Patent Blue V

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Patent Blue V
Chemical structure
Other names
  • Food Blue 5
  • Sulphan Blue
  • Acid Blue 3
  • L-Blau 3
  • C-Blau 20
  • Patentblau V
  • Sky Blue
  • C.I. 42051
  • E 131
3D model (JSmol)
ECHA InfoCard 100.020.522 Edit this at Wikidata
EC Number
  • 243-654-4
  • Calcium salt: 222-573-8
E number E131 (colours)
  • InChI=1S/C27H32N2O7S2/c1-5-28(6-2)21-13-9-19(10-14-21)27(20-11-15-22(16-12-20)29(7-3)8-4)23-17-24(30)26(38(34,35)36)18-25(23)37(31,32)33/h9-18H,5-8H2,1-4H3,(H2-,30,31,32,33,34,35,36)/p+1
  • InChI=1/C27H32N2O7S2/c1-5-28(6-2)21-13-9-19(10-14-21)27(20-11-15-22(16-12-20)29(7-3)8-4)23-17-24(30)26(38(34,35)36)18-25(23)37(31,32)33/h9-18H,5-8H2,1-4H3,(H2-,30,31,32,33,34,35,36)/p+1
  • CCN(CC)c1ccc(cc1)C(=C2C=CC(=[N+](CC)CC)C=C2)c3cc(c(cc3S(=O)(=O)O)S(=O)(=O)O)O
V04CX09 (WHO)
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).

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-(α-(4-diethylaminophenyl)-5-hydroxy- 2,4-disulfophenylmethylidene)-2,5-cyclohexadien-1-ylidene] diethylammonium hydroxide inner salt.

Use as dye[edit]

Candies with Patent Blue V (E131) added

It is not widely used, but in Europe it can be found in Scotch eggs,[citation needed] certain jelly sweets, blue Curaçao, certain gelatin desserts, among others. An important advantage is the very deep color it produces even at low concentration, a disadvantage is that it fades fairly quickly when exposed to light.

In medicine, Patent Blue V is used in lymphangiography and sentinel node biopsy as a dye to color lymph vessels.[2][clarification needed] It is also used in dental disclosing tablets as a stain to show dental plaque on teeth.

The color of the dye is pH-dependent. In aqueous solution, its color will vary from a deep blue in alkaline or weakly acidic medium to a yellow–orange in stronger acidic conditions. It is useful as a pH indicator for the range 0.8–3.0.[3] The structure is also redox-sensitive, changing from a reduced yellow form to an oxidized red form in solution. The reduction potential of around 0.77 volts is similar to that of other triphenylmethane dyes. It is usable as a reversible redox indicator in some analytical methods.[4]

Because of its pH-dependent color, Patent Blue V was included in chemistry sets from Salter Science in the 1970s and 80s under the name Sky Blue.


Patent Blue V is banned as a food dye in Australia and US, because health officials in these countries suspect that it may cause allergic reactions, with symptoms ranging from itching[5] and nettle rash to nausea, hypotension, and in rare cases anaphylactic shock; it is therefore not recommended in those countries for children.


  1. ^ Gessner, Thomas; Mayer, Udo (2000). "Triarylmethane and Diarylmethane Dyes". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. doi:10.1002/14356007.a27_179. ISBN 978-3527306732.
  2. ^ Erratum - 44 (4): 649 - The Journal of Nuclear Medicine
  3. ^ Sabnis, R. W. (2008). "Patent Blue V". Handbook of Acid–Base Indicators. CRC Press. pp. 295–297. ISBN 9780849382185.
  4. ^ Yoe, John H.; Boyd, George R. (1939). "Patent blue V as a pH and redox indicator". Ind. Eng. Chem. Anal. Ed. 11 (9): 492–493. doi:10.1021/ac50137a008.
  5. ^ L. Barthelmes, A. Goyal, R.G. Newcombe, F. McNeill, R.E. Mansel, Adverse reactions to patent blue V dye – The NEW START and ALMANAC experience, European Journal of Surgical Oncology (EJSO), Volume 36, Issue 4, April 2010, Pages 399-403, doi:10.1016/j.ejso.2009.10.007.