FD&C Blue No. 2 lake also known as Indigotine or Indigo Blue, is a synthetic dye derived from indigo, a natural colorant substance.

The name describes the structure of the molecule.

• FD&C indicates that the color is approved by the United States Food and Drug Administration (FDA) for use in foods, drugs, and cosmetics. (1987)
• BLUE #2 refers to a particular blue color, also known as Indigotine or Indigo Blue, which is a synthetic dye.
• lake  indicates that the color has been combined with a substance (often a calcium or aluminum salt) to make it water-insoluble, often used in products that are not meant to dissolve immediately in water.

Description of raw materials used in production. 

• Indigotine can be both synthetically produced and extracted from plants of the Indigofera genus. The synthetic version, which is more commonly used, is often derived from aniline-based compounds.

Step-by-step summary of industrial production process.

• Preparation of Aniline. Aniline is prepared through a process involving the nitration of benzene, followed by a reduction to produce aniline.
• Production of Indoxyl. The aniline is converted into indoxyl, which is the direct precursor of Indigotine.
• Oxidation to Indigotine. The indoxyl is then oxidized to produce the characteristic blue color of Indigotine.
• Purification. The dye is then purified through various methods, including precipitation, filtration, and washing.
• Drying and Granulation. The purified Indigotine is dried and then processed to produce a powder or granules, depending on the specifications of the final product.

Indigotine is a disodium salt, synthetic blue dye also called Acid Blue 74, CI 73000, Indigo carmine and others. It is produced by the condensation of 2-formylbenzenesulphonic acid and aniline with a subsequent oxidation process. Mixed with tartrazine (E102) it can produce various shades of green.

It appears as a blue powder

A cosa serve e dove si usa

Food

Ingredient included in the list of European food additives as E132, dye.

The  EFSA Panel  on  Food  Additives  and  Nutrient  Sources  added  to  Food concluded that Indigo Carmine is poorly absorbed by the human body and does not raise genotoxicity concerns. No adverse effects were identified in subacute, chronic, reproductive and developmental toxicity studies, and no changes in haematological and biological parameters were identified in chronic toxicity studies at doses less than or equal to 500 mg/kg body weight/day (1).

The  Panel  confirmed  the  ADI  of  5  mg/kg  bw/day  for  Indigo  Carmine  of  at  least  93%  purity manufactured  using  the  same  or  equivalent  manufacturing  process  resulting  in  material  tested in Borzelleca  et  al.  studies.  Given  the  uncertainties  in  the  database,  the  Panel  was not  able  to conclude whether  this  ADI  should  apply  to  Indigo  Carmine  with  lower  purity  manufactured  using these  same processes or material manufactured using a different but not equivalent process. Furthermore, the Panel concluded that the current specifications should be revised in order to restrict the Indigo Carmine (E 132) permitted as food additive to that for which the ADI is applicable.  The  Panel  concluded  that  at  the  maximum  permitted  level  of  use,  exposure  estimates  of  Indigo Carmine (E 132) would exceed the ADI for toddlers and children at the high level. Exposure estimates using  the  available  usage  and  analytical data  did  not  show  an  exceedance  of the  ADI  for  any population groups.The Panel noted that the three main contributing food categories for age-groups where MPL scenario estimates exceeded the ADI showed high number of analytical data far below MPL or had no use level reported combined with no detection in limited analytical data. The Panel therefore considered that it is not likely that the ADI will be exceeded (1).

Medical

It is used in medicine in intravenous administration to examine sections of the human body to be explored with endoscopic probes, is usually biologically safe and inert and can rarely cause adverse reactions, except in rare exceptions (2).

For more information:   Indigotine studies

Appearance	Blu powder
Melting Point	>300°C
Density	1.01 g/mL at 20°C
PSA	189.36000
LogP	3.06060
Refraction Index	n20/D 1.335
Water Solubility	1 g/100 mL (25 ºC)
Safety

