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{{short description|Chemical compound, food additive and dye}}
{{Chembox new
{{chembox
| Name = '''Indigo '''
| Watchedfields = changed
| ImageFile = Indian indigo dye lump.jpg
| verifiedrevid = 477315345
| ImageSize = 200px
| Name = Indigo
| ImageName = Lump of Indian indigo dye
| ImageFile = Indian indigo dye lump.jpg
| ImageFile1 = Indigo.svg
| ImageName = Piece of indigo plant dye from India, {{circa|{{convert|2+1/2|cm|0}}}} square.
| ImageSize1 = 200px
| ImageAlt = Lump of Indian indigo dye
| ImageName1 = Indigo
| ImageFile1 = Indigo skeletal.svg
| OtherNames = 2,2’-Bis(2,3-dihydro-3- oxoindolyliden), Indigotin
| ImageSize1 = 220px
| Section1 = {{Chembox Identifiers
| ImageAlt1 = Skeletal formula of indigo dye
| SMILES = O=c3c(=c2c1ccccc1c2=O)c4ccccc34
| ImageFile2 = Indigo dye 3D ball.png
| CASNo = 482-89-3
| RTECS = DU2988400 | ImageSize2 = 220px
| ImageAlt2 = Ball-and-stick model of the indigo dye molecule
| InChI=1/C16H10N2O2/c19-15-9-5-1-3-7-
| PIN = --3,3'(1''H'',1'''H'')-dione
11(9)17-13(15)14-16(20)10-6-2-4
| OtherNames = 2,2'-Bis(2,3-dihydro-3-oxoindolyliden), Indigotin
-8-12(10)18-14/h1-8,17-18H/b14-13+
| Section1 = {{Chembox Identifiers
| SMILES = c1ccc2c(c1)C(=O)/C(=C\3/C(=O)c4ccccc4N3)/N2
| ChemSpiderID_Ref = {{chemspidercite|correct|chemspider}}
| ChemSpiderID = 4477009
| UNII_Ref = {{fdacite|correct|FDA}}
| UNII = 1G5BK41P4F
| PubChem = 10215
| InChIKey = COHYTHOBJLSHDF-BUHFOSPRBQ
| ChEMBL_Ref = {{ebicite|correct|EBI}}
| ChEMBL = 599552
| StdInChI_Ref = {{stdinchicite|correct|chemspider}}
| StdInChI = 1S/C16H10N2O2/c19-15-9-5-1-3-7-11(9)17-13(15)14-16(20)10-6-2-4-8-12(10)18-14/h1-8,17-18H/b14-13+
| StdInChIKey_Ref = {{stdinchicite|correct|chemspider}}
| StdInChIKey = COHYTHOBJLSHDF-BUHFOSPRSA-N
| CASNo_Ref = {{cascite|correct|CAS}}
| CASNo = 64784-13-0
| RTECS = DU2988400
| InChI=1/C16H10N2O2/c19-15-9-5-1-3-7-11(9)17-13(15)14-16(20)10-6-2-4-8-12(10)18-14/h1-8,17-18H/b14-13+
}} }}
| Section2 = {{Chembox Properties | Section2 = {{Chembox Properties
| Formula = C<sub>16</sub>H<sub>10</sub>N<sub>2</sub>O<sub>2</sub> | Formula = C<sub>16</sub>H<sub>10</sub>N<sub>2</sub>O<sub>2</sub>
| MolarMass = 262.27&nbsp;g/mol | MolarMass = 262.27&nbsp;g/mol
| Appearance = dark blue crystalline powder | Appearance = dark blue crystalline powder
| Density = 1.199&nbsp;g/cm<sup>3</sup> | Density = 1.199&nbsp;g/cm<sup>3</sup>
| Solubility = insoluble at 20&nbsp;°C | Solubility = 990 µg/L (at 25 °C)
| MeltingPt = 390&ndash;392&nbsp;°C | MeltingPtC = 390 to 392
| BoilingPt = | BoilingPt = decomposes
}} }}
| Section7 = {{Chembox Hazards | Section7 = {{Chembox Hazards
| ExternalMSDS = | ExternalSDS =
| GHSPictograms =
| EUClass = 207-586-9
| GHSSignalWord =
| RPhrases = {{R36/37/38}}
| SPhrases = {{S26}}-{{S36}} | HPhrases = {{H-phrases}}
| PPhrases = {{P-phrases}}
| FlashPt =
| GHS_ref =<!-- none found in PubChem 10-12-2021 -->
| Autoignition =
| FlashPt =
| AutoignitionPt =
}} }}
| Section8 = {{Chembox Related | Section8 = {{Chembox Related
| OtherCpds = ]<br />]<br />] | OtherCompounds = ]<br>]<br>]
}} }}
}} }}
'''Indigo dye''' is ] with a distinctive blue color (see ]). The chemical compound that constitutes the indigo dye is called indigotin. The ancients extracted the natural dye from several species of ] as well as one of the two famous ], but nearly all indigo produced today is ]. Historically, indigo played an important role in many countries' economies because natural blue dyes are rare.


'''Indigo dye''' is an ] with a distinctive ]. Indigo is a ] extracted from the leaves of some plants of the ] genus, in particular '']''. Dye-bearing ''Indigofera'' plants were commonly grown and used throughout the world, particularly in Asia, with the production of indigo dyestuff economically important due to the historical rarity of other blue dyestuffs.<ref name=Ullmann/>
Among other uses, it is used in the production of ] ] for blue ]. The form of indigo used in food is called "indigotine", and is listed in the ] as ] Blue No. 2, and in the ] as ]: E132.


Most indigo ] produced today is ], constituting around 80,000 tonnes each year, as of 2023.<ref>{{cite journal |last1=Linke |first1=Julia A. |last2=Rayat |first2=Andrea |last3=Ward |first3=John M. |date=2023 |title=Production of indigo by recombinant bacteria |journal=Bioresources and Bioprocessing |volume=10 |issue=1 |pages=20 |issn=2197-4365 |pmid=36936720 |doi=10.1186/s40643-023-00626-7 |doi-access=free |pmc=10011309 }}</ref> It is most commonly associated with the production of ] cloth and ], where its properties allow for effects such as ] and ] to be applied quickly.
== Sources and uses ==
]
A variety of plants have provided indigo throughout history, but most natural indigo is obtained from those in the genus '']'', which are native to the ]. In temperate climates indigo can also be obtained from ] (''Isatis tinctoria'') and ] (''Polygonum tinctorum''), although the ''Indigofera'' species yield more dye. The primary commercial indigo species in ] was true indigo ('']'', also known as '']''). In ] and ] the two species ''Indigofera suffruticosa''
(Anil) and ''Indigofera arrecta'' (Natal indigo) were the most important.


==Uses==
Dye was obtained from the processing of the plant's leaves. These were soaked in water and ] in order to convert the ] ] naturally present in the plant to the blue dye ]. The precipitate from the fermented leaf solution was mixed with a strong base such as ], pressed into cakes, dried, and powdered. The powder was then mixed with various other substances to produce different shades of blue and purple.
]
The primary use for indigo is as a dye for cotton yarn, mainly used in the production of denim cloth suitable for blue jeans; on average, a pair of blue jeans requires {{convert|3|g}} to {{convert|12|g}} of dye. Smaller quantities are used in the dyeing of wool and silk.


], also known as indigo, is an indigo derivative which is also used as a colorant. About 20,000 tonnes are produced annually, again mainly for the production of blue jeans.<ref name=Ullmann>{{Ullmann|doi=10.1002/14356007.a14_149.pub2|title=Indigo and Indigo Colorants|year=2004|last1=Steingruber|first1=Elmar|isbn=3527306730}}</ref> It is also used as a food colorant, and is listed in the United States as ] Blue No.&nbsp;2.
Natural indigo was the only source of the dye until July 1897. Within a short time, however, synthetic indigo almost completely superseded natural indigo, and today nearly all indigo produced is synthetic.


==Sources==
In the ], the primary use for indigo is as a dye for cotton work clothes and blue jeans.


===Natural sources===
For many years indigo was also used to produce deep navy blue colors on wool. Indigo does not bond strongly to wool fibers, and wear and repeated washing slowly removes the dye.
{{see also|#Indigo derivatives}}
A variety of plants have provided indigo throughout history, but most natural indigo was obtained from those in the genus ''Indigofera'', which are native to the tropics, notably the Indian Subcontinent. The primary commercial indigo species in Asia was true indigo ('']'', also known as ''I. sumatrana''). A common alternative used in the relatively colder subtropical locations such as Japan's {{lang|ja-Latn|Ryukyu|italic=no}} Islands and Taiwan is '']''.


