Food Coloring: Patent Blue V is used as a food additive and has the E number E131. Medicine: It is used in lymphangiography and sentinel node biopsy to color lymph vessels. Dental: Patent Blue V is used in dental disclosing tablets to show dental plaque on teeth. The color of Patent Blue V is pH-dependent. In aqueous solution: It appears deep blue in alkaline or weakly acidic medium. It changes to yellow–orange in stronger acidic conditions. It is useful as a pH indicator for the range 0.8–3.0.