• Molecular Formula  C₁₆H₈N₂Na₂O₈S₂
• Molecular Weight    466,346
• Exact Mass   465.951752
• CAS   860-22-0
• UNII    D3741U8K7L
• EC Number   212-728-8
• DSSTox Substance ID  DTXSID1020190    DTXSID0044288    DTXSID6044224
• IUPAC  disodium;2-(3-hydroxy-5-sulfonato-1H-indol-2-yl)-3-oxoindole-5-sulfonate
• InChI=1S/C16H10N2O8S2.2Na/c19-15-9-5-7(27(21,22)23)1-3-11(9)17-13(15)14-16(20)10-6-8(28(24,25)26)2-4-12(10)18-14;;/h1-6,17,19H,(H,21,22,23)(H,24,25,26);;/q;2*+1/p-2  
• InChl Key      JFVXEJADITYJHK-UHFFFAOYSA-L
• SMILES   C1=CC2=C(C=C1S(=O)(=O)[O-])C(=C(N2)C3=NC4=C(C3=O)C=C(C=C4)S(=O)(=O)[O-])O.[Na+].[Na+]
• MDL number  MFCD00005723
• PubChem Substance ID    24847997
• ChEBI  31695
• Colour Index Number   73015    
• Beilstein      4103904
• RTECS   DU3000000
• NACRES   NA.47   
• NCI   C83799

Synonyms :

• Sodium indigotindisulfonate
• disodium;2-(3-hydroxy-5-sulfonato-1H-indol-2-yl)-3-oxoindole-5-sulfonate
• (delta(sup 2,2')-Biindoline)-5,5'-disulfonic acid, 3,3'-dioxo-, disodium salt
• FD C Blue 2
• Sachsischblau
• Murabba
• Indigotine Lake
• Indigo Extract
• Indigo carmine
• Acid Blue 74
• Indigo carmine
• Indigocarmin
• Indigotine
• C.I. ACID BLUE 74
• Indigotindisulfonate sodium
• FD&C Blue No. 2
• Food Blue No. 2
• C.I. Food Blue 1
• C.I. Natural Blue 2
• Amacid Brilliant Blue

References_____________________________________________________________________

(1) EFSA Panel on Food Additives and Nutrient Sources added to Food (ANS), 2014. Scientific Opinion on the re‐evaluation of Indigo Carmine (E 132) as a food additive. EFSA Journal, 12(7), p.3768.

Abstract. The ANS Panel provides a scientific opinion re-evaluating the safety of Indigo Carmine (E132). The Panel observed that Indigo Carmine was poorly absorbed and does not raise concern for genotoxicity. No adverse effects in subacute, chronic, reproduction and developmental toxicity studies, and no modifications of haematological and biological parameters in chronic toxicity studies have been identified at doses less than or equal to 500 mg/kg bw/day. The only report of an adverse effect was in testis with a LOAEL of 17 mg/kg bw/day which would give rise to a safety concern if confirmed. The Panel considered that this study has shortcomings since it is not clear to the Panel whether the adverse effects observed were due to the food additive itself or to impurities and/or contaminants present in the material tested and/or to the conduct of the study. The Panel considered that the current ADI of 5 mg/kg bw/day for Indigo Carmine was applicable to a material with the same purity and manufacturing process as material used in studies without adverse effects on testis (93% pure colouring and 7% volatile matter) and concluded that any extension of this ADI to Indigo Carmine of lower purity and/or manufactured using a different process would require new data which would need to address the adverse effects on testis. The Panel noted that at the MPL, exposure estimates of Indigo Carmine would exceed the ADI for toddlers and children at the high level. Exposure estimates using the available usage and analytical data did not show an exceedance of the ADI for any population groups.

(2) Nandate K, Voelzke BB. Severe Hypotension, Hypoxia, and Subcutaneous Erythema Induced by Indigo Carmine Administration during Open Prostatectomy. Case Rep Urol. 2016;2016:5237387. doi: 10.1155/2016/5237387. 

Abstract. Indigo carmine (also known as 5,5′-indigodisulfonic acid sodium salt or indigotine) is a blue dye that is administered intravenously to examine the urinary tract and usually is biologically safe and inert. Indigo carmine rarely may cause adverse reactions. We treated a 66-year-old man who had general anesthesia and radical retropubic prostatectomy for prostate cancer. He had a previous history of allergy to bee sting with nausea, vomiting, and dizziness. Within 1 minute after injection of indigo carmine for evaluation of the ureters, the patient developed hypotension to 40 mmHg, severe hypoxia (the value of SpO2 (peripheral capillary oxygen saturation) was 75% on 40% inspired oxygen concentration), poor air movement and bilateral diffuse wheezing on auscultation, and marked subcutaneous erythema at the upper extremities. After treatment with 100% oxygen, epinephrine (total, 1.5 mg), hydrocortisone (100 mg), diphenhydramine (50 mg), albuterol nebulizer (0.083%), and continuous infusion of epinephrine (0.15 μg/kg/min), the vital signs became stable, and he recovered completely. In summary, indigo carmine rarely may cause life-threatening anaphylactic or anaphylactoid reaction that may necessitate rapid treatment to stabilize cardiovascular, hemodynamic, and pulmonary function.