Until the introduction of ''Indigofera'' species from the south, '']'' (dyer's knotweed) was the most important blue dyestuff in East Asia; however, the crop produced less dyestuff than the average crop of indigo, and was quickly surpassed in favour of the more economical ''Indigofera tinctoria'' plant. In ] and South America, the species grown is '']'', also known as ''anil'', and in India, an important species was '']'', Natal indigo. In Europe, '']'', commonly known as woad, was used for dyeing fabrics blue, containing the same dyeing compounds as indigo, also referred to as indigo.
Indigo is also used as a ], and is listed in the USA as FD&C Blue No. 2. The for FD&C Blue No. 2 includes three substances, of which the major one is the sodium salt of .


Several plants contain indigo, which, when exposed to an oxidizing source such as atmospheric oxygen, reacts to produce indigo dye; however, the relatively low concentrations of indigo in these plants make them difficult to work with, with the color more easily tainted by other dye substances also present in these plants, typically leading to a greenish tinge.
Indigotinesulfonate is also used as a dye in ] testing, as a ] for the detection of ]s and ]s and in the testing of milk.


The precursor to indigo is ], a colorless, water-soluble derivative of the amino acid ], and ''Indigofera'' leaves contain as much as 0.2–0.8% of this compound. Pressing cut leaves into a vat and soaking hydrolyzes the indican, releasing β-{{sc|D}}-] and ]. The indoxyl dimerizes in the mixture, and after 12–15 hours of ] yields the yellow, water-soluble ]. Subsequent exposure to air forms the blue, water-insoluble indigo dye.<ref>{{cite book |last=Schorlemmer |first=Carl |author-link=Carl Schorlemmer |title=A Manual of the Chemistry of the Carbon compounds; or, Organic Chemistry |url=https://archive.org/details/amanualchemistr02schogoog |location=London |year=1874}} Quoted in the ], second edition, 1989</ref><ref>{{cite book |last1=Freeman |first1=H. S. |title=Colorants for non-textile applications |last2=Peters |first2=A. T. |publisher=Elsevier |year=2000 |isbn=978-0-444-82888-0 |location=Amsterdam New York |pages=382–455 |chapter=9 - Natural Dyes}}</ref> The dye precipitates from the fermented leaf solution upon oxidation, but may also be precipitated when mixed with a strong base<ref>{{cite web |url=https://www.coyuchi.com/the-naturalista/indigo_dyeing/ |title=Indigo Dyeing |website=Coyuchi Inc. |language=en |access-date=2019-05-24 |archive-date=2019-05-24 |archive-url=https://web.archive.org/web/20190524001631/https://www.coyuchi.com/the-naturalista/indigo_dyeing/ |url-status=dead}}</ref> such as ]. The solids are filtered, pressed into cakes, dried, and powdered. The powder is then mixed with various other substances to produce different shades of blue and purple.
== History ==
===Classical antiquity===


Natural sources of indigo also include mollusks: the '']'' genus of sea snails produces a mixture of indigo and ] (red), which together produce a range of purple hues known as ]. Light exposure during part of the dyeing process can convert the dibromoindigo into indigo, resulting in blue hues known as royal blue, hyacinth purple, or ].
Indigo, a blue pigment and a dye, was used as a dye in ], which was also the earliest major center for its production and processing.<ref name=k&c>Kriger & Connah, page 120</ref> The ''Indigofera tinctoria'' variety of Indigo was domecticated in India.<ref name=k&c/> Indigo, used as a dye, made its way to the ] and the ] via various trade routes, and was valued as a luxury product.<ref name=k&c/>


===Chemical synthesis===
Indigo is among the oldest dyes to be used for textile dyeing and printing. Many Asian countries, such as ], ], ] and ]n nations have used indigo as a dye (particularly ] dye) for centuries. The dye was also known to ancient civilizations in ], ], ], ], ], ], ], ], and ].
{{Multiple image|direction=vertical|width=330
|image1=Indigo.Baeyer-Drewson.Synthesis.svg|caption1=Heumann's synthesis of indigo
|image2=Indigo Synthesis V.1.svg|caption2=Pfleger's synthesis of indigo}}


Given its economic importance, indigo has been prepared by many methods. The ] dates back to 1882. It involves an aldol condensation of o-nitrobenzaldehyde with acetone, followed by cyclization and oxidative dimerization to indigo. This route was highly useful for obtaining indigo and many of its derivatives on the laboratory scale, but proved impractical for industrial-scale synthesis. Johannes Pfleger<ref name="history.evonik.com"/> and {{ill|Karl Heumann|de}} eventually came up with industrial mass production synthesis from ] by using ] as a catalyst. The method was discovered by an accident by Karl Heumann in Zurich which involved a broken thermometer.<ref name="ingenious.org.uk"/>
India is believed to be the oldest center of indigo dyeing in the Old World. It was a primary supplier of indigo to Europe as early as the Greco-Roman era. The association of India with indigo is reflected in the Greek word for the dye, which was ''indikon''. The Romans used the term ''indicum'', which passed into Italian dialect and eventually into English as the word ''indigo''.


The first commercially practical route of producing indigo is credited to Pfleger in 1901. In this process, ] is treated with a molten mixture of ], ], and ]. This highly sensitive melt produces ], which is subsequently oxidized in air to form indigo. Variations of this method are still in use today. An alternative and also viable route to indigo is credited to Heumann in 1897. It involves heating ''N''-(2-carboxyphenyl)glycine to {{convert|200|°C}} in an inert atmosphere with sodium hydroxide. The process is easier than the Pfleger method, but the precursors are more expensive. Indoxyl-2-carboxylic acid is generated. This material readily decarboxylates to give indoxyl, which oxidizes in air to form indigo.<ref name=Ullmann/> The preparation of indigo dye is practised in college laboratory classes according to the original Baeyer-Drewsen route.<ref>{{cite journal |last1=McKee |first1=James R. |last2=Zanger |first2=Murray |year=1991 |title=A microscale synthesis of indigo: Vat dyeing |journal=Journal of Chemical Education |volume=68 |issue=10 |page=A242 |doi=10.1021/ed068pA242 |bibcode=1991JChEd..68..242M}}</ref>
In Mesopotamia, a Neo-Babylonian ] tablet of the 7th century BC gives a recipe for the dyeing of wool, where ]-colored wool (uqnatu) is produced by repeated immersion and airing of the cloth. Indigo was most probably imported from India.


==History==
The Romans used indigo as a pigment for painting and for medicinal and cosmetic purposes. It was a luxury item imported to the Mediterranean from India by Arab merchants.
], Germany]]


<!-- Section removed from ], contains much redundancy with this section. (What does that mean?)
Indigo remained a rare commodity in Europe throughout the Middle Ages. ], a chemically identical dye derived from the plant ''] tinctoria'' (]), was used instead.
(BEGINNING OF SECTION "REMOVED" from THIS article)
===Indigo versus woad===
] used natural ultramarine in his paintings, as in his '']''. The expense was probably borne by his wealthy patron ].<ref>{{cite web |title=Girl with a Pearl Earring |url=http://www.essentialvermeer.com/catalogue/girl_with_a_pearl_earring.html |publisher=essentialvermeer.com |url-status=live |archive-url=https://web.archive.org/web/20150630060601/http://www.essentialvermeer.com/catalogue/girl_with_a_pearl_earring.html |archive-date=2015-06-30}}</ref>]]


In Europe, ], or woad, had been the main source of ], and the most readily-available source; the plant was processed into a paste called pastel. This industry was threatened in the 15th century by the arrival from India of the same dye (]), obtained from a shrub widely grown in Asia, '']''. The plant produced indigo dye in greater and more colourfast quantities than woad, making its introduction a major source of competition for European-produced indigo dye.
In the late fifteenth century, the ] explorer ] discovered a sea route to India. This led to the establishment of direct trade with India, the ], China, and Japan. Importers could now avoid the heavy duties imposed by ]n, ]ine, and Greek middlemen and the lengthy and dangerous land routes which had previously been used. Consequently, the importation and use of indigo in Europe rose significantly. Much European indigo from Asia arrived through ports in Portugal, the ], and England. ] imported the dye from its colonies in South America. Many indigo plantations were established by European powers in tropical climates; it was a major crop in ] and ], with much or all of the labor performed by enslaved Africans and African-Americans. Indigo plantations also thrived in the ]. However, ] and ] outlawed imported indigo in the 1500s to protect the local woad dye industry.


