Specific examples of the compound represented by formula (A-IV) include diethylenetriaminepentaacetic acid, triethylenetetraminehexaacetic cid, diethylenetriaminepentamethylenephosphonic acid and triethylenetetraminehexamethylenephosphonic acid, and their salts (e.g., alkali metal salts such as potassium, sodium and lithium).