E104 or Quinoline Yellow is a chemical compound, derived from quinophthalone, a dye used in a variety of industries: 

• Food
• Cosmetics. Nail polishes, cold soaps, liquid detergents.
• Pharmaceuticals. Tablets
• Textile. Fabrics
• Plastic
• etc

The synthesis process takes place in different steps:

• Preparation of the diazonium salt. An aromatic amine is treated with nitrous acid (usually generated in situ from sodium nitrite and a strong acid) to form a diazonium salt. This reaction typically takes place in an aqueous and acidic environment and at low temperatures to stabilise the diazonium group.
• Coupling reaction. The diazonium salt is then reacted with a phenol or aniline in a basic or slightly acidic solution to form the azo dye. 
• Isolation and purification. The resulting dye is isolated, usually by filtration, and then purified, often by recrystallisation.

What it is for and where

Cosmetics

It is a restricted ingredient as IV/82 a Relevant Item in the Annexes of the European Cosmetics Regulation 1223/2009. Ingredient or substance at risk: 1H-Indene-1,3(2H)-dione, 2-(2-quinolinyl)-, sulfonated, sodium salts

Cosmetics - INCI Functions

Colorant. This ingredient has the function of colouring the solution in which it is inserted in a temporary, semi-permanent or permanent manner, either alone or in the presence of the complementary components added for colouring.

It is also used to add color to non-food producing animal feed at a maximum dose of 500 mg/kg of feed.

Labeled with the number E104 in the list of European food color additives and internationally known as CI 47005.

Safety

Regulatory data on the genotoxicity and/or mutagenicity of this compound are still controversial. This study finds that, even with in vitro testing, this dye could cause harmful effects in humans if it is metabolized or absorbed through the skin (1).

Quinoline yellow studies

Typical optimum characteristics of the commercial colour Chinoline Yellow

• Molecular Formula : C₁₈H₉NNa₂O₈S₂
• Molecular Weight : 477.369 g/mol
• CAS : 100208-62-6    8004-92-0
• EC Number 616-849-0
• UNII  35SW5USQ3G
• InChI=1S/C18H11NO8S2.2Na/c20-17-11-3-1-2-4-12(11)18(21)15(17)13-6-5-9-7-10(28(22,23)24)8-14(16(9)19-13)29(25,26)27;;/h1-8,15H,(H,22,23,24)(H,25,26,27);;/q;2*+1/p-2
• InChl Key      FZUOVNMHEAPVBW-UHFFFAOYSA-L
• SMILES  C1=CC=C2C(=C1)C(=O)C(C2=O)C3=NC4=C(C=C(C=C4C=C3)S(=O)(=O)[O-])S(=O)(=O)[O-].[Na+].[Na+]
• IUPAC  disodium;2-(1,3-dioxoinden-2-yl)quinoline-6,8-disulfonate
• DSSTox Substance ID  DTXSID0041721     DTXSID70873122
• NACRES  NA.47

Synonyms : 

• CI 47005
• E104
• Acid Yellow 3
• Yellow 13
• Yellow 10
• Lemon Yellow ZN 3
• Quinoline Yellow S
• Quinoline yellow WS
• Sodium 2-(1,3-dioxo-2,3-dihydro-1H-inden-2-yl)quinoline-6,8-disulfonate
• 2-(2-Quinolyl)-1,3-indandione disulfonic acid disodium salt
• dipotassium 2-(1,3-dioxo-2H-inden-2-yl)quinoline-6,8-disulfonate
• disodium 2-(1,3-dioxo-2H-inden-2-yl)quinoline-6,8-disulfonate
• 2-(1,3-Dioxoindan-2-yl)quinoline-6,8-bis(sulfonic acid sodium) salt
• disodium 2-(1,3-dioxo-2,3-dihydro-1h-inden-2-yl)-6,8-quinolinedisulfonate
• disodium 2-(1,3-dioxo-2,3-dihydro-1H-inden-2-yl)quinoline-6,8-disulfonate

References______________________________________________________________

(1) Chequer FM, Venâncio Vde P, de Souza Prado MR, Campos da Silva e Cunha Junior LR, Lizier TM, Zanoni MV, Rodríguez Burbano R, Bianchi ML, Antunes LM. The cosmetic dye quinoline yellow causes DNA damage in vitro. Mutat Res Genet Toxicol Environ Mutagen. 2015 Jan 1;777:54-61. doi: 10.1016/j.mrgentox.2014.11.003.

Macioszek VK, Kononowicz AK. The evaluation of the genotoxicity of two commonly used food colors: Quinoline Yellow (E 104) and Brilliant Black BN (E 151). Cell Mol Biol Lett. 2004;9(1):107-22.

Abstract. Additives, especially colors, are in widespread use in the food industry. With the exception of the quinolines, food colors are relatively weak mutagens and are certified as safe additives despite reports that some people have allergic reactions to them. The number of food additives is still on the increase, and research on their potential mutagenic/carcinogenic activity in vivo is very expensive. Using two different cellular model systems, human lymphocytes in vitro and Vicia faba root tip meristems of in vivo, we evaluated the potential cytological and genotoxic effects of two dyes: Quinoline Yellow (E 104) and Brilliant Black BN (E 151). Two relatively new, very sensitive and rapid tests - the micronucleus and Comet assays - were used in this study. The data provided in this paper showed the genotoxic effects of the two analyzed food colors, and confirmed the diagnostic value of the MN and Comet assays for screening potentially genotoxic substances.