In 1498, ] opened a trade route to import indigo from India to Europe. In India, the indigo leaves were soaked in water, fermented, pressed into cakes, dried into bricks, then carried to the ports London, Marseille, Genoa, and Bruges.<ref name=autogenerated3>Eva Heller, ''Psychologie de la couleur effets et symboliques'' p. 21</ref> Later, in the 17th century, the British, Spanish, and Dutch established indigo plantations in Jamaica, South Carolina, the Virgin Islands and South America, and began to import American indigo to Europe. Countries with large and prosperous pastel industries attempted to block the import and use of indigo; one government in Germany outlawed the use of indigo in 1577, describing it as a "pernicious, deceitful and corrosive substance, the Devil's dye."<ref> {{webarchive|url=https://web.archive.org/web/20121106154802/http://www.gutenberg.org/files/13242/13242-8.txt |date=2012-11-06}}.</ref><ref>D G Schreber, ''Historische, physische und economische Beschreibung des Waidtes'', 1752, the appendix; Thorpe JF and Ingold CK, 1923, ''Synthetic colouring matters – vat colours'' (London: Longmans, Green), p. 23</ref> In France, ], in an edict of 1609, forbade under pain of death the use of "the false and pernicious Indian drug".<ref>{{cite book |last=Foucaud |first=Édouard |editor-last=Frost |editor-first=John |title=The book of illustrious mechanics of Europe and America |publisher=D. Appleton |year=1846 |page= |url=https://archive.org/details/bookillustrious00foucgoog}}</ref> It was forbidden in England until 1611, when British traders established their own indigo industry in India and began to import it into Europe.<ref name=autogenerated4>Eva Heller, ''Psychologie de la couleur – effets et symboliques'' p. 28</ref>
].]]
Indigo was the foundation of centuries-old textile traditions throughout ]. From the ] nomads of the ] to ], clothes dyed with indigo signified wealth. Women dyed the cloth in most areas, with the ] of ] and the Manding of ] particularly well known for their expertise. Among the ] male dyers working at communal dye pits were the basis of the wealth of the ancient city of ], and can still be seen plying their trade today at the same pits.


The efforts to block indigo were in vain; the quality of indigo blue was too high and the price too low for pastel made from woad to compete. In 1737, both the French and German governments finally allowed the use of indigo. This ruined the dye industries in Toulouse and the other cities that produced pastel, but created a thriving new indigo commerce to seaports such as Bordeaux, Nantes and Marseille.<ref>F. Lauterbach, ''Der Kampf des Waides mit dem Indigo'', Leipzig, p. 25. Cited by Michel Pastoureau, Bleu – Histoire d'une couleur, pp.&nbsp;108–13.</ref>
In Japan, indigo became especially important in the ] when it was forbidden to use silk, so the Japanese began to import and plant cotton. It was difficult to dye the cotton fiber except with indigo. Even today indigo is very much appreciated as a color for the summer ] Yukata, as this traditional clothing recalls Nature and the blue sea.
<!--
===Middle Age===
-->


Another war of the blues took place at the end of the 19th century, between indigo and ], discovered in 1868 by the German chemist Johann Friedrich Wilhelm Adolf von Baeyer. The German chemical firm ] put the new dye on the market in 1897, in direct competition with the British-run indigo industry in India, which produced most of the world's indigo. In 1897 Britain sold ten thousand tons of natural indigo on the world market, while BASF sold six hundred tons of synthetic indigo. The British industry took measures to ensure their economic viability with the new BASF dye, but it was unable to compete; the synthetic indigo was more pure, made a more lasting blue, and was not dependent upon good or bad harvests. In 1911, India sold only 660 tons of natural indigo, while BASF sold 22,000 tons of synthetic indigo. In 2002, more than 38,000 tons of synthetic indigo was produced, often for the production of blue jeans.<ref name=Ullmann>{{Ullmann|first=Elmar|last=Steingruber|title=Indigo and Indigo Colorants|date=2004|doi=10.1002/14356007.a14_149.pub2}}</ref>
===Late 19th century===


<gallery mode="packed" heights="150px">
In ] the ] chemist ] began working with indigo. His work culminated in the first synthesis of indigo in ] from Istatine, a second synthesis in ] from o-nitrobenzaldehyde and ] upon addition of dilute sodium hydroxide, barium hydroxide, or ammonia and the announcement of its chemical structure three years later.
File:Isatis tinctoria02.JPG|'']'', or woad, was the main source of blue dye in Europe from ancient times until the arrival of indigo from Asia and America. It was processed into a paste called pastel.
File:Schreber woad mill 1752.JPG|A woad mill in ], in Germany, in 1752. The woad industry was already on its way to extinction, unable to compete with indigo blue.
File:The Hunt of the Unicorn Tapestry 1.jpg|A Dutch tapestry from 1495 to 1505. The blue colour comes from ].
File:Indigofera tinctoria1.jpg|'']'', a tropical shrub, is the main source of indigo dye. The chemical composition of indigo dye is the same as that of woad, but the colour is more intense.
File:Indigo-Historische Farbstoffsammlung.jpg|Cakes of indigo. The leaf has been soaked in water, fermented, mixed with lye or another base, then pressed into cakes and dried, ready for export.
</gallery>
(END OF SECTION "REMOVED" from THIS article) -->


The oldest known fabric dyed indigo, dated to 6,000 years ago, was discovered in ], Peru.<ref>{{cite journal |vauthors=Splitstoser JC, Dillehay TD, Wouters J, Claro A |date=2016-09-14 |title=Early pre-Hispanic use of indigo blue in Peru |journal=] |volume=2 |issue=9 |pages=e1501623 |doi=10.1126/sciadv.1501623 |pmid=27652337 |pmc=5023320 |bibcode=2016SciA....2E1623S}}</ref> Many Asian countries, such as ], China, Japan, and ]n nations have used indigo as a dye (particularly for ]) for centuries. The dye was also known to ancient civilizations in ], ], ], ], ], ], and ]. Indigo was also cultivated in India, which was also the earliest major center for its production and processing.<ref name=k&c>Kriger & Connah, page 120</ref> The '']'' species was domesticated in India.<ref name=k&c/> Indigo, used as a dye, made its way to the ] and the ], where it was valued as a luxury product.<ref name=k&c/>
The production of o-nitrobenzaldehyde was too complicated for a commercial product so the search for alternative starting materials was crucial for BASF and Hoechst. The synthesis of N-(2-Carboxyphenyl)glycine starting from the easy to obtain ] gave the development of a synthesis a boost.


]
] developed a commercially feasible manufacturing process that was in use by ], and by ] natural indigo had been almost entirely replaced by synthetic indigo. In ], 17,000 tons of synthetic indigo were produced worldwide.
In Mesopotamia, a neo-Babylonian ] tablet of the seventh century BC gives a recipe for the dyeing of wool, where ]-colored wool (''uqnatu'') is produced by repeated immersion and airing of the cloth.<ref name="StClair"/> Indigo was most probably imported from India. The Romans used indigo as a pigment for painting and for medicinal and cosmetic purposes. It was a luxury item imported to the Mediterranean from India by Arab merchants.


India was a primary supplier of indigo to Europe as early as the Greco-Roman era. The association of India with indigo is reflected in the Greek word for the dye, ''indikón'' ({{lang|grc|Ἰνδικόν}}, Indian).<ref name="StClair">{{cite book |title=The Secret Lives of Colour |last=St. Clair |first=Kassia |publisher=John Murray |year=2016 |isbn=9781473630819 |location=London |page=189 |oclc=936144129}}</ref> The Romans ]ized the term to ''indicum'', which passed into ] and eventually into English as the word indigo.
In the nineteenth century, the British obtained much indigo from ]. With the coming of the synthetic substitute, the demand for natural indigo dropped and indigo farming became unprofitable.


In ] indigo cultivators revolted against exploitative working conditions created by European merchants and planters in what became known as the ] in 1859. The Bengali play '']'' by Indian playwright ] was a fictionalized retelling of the revolt.
In literature, the play ] by ] is based on the indigo slavery and forceful cultivation of indigo in India. It played an essential part in the ]i indigo revolt of 1858 called ].


], India, drawn by {{Interlanguage link|Émile Thérond|fr|3=Émile Thérond}}, 19th century]]
== Developments in dyeing technology ==
The demand for indigo in the 19th century is indicated by the fact that in 1897, {{convert|7000|sqkm|sqmi|abbr=on}} were dedicated to the cultivation of indican-producing plants, mainly in ]. By comparison, the country of ] is {{convert|2,586|sqkm|sqmi|abbr=on}}.<ref name=Ullmann/>
]
Indigo is a challenging dye to use because it is not ] in ]; to be dissolved, it must undergo a chemical change (]). When a submerged fabric is removed from the dyebath, the indigo quickly combines with ] in the air and reverts to its insoluble form. When it first became widely available in Europe in the sixteenth century, European dyers and printers struggled with indigo because of this distinctive property. It also required several chemical manipulations, some involving toxic materials, and had many opportunities to injure workers. In fact, during the 19th century, English poet William Wordsworth referred to the plight of indigo dye workers of his hometown of Cockermouth in his autobiographical poem "The Prelude". Speaking of their dire working conditions and the empathy that he feels for them, he writes, "Doubtless, I should have then made common cause/ With some who perished; haply perished too,/ A poor mistaken and bewildered offering - / Unknown to those bare souls of miller blue."


