Patent Blue V

Patent Blue V, also called Food Blue 5 or Sulphan Blue, is a dark bluish synthetic dye used as a food coloring. As a food additive, it has E number E131. It is a sodium or calcium salt of [4-(α-(4-diethylaminophenyl)-5-hydroxy- 2,4-disulfophenyl-methylidene)-2,5-cyclohexadien-1-ylidene] diethylammonium hydroxide inner salt. It has the appearance of a violet powder.

It is not widely used, but can be found in Scotch eggs. Patent Blue V is banned as a food dye in Australia, USA, and Norway.

In medicine, Patent Blue V is used in lymphangiography as a dye to color lymph vessels. It is also used in dental disclosing tablets as a stain to show dental plaque on teeth.

It may cause allergic reactions, with symptoms ranging from itching and nettle rash to nausea, hypotension, and in rare cases anaphylactic shock; it is not recommended for children.