogen atom signify a morpholine ring, and wherein M is the required stoichiometric cationic equivalent for balancing the anionic charge in formula (1) and is a combination of Mg 2+ together with at least 1 further cation selected from the group consisting of H + , alkali metal cation, alkaline earth metal cation other than Mg 2+ , ammonium, mono-C 1 -C 4 -alkyl-di-C 2 -C 3 -hydroxyalkyl ammonium, di-C 1 -C 4 -alkyl-mono-C 2 -C 3 -hydroxyalkyl ammonium, ammonium which is mono-, di- or trisubstituted by a C 2 -C 3 hydroxyalkyl radical and mixtures thereof.