In Europe, indigo remained a rare commodity throughout the Middle Ages. A chemically identical dye derived from the woad plant ''(])'' was used instead. In the late 15th century, the ] explorer ] discovered a sea route to India. This led to the establishment of direct trade with India, the ], China, and Japan. Importers could now avoid the heavy duties imposed by ]n, ]ine, and Greek middlemen and the lengthy and dangerous land routes which had previously been used. Consequently, the importation and use of indigo in Europe rose significantly. Much European indigo from Asia arrived through ports in Portugal, the Netherlands, and England. Many indigo plantations were established by European powers in tropical climates. Spain imported the dye from its colonies in Central and South America, and it was a major crop in ] and Jamaica, with much or all of the labor performed by enslaved Africans and African Americans. In the Spanish colonial era, intensive production of indigo for the world market in the region of modern El Salvador entailed such unhealthy conditions that the local indigenous population, forced to labor in pestilential conditions, was decimated.<ref>Fowler, Walter (6 August 1991). ''The Formation of Complex Society in Southeastern Mesoamerica''. CRC Press.</ref> Indigo plantations also thrived in the ]. However, France and Germany outlawed imported indigo in the 16th century to protect the local woad dye industry. In central Europe, indigo resist dyeing is a centuries-old skill that has received UNESCO Intangible Cultural Heritage of Humanity recognition.<ref>{{cite web |last=Denisyuk |first=Yulia |title=Europe's secret dyeing formula |url=https://www.bbc.com/travel/article/20230420-blaudruck-europes-secret-dyeing-formula |access-date=2023-04-21 |website=bbc.com |language=en}}</ref>
A preindustrial process for dyeing with indigo, used in Europe, was to dissolve the indigo in stale urine.
Urine ] the water-insoluble indigo to a soluble substance known as ''indigo white'' or ''leucoindigo'', which is yellow, but because small amounts are easily oxidizied to indigo, the solution is often yellow-green. Fabric dyed in the solution turns blue after the indigo white ] and returns to indigo. Synthetic ] to replace urine became available in the 1800s.


] used "indigo" to describe one of the two new ]s he added to the five he had originally named, in his revised account of the rainbow in ''Lectiones Opticae'' of 1675.<ref>Quoted in {{cite book |last=Hentschel |first=Klaus |title=Mapping the spectrum: techniques of visual representation in research and teaching |publisher=Oxford University Press |location=Oxford, England |year=2002 |page=28 |isbn=978-0-19-850953-0}}</ref>
Another preindustrial method, used in ], was to dissolve the indigo in a heated vat in which a culture of ], ] bacteria was maintained. Some species of such bacteria generate ] as a metabolic product, which can convert insoluble indigo into soluble indigo white. Cloth dyed in such a vat was decorated with the techniques of ] (tie-dye), ], ], and ]. Examples of clothing and banners dyed with these techniques can be seen in the works of ] and other artists.


Because of its high value as a trading commodity, indigo was often referred to as blue gold.<ref>{{cite web |title=History of Indigo & Indigo Dyeing |url=http://www.wildcolours.co.uk/html/indigo_history.html |website=wildcolours.co.uk |publisher=Wild Colours and natural Dyes |access-date=30 December 2015 |quote=Indigo was often referred to as Blue Gold as it was an ideal trading commodity; high value, compact and long lasting}}</ref>
Two different methods for the direct application of indigo were developed in England in the eighteenth century and remained in use well into the nineteenth century. The first method, known as ''pencil blue'' because it was most often applied by pencil or brush, could be used to achieve dark hues. ] and a thickener were added to the indigo vat. The arsenic compound delayed the oxidation of the indigo long enough to paint the dye onto fabrics.


] wearing the indigo-dyed ]]]
]
Throughout West Africa, Indigo was the foundation of centuries-old textile traditions. From the ] nomads of the ] to ], clothes dyed with indigo signified wealth. Women dyed the cloth in most areas, with the ] of ] and the ] of ] particularly well known for their expertise. Among the ] male dyers, working at communal dye pits was the basis of the wealth of the ancient city of ], and they can still be seen plying their trade today at the same pits.<ref>Kriger, Colleen E. & Connah, Graham (2006). ''Cloth in West African History''. Rowman Altamira. {{ISBN|0-7591-0422-0}}.</ref> The Tuareg are sometimes called the "Blue People" because the ] pigment in the cloth of their traditional robes and turbans stained their skin dark blue.<ref name="GearonEamonn">Gearon, Eamonn, (2011) ''The Sahara: A Cultural History'' Oxford University Press, p. 239</ref>
The second method was known as ''china blue'' due to its resemblance to Chinese blue-and-white porcelain. Instead of using an indigo solution directly, the process involved printing the insoluble form of indigo onto the fabric. The indigo was then reduced in a sequence of baths of ], with air-oxidation between each immersion. The china blue process could make sharp designs, but it could not produce the dark hues possible with the pencil blue method.


]}} dying technique traditionally used in Japan; cloth ready for dyeing in indigo, and after dyeing]]
Around ] the ''glucose process'' was developed. It finally enabled the direct printing of indigo onto fabric and could produce inexpensive dark indigo prints unattainable with the china blue method.
In Japan, indigo became especially important during the ]. This was due to a growing textiles industry,<ref name="Ikegami2005">{{cite book |author=Eiko Ikegami |title=Bonds of Civility: Aesthetic Networks and the Political Origins of Japanese Culture |url=https://books.google.com/books?id=KsN81J1s70kC&pg=PA284 |date=28 February 2005 |publisher=Cambridge University Press |isbn=978-0-521-60115-3 |page=284}}</ref> and because commoners had been banned from wearing silk,<ref name="Sagers2018">{{cite book |author=John H. Sagers |title=Confucian Capitalism: Shibusawa Eiichi, Business Ethics, and Economic Development in Meiji Japan |url=https://books.google.com/books?id=1-1lDwAAQBAJ&pg=PA27 |date=20 July 2018 |publisher=Springer |isbn=978-3-319-76372-9 |page=27}}</ref> leading to the increasing cultivation of cotton, and consequently indigo – one of the few substances that could dye it.<ref name="Wassenaar2011">{{cite book |author=Trudy M. Wassenaar |title=Bacteria: The Benign, the Bad, and the Beautiful |url=https://books.google.com/books?id=YGtR43I93iMC&pg=PA105 |date=3 November 2011 |publisher=John Wiley & Sons |isbn=978-1-118-14338-4 |page=105}}</ref>


In North America, indigo was introduced into colonial South Carolina by ], where it became the colony's second-most important cash crop (after rice).<ref>{{cite web |title=Eliza Lucas Pinckney: Indigo in the Atlantic World |author=Eliza Layne Martin |url=http://cwh.ucsc.edu/SocialBiog.Martin.pdf |access-date=2013-08-24 |url-status=dead |archive-url=https://web.archive.org/web/20100607061823/http://cwh.ucsc.edu/SocialBiog.Martin.pdf |archive-date=2010-06-07}}</ref> As a major export crop, indigo supported plantation slavery there.<ref>Andrea Feeser, ''Red, White, and Black Make Blue: Indigo in the Fabric of Colonial South Carolina Life'' (University of Georgia Press; 2013)</ref> In the May and June 1755 issues of ''],'' there appeared a detailed account of the cultivation of indigo, accompanied by drawings of necessary equipment and a prospective budget for starting such an operation, authored by South Carolina planter ]. It later appeared as a book.<ref>{{cite journal |last1=Jones |first1=Claude E. |year=1958 |title=Charles Woodmason as a Poet |journal=The South Carolina Historical Magazine |volume=59 |issue=4 |pages=189–194}}</ref><ref>{{cite book |last=Shields |first=David S. |title=Oracles of Empire: Poetry, Politics, and Commerce in British America, 1690-1750 |year=2010 |publisher=University of Chicago Press |pages=69, 249}}</ref> By 1775, indigo production in South Carolina exceeded 1,222,000 pounds.<ref>{{cite book |editor-last=Edgar |editor-first=Walter B. |title=The South Carolina Encyclopedia |year=2006 |publisher=University of South Carolina Press |page=9}}</ref> When ] sailed to France in November 1776 to enlist France's support for the ], 35 barrels of indigo were on board the ], the sale of which would help fund the war effort.<ref>{{cite book |last=Schoenbrun |first=David |date=1976 |title=Triumph in Paris: The Exploits of Benjamin Franklin |publisher=Harper & Row |location=New York |isbn=978-0-06-013854-7 |page=51}}</ref> In colonial North America, three commercially important species are found: the native ''I. caroliniana'', and the introduced ''I. tinctoria'' and ''I. suffruticosa''.<ref>{{cite journal |journal=Economic Botany |volume=33 |issue=2 |year=1979 |pages=128–134 |title=The indigo of commerce in colonial North America |author=David H. Rembert Jr. |doi=10.1007/BF02858281 |s2cid=2488865}}</ref>
Since 2004 ] indigo, or instant indigo, has become available. In this method the indigo has already been reduced, and then freeze dried into a crystal. The crystals are added to warm water to create the dye pot. As in a standard indigo dye pot, care has to be taken to avoid mixing in oxygen. Freeze dried indigo is simple to use, and the crystals can be stored indefinitely as long as they are not exposed to moisture. <ref>{{cite web |url= http://www.paradisefibers.com/instantindigo.htm|title=Directions for Instant Indigo |accessdate=2008-05-06 |author= Judith McKenzie McCuin}}</ref>


