. . . . "xy, carboxyl, or the radical Z stands for hydrogen, halogen such as chlorine or bromine, an alkyl radical, preferably up to 3 carbon atoms such as methyl, ethyl or propyl, a nitro group or a sulphonic acid group, an alkoxy radical having preferably up to 3 carbon atoms, such as 'methoxy, ethoxy or carboxyl; R and R each represent a carboxyalkyl radical preferably having up to 5 carbon atoms, or sa" .