X 1259 (OTTO) Xylenol orange tetrasodium salt, ACS reagent Cas 3618-43-7 - used as a fluorescent marker of bone growth. X 1259 (OTTO) Xylenol orange tetrasodium salt,ACS reagent Cas 3618-43-7 - used in FBX (ferrous benzoic xylenon orange) gel preparation to study the effects of MRI (magnetic resonance imaging) parameters on image quality.