===Synthetic development===
== Chemical properties ==
] ] plant (1890)]]
In 1865 the German chemist ] began working on the synthesis of indigo. He described his first synthesis of indigo in 1878 (from ]) and a second synthesis in 1880 (from ]). (It was not until 1883 that Baeyer finally determined the structure of indigo.<ref>Adolf Baeyer (1883) , ''Berichte der Deutschen chemischen Gesellschaft zu Berlin'', '''16''' : 2188-2204; see especially p. 2204.</ref>) The synthesis of indigo remained impractical, so the search for alternative starting materials at ] (BASF) and Hoechst continued. Johannes Pfleger<ref name="history.evonik.com">{{cite web |url=https://history.evonik.com/sites/geschichte/en/personalities/pfleger-johannes/pages/default.aspx |title=Johannes Pfleger - Das Evonik Geschichtsportal - Die Geschichte von Evonik Industries |website=history.evonik.com |access-date=7 June 2020 |archive-date=1 August 2020 |archive-url=https://web.archive.org/web/20200801202949/https://history.evonik.com/sites/geschichte/en/personalities/pfleger-johannes/pages/default.aspx |url-status=dead}}</ref> and ] eventually came up with industrial mass production synthesis.<ref name="ingenious.org.uk">{{cite web |url=http://www.ingenious.org.uk/site.asp?s=RM&Param=1&SubParam=1&Content=1&ArticleID=%7BCBDF1082-9F5C-498F-A769-B33A7DA83B30%7D&ArticleID2=%7B3C4444FC-FC4D-4498-B0B4-8B8A47C5BA76%7D&MenuLinkID=%7BA54FA022-17E2-483C-B937-DEC8B8964C33%7D |title=The Synthesis of Indigo |access-date=2015-01-05 |archive-url=https://web.archive.org/web/20160304084155/http://www.ingenious.org.uk/site.asp?s=RM&Param=1&SubParam=1&Content=1&ArticleID=%7BCBDF1082-9F5C-498F-A769-B33A7DA83B30%7D&ArticleID2=%7B3C4444FC-FC4D-4498-B0B4-8B8A47C5BA76%7D&MenuLinkID=%7BA54FA022-17E2-483C-B937-DEC8B8964C33%7D |archive-date=2016-03-04 |url-status=dead}}</ref>
Indigo is a dark blue crystalline powder that ] at 390°&ndash;392°C. It is insoluble in water, ], or ] but soluble in ], ], or concentrated ]. The chemical structure of indigo corresponds to the formula ]<sub>16</sub>]<sub>10</sub>]<sub>2</sub>]<sub>2</sub>.


The synthesis of N-(2-carboxyphenyl)glycine from the easy to obtain ] provided a new and economically attractive route. BASF developed a commercially feasible manufacturing process that was in use by 1897, at which time 19,000 tons of indigo were being produced from plant sources. This had dropped to 1,000 tons by 1914 and continued to contract. By 2011, 50,000 tons of synthetic indigo were being produced worldwide.<ref>{{cite journal |title=Chemists go green to make better blue jeans |journal=Nature |year=2018 |volume=553 |issue=7687 |page=128 |doi=10.1038/d41586-018-00103-8 |bibcode=2018Natur.553..128. |doi-access=free}}</ref>
The naturally occurring substance is ], which is colorless and soluble in water. ] can easily be ] to β-D-] and ]. Mild ], such as by exposure to air, converts indoxyl to indigo.


==Dyeing technology==
The manufacturing process developed in the late 1800s is still in use throughout the world. In this process, ] is ] by the fusion of ] phenylglycinate in a mixture of ] and sodamide.
]
]


===Indigo white===
]
Indigo is a challenging dye because it is not ] in water. To be dissolved, it must undergo a chemical change (]). Reduction converts indigo into "white indigo" (]-indigo). When a submerged fabric is removed from the dyebath, the white indigo quickly combines with ] in the air and reverts to the insoluble, intensely colored indigo. When it first became widely available in Europe in the 16th century, European dyers and printers struggled with indigo because of this distinctive property. It also required several chemical manipulations, some involving toxic materials, and presented many opportunities to injure workers. In the 19th century, English poet ] referred to the plight of indigo dye workers of his hometown of Cockermouth in his autobiographical poem ''The Prelude''. Speaking of their dire working conditions and the empathy that he felt for them, he wrote:
{{Poem quote|
Doubtless, I should have then made common cause
With some who perished; haply perished too
A poor mistaken and bewildered offering
Unknown to those bare souls of miller blue}}


A pre-industrial process for production of indigo white, used in Europe, was to dissolve the indigo in stale urine, which contains ammonia. A more convenient reductive agent is zinc. Another pre-industrial method, used in Japan, was to dissolve the indigo in a heated vat in which a culture of ], ] bacteria was maintained. Some species of such bacteria generate ] as a metabolic product, which convert insoluble indigo into soluble indigo white. Cloth dyed in such a vat was decorated with the techniques of '']'' (]), '']'', '']'', and '']''. Examples of clothing and banners dyed with these techniques can be seen in the works of ] and other artists.
Several simpler compounds can be produced by decomposing indigo; these compounds include ] and ]. The only chemical reaction of practical importance is its reduction by ] to indigo white. The indigo white is reoxidized to indigo after it has been applied to the fabric.


===Direct printing===
]
Two different methods for the direct application of indigo were developed in England in the 18th century and remained in use well into the 19th century. The first method, known as 'pencil blue' because it was most often applied by pencil or brush, could be used to achieve dark hues. ] and a thickener were added to the indigo vat. The arsenic compound delayed the oxidation of the indigo long enough to paint the dye onto fabrics.{{Citation needed|date=May 2024}}


]
Indigo treated with sulfuric acid produces a blue-green color. It became available in the mid-1700s. Sulfonated indigo is also referred to as ''Saxon blue'' or '']''.
The second method was known as 'China blue' due to its resemblance to Chinese blue-and-white porcelain. Instead of using an indigo solution directly, the process involved printing the insoluble form of indigo onto the fabric. The indigo was then reduced in a sequence of baths of ], with air oxidation between each immersion. The China blue process could make sharp designs, but it could not produce the dark hues possible with the pencil blue method.


Around 1880, the 'glucose process' was developed. It finally enabled the direct printing of indigo onto fabric and could produce inexpensive dark indigo prints unattainable with the China blue method.
'']'' was a valuable purple dye in antiquity. It was made from excretions of a common ] snail. In ] its structure was shown to be 6,6′-dibromoindigo. It has never been produced synthetically on a commercial basis.


Since 2004, ] indigo, or instant indigo, has become available. In this method, the indigo has already been reduced, and then freeze-dried into a crystal. The crystals are added to warm water to create the dye pot. As in a standard indigo dye pot, care has to be taken to avoid mixing in oxygen. Freeze-dried indigo is simple to use, and the crystals can be stored indefinitely as long as they are not exposed to moisture.<ref>{{cite web |title=Directions for Instant Indigo |author=Judith McKenzie McCuin |url=http://www.paradisefibers.com/instantindigo.htm |access-date=2008-05-06 |url-status=dead |archive-url=https://web.archive.org/web/20041116195957/http://paradisefibers.com/instantindigo.htm |archive-date=2004-11-16}}</ref>
The ] structure of indigo is {{SMILES|S=O=c3c(=c2c1ccccc1c2=O)c4ccccc34}} and its CAS number is {{CASREF|CAS=12626-73-2}}.


==Chemical synthesis== ==Chemical properties==
]
The ] is a method dating back to 1882. Indigo may be synthetically manufactured in a number of different ways. The original method, first used to synthesise indigo by ] in 1897, involves heating N-(2-Carboxyphenyl)glycine
Indigo dye is a dark blue crystalline powder that ] at {{convert|390|–|392|°C}}. It is insoluble in water, ], or ], but soluble in ], ], ], and concentrated ]. The ] of indigo is ]<sub>16</sub>]<sub>10</sub>]<sub>2</sub>]<sub>2</sub>.
acid to 200°C in an inert atmosphere with ]. This produces indoxyl-2-carboxylic acid, a material that readily decarboxylates and oxidises in air to form indigo.


