Patent Blue V, also called Food Blue 5, Sulphan Blue, Acid Blue 3, L-Blau 3, C-Blau 20, Patentblau V, Sky Blue, or C.I. 42051, is a sky blue synthetic triphenylmethane dye used as a food coloring.[1] As a food additive, it has E number E131. It is a sodium or calcium salt of [4-(�±-(4-diethylaminophenyl)-5-hydroxy- 2,4-disulfophenylmethylidene)-2,5-cyclohexadien-1-ylidene] diethylammonium hydroxide inner salt. In medicine, Patent Blue V is used in lymphangiography and sentinel node biopsy as a dye to color lymph vessels. It is also used in dental disclosing tablets as a stain to show dental plaque on teeth. E number: E131 (colours) EC Number: 243-654-4; Calcium salt: 222-573... CAS Number: 3536-49-0