xy-2-hydroxypropanesulfonic acid and salts thereof): wherein R7 is a hydrogen atom or an alkyl group having 1 to 8 carbon atoms, a to d, which may be the same or different, represent 0 or integers of 1 to 100 (with the proviso that a+b+c+d=0 to 100), an (OC2H4) unit and an (OC3H6) unit are combined in any order, Y and Z are sulfonic acid groups or hydroxyl groups, and at least one of Y and Z is a