The molecule absorbs light in the orange part of the spectrum (''λ''<sub>max</sub>=613&nbsp;nm).<ref>{{cite journal |last1=Wouten |first1=J. |last2=Verhecken |first2=A. |year=1991 |title=High-performance liquid chromatography of blue and purple indigoid natural dyes |journal=Journal of the Society of Dyers and Colourists |volume=107 |issue=7–8 |pages=266–269 |doi=10.1111/j.1478-4408.1991.tb01351.x}}</ref> The compound owes its deep color to the conjugation of the ]s, i.e. the double bonds within the molecule are adjacent and the molecule is planar. In indigo white, the conjugation is interrupted because the molecule is non-planar.
]


===Indigo derivatives===
The modern synthesis of indigo is slightly different from that route originally used and its discovery is credited to Pfleger in 1901. In this process, N-phenylglycine is treated with an alkaline melt of sodium and potassium hydroxides containing ]. This produces ], which is subsequently oxidised in air to form indigo.
]
]
The benzene rings in indigo can be modified to give a variety of related dyestuffs. ], where the two NH groups are replaced by S atoms, is deep red. ] is a dull purple dye that is secreted by a common Mediterranean snail. It was highly prized in antiquity. In 1909, its structure was shown to be ] (red). 6-bromoindigo (purple) is a component as well.<ref name="indigo-bromo">{{cite journal |last1=Ramig |first1=Keith |last2=Lavinda |first2=Olga |last3=Szalda |first3=David J. |last4=Mironova |first4=Irina |last5=Karimi |first5=Sasan |last6=Pozzi |first6=Federica |last7=Shah |first7=Nilam |last8=Samson |first8=Jacopo |last9=Ajiki |first9=Hiroko |last10=Massa |first10=Lou |last11=Mantzouris |first11=Dimitrios |last12=Karapanagiotis |first12=Ioannis |last13=Cooksey |first13=Christopher |title=The nature of thermochromic effects in dyeings with indigo, 6-bromoindigo, and 6,6'-dibromoindigo, components of Tyrian purple |journal=Dyes and Pigments |date=June 2015 |volume=117 |pages=37–48 |doi=10.1016/j.dyepig.2015.01.025}}</ref> It has never been produced on a commercial basis. The related ] (5,7,5',7'-tetrabromoindigo) is, however, of commercial value.


Indigo and its derivatives featuring intra- and intermolecular hydrogen bonding have very low solubility in organic solvents. They can be made soluble using transient ]s such as the ], which suppresses intermolecular bonding.<ref>{{cite journal |last1=Głowacki |first1=Eric Daniel |last2=Voss |first2=Gundula |last3=Demirak |first3=Kadir |last4=Havlicek |first4=Marek |last5=Sünger |first5=Nevsal |last6=Okur |first6=Aysu Ceren |last7=Monkowius |first7=Uwe |last8=Gąsiorowski |first8=Jacek |last9=Leonata |first9=Lucia |last10=Sariciftcia |first10=Niyazi Serdar |display-authors=5 |date=2013 |title=A facile protection–deprotection route for obtaining indigo pigments as thin films and their applications in organic bulk heterojunctions |journal=Chemical Communications |volume=49 |issue=54 |pages=6063–6065 |doi=10.1039/C3CC42889C |pmid=23723050}}</ref> Heating of the tBOC indigo results in efficient thermal deprotection and regeneration of the parent H-bonded pigment.
]


Treatment with sulfuric acid converts indigo into a blue-green derivative called ] (sulfonated indigo). It became available in the mid-18th century. It is used as a colorant for food, pharmaceuticals, and cosmetics.
== External links ==

<!--===========================({{NoMoreLinks}})===============================
==Indigo as an organic semiconductor==
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Indigo and some of its derivatives are known to be ambipolar ] when deposited as thin films by vacuum evaporation.<ref>{{cite journal |doi=10.1002/adma.201102619 |pmid=22109816 |title=Indigo - A Natural Pigment for High Performance Ambipolar Organic Field Effect Transistors and Circuits |year=2012 |last1=Irimia-Vladu |first1=Mihai |last2=Głowacki |first2=Eric D. |last3=Troshin |first3=Pavel A. |last4=Schwabegger |first4=Günther |last5=Leonat |first5=Lucia |last6=Susarova |first6=Diana K. |last7=Krystal |first7=Olga |last8=Ullah |first8=Mujeeb |last9=Kanbur |first9=Yasin |last10=Bodea |first10=Marius A. |last11=Razum |first11=Vladimir F. |last12=Sitter |first12=Helmut |last13=Bauer |first13=Siegfried |author14-link=Niyazi Serdar Sarıçiftçi |last14=Sarıçiftçi |first14=Niyazi Serdar |journal=Advanced Materials |volume=24 |issue=3 |pages=375–380 |bibcode=2012AdM....24..375I |s2cid=205241976}}</ref>
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==Safety and the environment==
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Indigo has a low oral toxicity, with an {{LD50}} of 5&nbsp;g/kg (0.5% of total mass) in mammals.<ref name=Ullmann/> In 2009, large spills of blue dyes had been reported downstream of a blue jeans manufacturer in ].<ref name="st2009">{{cite web |title=Gap alarm |date=2009-08-09 |newspaper=] |url=http://www.timesonline.co.uk/tol/news/world/africa/article6788728.ece |archive-url=https://archive.today/20100528081251/http://www.timesonline.co.uk/tol/news/world/africa/article6788728.ece |url-status=dead |archive-date=May 28, 2010 |access-date=2011-08-16}}</ref>
| See ] and ] for details. |

| |
The compound has been found to act as an ] of the ].<ref name="pmid12540743">{{cite journal |vauthors=Denison MS, Nagy SR |title=Activation of the aryl hydrocarbon receptor by structurally diverse exogenous and endogenous chemicals |journal=Annu. Rev. Pharmacol. Toxicol. |volume=43 |pages=309–334 |year=2003 |pmid=12540743 |doi=10.1146/annurev.pharmtox.43.100901.135828}}</ref>
| If there are already plentiful links, please propose additions or |

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==See also==
| to the appropriate category at the Open Directory Project (www.dmoz.org)|
* ]
| and link back to that category using the {{dmoz}} template. |
* ]
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* ]
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* '']''
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==References== ==References==
{{reflist}} {{Reflist}}

==Further reading== ==Further reading==
*{{cite book |title = Indigo | first = Jenny | last = Balfour-Paul | publisher = British Museum Press | location = London | year= 1998 | pages = 264 pages}} *{{cite book |last=Balfour-Paul |first=Jenny |title=Indigo: Egyptian Mummies to Blue Jeans |publisher=British Museum Press |year=2016 |isbn=978-0-7141-1776-8 |location=London |pages=264 pages}}
*{{cite journal |title = The natural constituents of historical textile dyes | last = Ferreira | first = E.S.B. | coauthors = Hulme A. N., McNab H., Quye A. | journal = Chemical Society reviews | volume = 33 | issue = 6 | year= 2004 | doi = 10.1039/b305697j | pages = 329}} *{{cite journal |last1=Ferreira |first1=E.S.B. |last2=Hulme A. N. |author2-link=Alison Hulme |last3=McNab H. |last4=Quye A. |year=2004 |title=The natural constituents of historical textile dyes |url=http://eprints.gla.ac.uk/109139/1/109148.pdf |journal=Chemical Society Reviews |volume=33 |issue=6 |pages=329–36 |doi=10.1039/b305697j |pmid=15280965}}
*Paul, Jenny Balfour. 2020. "Indigo and Blue: A Marriage Made in Heaven." ''Textile Museum Journal'' 47 (January): 160–85.
*{{cite journal |title = The chemistry of plant and animal dyes | last = Sequin-Frey | first = Margareta | journal = Journal of Chemical Education | volume = 58 | issue = 4 | year= 1981 | url = http://jchemed.chem.wisc.edu/Journal/Issues/1981/Apr/jceSubscriber/JCE1981p0301.pdf}}
*{{cite journal |last=Sequin-Frey |first=Margareta |year=1981 |title=The chemistry of plant and animal dyes |url=http://jchemed.chem.wisc.edu/Journal/Issues/1981/Apr/jceSubscriber/JCE1981p0301.pdf |journal=Journal of Chemical Education |volume=58 |issue=4 |pages=301 |bibcode=1981JChEd..58..301S |doi=10.1021/ed058p301}}


==External links==
{{Dyeing}}
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{{Aryl hydrocarbon receptor modulators}}
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Latest revision as of 17:44, 1 December 2024

