A compound having the formula ##STR13## wherein R is a radical consisting of 4-22 carbon atoms chosen from the group consisting of alkyl, alkenyl and arylalkenyl, which can be substituted by carboxy, alkoxycarbonyl, hydroxy, cyano, chlorine,bromine or sulfonato groups; andM is chosen from the group consisting of alkali metal, pyridinum or ammonium.12.