Chemical compound, food additive and dye
Indigo
Lump of Indian indigo dye
Skeletal formula of indigo dye
Ball-and-stick model of the indigo dye molecule
Names
Preferred IUPAC name --3,3'(1H,1'H)-dione
Other names 2,2'-Bis(2,3-dihydro-3-oxoindolyliden), Indigotin
Identifiers
CAS Number
3D model (JSmol)
ChEMBL
ChemSpider
ECHA InfoCard 100.006.898 Edit this at Wikidata
PubChem CID
RTECS number
  • DU2988400
UNII
CompTox Dashboard (EPA)
InChI
  • InChI=1S/C16H10N2O2/c19-15-9-5-1-3-7-11(9)17-13(15)14-16(20)10-6-2-4-8-12(10)18-14/h1-8,17-18H/b14-13+Key: COHYTHOBJLSHDF-BUHFOSPRSA-N
  • InChI=1/C16H10N2O2/c19-15-9-5-1-3-7-11(9)17-13(15)14-16(20)10-6-2-4-8-12(10)18-14/h1-8,17-18H/b14-13+Key: COHYTHOBJLSHDF-BUHFOSPRBQ
SMILES
  • c1ccc2c(c1)C(=O)/C(=C\3/C(=O)c4ccccc4N3)/N2
Properties
Chemical formula C16H10N2O2
Molar mass 262.27 g/mol
Appearance dark blue crystalline powder
Density 1.199 g/cm
Melting point 390 to 392 °C (734 to 738 °F; 663 to 665 K)
Boiling point decomposes
Solubility in water 990 µg/L (at 25 °C)
Related compounds
Related compounds Indoxyl
Tyrian purple
Indican
Except where otherwise noted, data are given for materials in their standard state (at 25 °C , 100 kPa). checkverify (what is  ?) Infobox references
Chemical compound

Indigo dye is an organic compound with a distinctive blue color. Indigo is a natural dye extracted from the leaves of some plants of the Indigofera genus, in particular Indigofera tinctoria. Dye-bearing Indigofera plants were commonly grown and used throughout the world, particularly in Asia, with the production of indigo dyestuff economically important due to the historical rarity of other blue dyestuffs.

Most indigo dye produced today is synthetic, constituting around 80,000 tonnes each year, as of 2023. It is most commonly associated with the production of denim cloth and blue jeans, where its properties allow for effects such as stone washing and acid washing to be applied quickly.

Uses

Indigo dye

The primary use for indigo is as a dye for cotton yarn, mainly used in the production of denim cloth suitable for blue jeans; on average, a pair of blue jeans requires 3 grams (0.11 oz) to 12 grams (0.42 oz) of dye. Smaller quantities are used in the dyeing of wool and silk.

Indigo carmine, also known as indigo, is an indigo derivative which is also used as a colorant. About 20,000 tonnes are produced annually, again mainly for the production of blue jeans. It is also used as a food colorant, and is listed in the United States as FD&C Blue No. 2.

Sources

Natural sources

A variety of plants have provided indigo throughout history, but most natural indigo was obtained from those in the genus Indigofera, which are native to the tropics, notably the Indian Subcontinent. The primary commercial indigo species in Asia was true indigo (Indigofera tinctoria, also known as I. sumatrana). A common alternative used in the relatively colder subtropical locations such as Japan's Ryukyu Islands and Taiwan is Strobilanthes cusia.

Until the introduction of Indigofera species from the south, Persicaria tinctoria (dyer's knotweed) was the most important blue dyestuff in East Asia; however, the crop produced less dyestuff than the average crop of indigo, and was quickly surpassed in favour of the more economical Indigofera tinctoria plant. In Central and South America, the species grown is Indigofera suffruticosa, also known as anil, and in India, an important species was Indigofera arrecta, Natal indigo. In Europe, Isatis tinctoria, commonly known as woad, was used for dyeing fabrics blue, containing the same dyeing compounds as indigo, also referred to as indigo.

Several plants contain indigo, which, when exposed to an oxidizing source such as atmospheric oxygen, reacts to produce indigo dye; however, the relatively low concentrations of indigo in these plants make them difficult to work with, with the color more easily tainted by other dye substances also present in these plants, typically leading to a greenish tinge.

The precursor to indigo is indican, a colorless, water-soluble derivative of the amino acid tryptophan, and Indigofera leaves contain as much as 0.2–0.8% of this compound. Pressing cut leaves into a vat and soaking hydrolyzes the indican, releasing β-D-glucose and indoxyl. The indoxyl dimerizes in the mixture, and after 12–15 hours of fermentation yields the yellow, water-soluble leucoindigo. Subsequent exposure to air forms the blue, water-insoluble indigo dye. The dye precipitates from the fermented leaf solution upon oxidation, but may also be precipitated when mixed with a strong base such as lye. The solids are filtered, pressed into cakes, dried, and powdered. The powder is then mixed with various other substances to produce different shades of blue and purple.

Natural sources of indigo also include mollusks: the Murex genus of sea snails produces a mixture of indigo and 6,6'-dibromoindigo (red), which together produce a range of purple hues known as Tyrian purple. Light exposure during part of the dyeing process can convert the dibromoindigo into indigo, resulting in blue hues known as royal blue, hyacinth purple, or tekhelet.

Chemical synthesis

Heumann's synthesis of indigoPfleger's synthesis of indigo

Given its economic importance, indigo has been prepared by many methods. The Baeyer–Drewsen indigo synthesis dates back to 1882. It involves an aldol condensation of o-nitrobenzaldehyde with acetone, followed by cyclization and oxidative dimerization to indigo. This route was highly useful for obtaining indigo and many of its derivatives on the laboratory scale, but proved impractical for industrial-scale synthesis. Johannes Pfleger and Karl Heumann [de] eventually came up with industrial mass production synthesis from aniline by using mercury as a catalyst. The method was discovered by an accident by Karl Heumann in Zurich which involved a broken thermometer.

The first commercially practical route of producing indigo is credited to Pfleger in 1901. In this process, N-phenylglycine is treated with a molten mixture of sodium hydroxide, potassium hydroxide, and sodamide. This highly sensitive melt produces indoxyl, which is subsequently oxidized in air to form indigo. Variations of this method are still in use today. An alternative and also viable route to indigo is credited to Heumann in 1897. It involves heating N-(2-carboxyphenyl)glycine to 200 °C (392 °F) in an inert atmosphere with sodium hydroxide. The process is easier than the Pfleger method, but the precursors are more expensive. Indoxyl-2-carboxylic acid is generated. This material readily decarboxylates to give indoxyl, which oxidizes in air to form indigo. The preparation of indigo dye is practised in college laboratory classes according to the original Baeyer-Drewsen route.

History

Indigo, historical dye collection of the Technical University of Dresden, Germany


The oldest known fabric dyed indigo, dated to 6,000 years ago, was discovered in Huaca Prieta, Peru. Many Asian countries, such as India, China, Japan, and Southeast Asian nations have used indigo as a dye (particularly for silk) for centuries. The dye was also known to ancient civilizations in Mesopotamia, Egypt, Britain, Mesoamerica, Peru, Iran, and West Africa. Indigo was also cultivated in India, which was also the earliest major center for its production and processing. The Indigofera tinctoria species was domesticated in India. Indigo, used as a dye, made its way to the Greeks and the Romans, where it was valued as a luxury product.

Cake of indigo, about 2 cm

In Mesopotamia, a neo-Babylonian cuneiform tablet of the seventh century BC gives a recipe for the dyeing of wool, where lapis-colored wool (uqnatu) is produced by repeated immersion and airing of the cloth. Indigo was most probably imported from India. The Romans used indigo as a pigment for painting and for medicinal and cosmetic purposes. It was a luxury item imported to the Mediterranean from India by Arab merchants.

India was a primary supplier of indigo to Europe as early as the Greco-Roman era. The association of India with indigo is reflected in the Greek word for the dye, indikón (Ἰνδικόν, Indian). The Romans latinized the term to indicum, which passed into Italian dialect and eventually into English as the word indigo.

In Bengal indigo cultivators revolted against exploitative working conditions created by European merchants and planters in what became known as the Indigo revolt in 1859. The Bengali play Nil Darpan by Indian playwright Dinabandhu Mitra was a fictionalized retelling of the revolt.

Indigo factory at Allahabad, India, drawn by Émile Thérond [fr], 19th century

The demand for indigo in the 19th century is indicated by the fact that in 1897, 7,000 km (2,700 sq mi) were dedicated to the cultivation of indican-producing plants, mainly in India. By comparison, the country of Luxembourg is 2,586 km (998 sq mi).

In Europe, indigo remained a rare commodity throughout the Middle Ages. A chemically identical dye derived from the woad plant (Isatis tinctoria) was used instead. In the late 15th century, the Portuguese explorer Vasco da Gama discovered a sea route to India. This led to the establishment of direct trade with India, the Spice Islands, China, and Japan. Importers could now avoid the heavy duties imposed by Persian, Levantine, and Greek middlemen and the lengthy and dangerous land routes which had previously been used. Consequently, the importation and use of indigo in Europe rose significantly. Much European indigo from Asia arrived through ports in Portugal, the Netherlands, and England. Many indigo plantations were established by European powers in tropical climates. Spain imported the dye from its colonies in Central and South America, and it was a major crop in Haiti and Jamaica, with much or all of the labor performed by enslaved Africans and African Americans. In the Spanish colonial era, intensive production of indigo for the world market in the region of modern El Salvador entailed such unhealthy conditions that the local indigenous population, forced to labor in pestilential conditions, was decimated. Indigo plantations also thrived in the Virgin Islands. However, France and Germany outlawed imported indigo in the 16th century to protect the local woad dye industry. In central Europe, indigo resist dyeing is a centuries-old skill that has received UNESCO Intangible Cultural Heritage of Humanity recognition.

Newton used "indigo" to describe one of the two new primary colors he added to the five he had originally named, in his revised account of the rainbow in Lectiones Opticae of 1675.

Because of its high value as a trading commodity, indigo was often referred to as blue gold.

Tuaregs wearing the indigo-dyed tagelmust

Throughout West Africa, Indigo was the foundation of centuries-old textile traditions. From the Tuareg nomads of the Sahara to Cameroon, clothes dyed with indigo signified wealth. Women dyed the cloth in most areas, with the Yoruba of Nigeria and the Mandinka of Mali particularly well known for their expertise. Among the Hausa male dyers, working at communal dye pits was the basis of the wealth of the ancient city of Kano, and they can still be seen plying their trade today at the same pits. The Tuareg are sometimes called the "Blue People" because the indigo pigment in the cloth of their traditional robes and turbans stained their skin dark blue.

Thread-tied shibori dying technique traditionally used in Japan; cloth ready for dyeing in indigo, and after dyeing

In Japan, indigo became especially important during the Edo period. This was due to a growing textiles industry, and because commoners had been banned from wearing silk, leading to the increasing cultivation of cotton, and consequently indigo – one of the few substances that could dye it.

In North America, indigo was introduced into colonial South Carolina by Eliza Lucas, where it became the colony's second-most important cash crop (after rice). As a major export crop, indigo supported plantation slavery there. In the May and June 1755 issues of The Gentleman's Magazine, there appeared a detailed account of the cultivation of indigo, accompanied by drawings of necessary equipment and a prospective budget for starting such an operation, authored by South Carolina planter Charles Woodmason. It later appeared as a book. By 1775, indigo production in South Carolina exceeded 1,222,000 pounds. When Benjamin Franklin sailed to France in November 1776 to enlist France's support for the American Revolutionary War, 35 barrels of indigo were on board the Reprisal, the sale of which would help fund the war effort. In colonial North America, three commercially important species are found: the native I. caroliniana, and the introduced I. tinctoria and I. suffruticosa.

Synthetic development

Production of Indigo dye in a BASF plant (1890)

In 1865 the German chemist Adolf von Baeyer began working on the synthesis of indigo. He described his first synthesis of indigo in 1878 (from isatin) and a second synthesis in 1880 (from 2-nitrobenzaldehyde). (It was not until 1883 that Baeyer finally determined the structure of indigo.) The synthesis of indigo remained impractical, so the search for alternative starting materials at Badische Anilin- und Soda-Fabrik (BASF) and Hoechst continued. Johannes Pfleger and Karl Heumann eventually came up with industrial mass production synthesis.

The synthesis of N-(2-carboxyphenyl)glycine from the easy to obtain aniline provided a new and economically attractive route. BASF developed a commercially feasible manufacturing process that was in use by 1897, at which time 19,000 tons of indigo were being produced from plant sources. This had dropped to 1,000 tons by 1914 and continued to contract. By 2011, 50,000 tons of synthetic indigo were being produced worldwide.

Dyeing technology

Indigo white (leuco-indigo)
Yarn dyed with indigo dye

Indigo white

Indigo is a challenging dye because it is not soluble in water. To be dissolved, it must undergo a chemical change (reduction). Reduction converts indigo into "white indigo" (leuco-indigo). When a submerged fabric is removed from the dyebath, the white indigo quickly combines with oxygen in the air and reverts to the insoluble, intensely colored indigo. When it first became widely available in Europe in the 16th century, European dyers and printers struggled with indigo because of this distinctive property. It also required several chemical manipulations, some involving toxic materials, and presented many opportunities to injure workers. In the 19th century, English poet William Wordsworth referred to the plight of indigo dye workers of his hometown of Cockermouth in his autobiographical poem The Prelude. Speaking of their dire working conditions and the empathy that he felt for them, he wrote:

Doubtless, I should have then made common cause
With some who perished; haply perished too
A poor mistaken and bewildered offering
Unknown to those bare souls of miller blue

A pre-industrial process for production of indigo white, used in Europe, was to dissolve the indigo in stale urine, which contains ammonia. A more convenient reductive agent is zinc. Another pre-industrial method, used in Japan, was to dissolve the indigo in a heated vat in which a culture of thermophilic, anaerobic bacteria was maintained. Some species of such bacteria generate hydrogen as a metabolic product, which convert insoluble indigo into soluble indigo white. Cloth dyed in such a vat was decorated with the techniques of shibori (tie-dye), kasuri, katazome, and tsutsugaki. Examples of clothing and banners dyed with these techniques can be seen in the works of Hokusai and other artists.

Direct printing

Two different methods for the direct application of indigo were developed in England in the 18th century and remained in use well into the 19th century. The first method, known as 'pencil blue' because it was most often applied by pencil or brush, could be used to achieve dark hues. Arsenic trisulfide and a thickener were added to the indigo vat. The arsenic compound delayed the oxidation of the indigo long enough to paint the dye onto fabrics.

Pot of freeze-dried indigo dye

The second method was known as 'China blue' due to its resemblance to Chinese blue-and-white porcelain. Instead of using an indigo solution directly, the process involved printing the insoluble form of indigo onto the fabric. The indigo was then reduced in a sequence of baths of iron(II) sulfate, with air oxidation between each immersion. The China blue process could make sharp designs, but it could not produce the dark hues possible with the pencil blue method.

Around 1880, the 'glucose process' was developed. It finally enabled the direct printing of indigo onto fabric and could produce inexpensive dark indigo prints unattainable with the China blue method.

Since 2004, freeze-dried indigo, or instant indigo, has become available. In this method, the indigo has already been reduced, and then freeze-dried into a crystal. The crystals are added to warm water to create the dye pot. As in a standard indigo dye pot, care has to be taken to avoid mixing in oxygen. Freeze-dried indigo is simple to use, and the crystals can be stored indefinitely as long as they are not exposed to moisture.

Chemical properties

Indigo, space-filling

Indigo dye is a dark blue crystalline powder that sublimes at 390–392 °C (734–738 °F). It is insoluble in water, alcohol, or ether, but soluble in DMSO, chloroform, nitrobenzene, and concentrated sulfuric acid. The chemical formula of indigo is C16H10N2O2.

The molecule absorbs light in the orange part of the spectrum (λmax=613 nm). The compound owes its deep color to the conjugation of the double bonds, i.e. the double bonds within the molecule are adjacent and the molecule is planar. In indigo white, the conjugation is interrupted because the molecule is non-planar.

Indigo derivatives

Structure of Tyrian purple
Structure of indigo carmine.

The benzene rings in indigo can be modified to give a variety of related dyestuffs. Thioindigo, where the two NH groups are replaced by S atoms, is deep red. Tyrian purple is a dull purple dye that is secreted by a common Mediterranean snail. It was highly prized in antiquity. In 1909, its structure was shown to be 6,6'-dibromoindigo (red). 6-bromoindigo (purple) is a component as well. It has never been produced on a commercial basis. The related Ciba blue (5,7,5',7'-tetrabromoindigo) is, however, of commercial value.

Indigo and its derivatives featuring intra- and intermolecular hydrogen bonding have very low solubility in organic solvents. They can be made soluble using transient protecting groups such as the tBOC group, which suppresses intermolecular bonding. Heating of the tBOC indigo results in efficient thermal deprotection and regeneration of the parent H-bonded pigment.

Treatment with sulfuric acid converts indigo into a blue-green derivative called indigo carmine (sulfonated indigo). It became available in the mid-18th century. It is used as a colorant for food, pharmaceuticals, and cosmetics.

Indigo as an organic semiconductor

Indigo and some of its derivatives are known to be ambipolar organic semiconductors when deposited as thin films by vacuum evaporation.

Safety and the environment

Indigo has a low oral toxicity, with an LD50 of 5 g/kg (0.5% of total mass) in mammals. In 2009, large spills of blue dyes had been reported downstream of a blue jeans manufacturer in Lesotho.

The compound has been found to act as an agonist of the aryl hydrocarbon receptor.

See also

References

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