Editing Tannin (section)

== Tests for tannins ==
There are three groups of methods for the analysis of tannins: precipitation of proteins or alkaloids, reaction with phenolic rings, and depolymerization.<ref>{{cite book | doi = 10.1007/978-1-4615-3476-1_15 | title=Plant Polyphenols | year=1992 | pages=259–280 | last1 = Scalbert | first1 = Augustin| chapter=Quantitative Methods for the Estimation of Tannins in Plant Tissues | isbn=978-1-4613-6540-2 }}</ref>

=== Alkaloid precipitation ===
Alkaloids such as [[caffeine]], [[cinchonine]], [[quinine]] or [[strychnine]], precipitates polyphenols and tannins. This property can be used in a quantitation method.<ref>Plant Polyphenols: Synthesis, Properties, Significance. Richard W. Hemingway, Peter E. Laks, Susan J. Branham (page 263)</ref>

=== Goldbeater's skin test ===
When [[goldbeater's skin]] or ox skin is dipped in [[Hydrochloric acid|HCl]], rinsed in water, soaked in the tannin solution for 5 minutes, washed in water, and then treated with 1% [[Iron(II) sulfate|FeSO<sub>4</sub>]] solution, it gives a blue black color if tannin was present.<ref>{{Cite book |last=Prakashan |first=Nirali |url=https://books.google.com/books?id=KwYIsLRyDp4C&dq=tannin+Goldbeater%27s+skin+test&pg=SL1-PA5 |title=Pharmacognosy |date=2009 |publisher=Nirali Prakashan |isbn=978-81-963961-5-2 |language=en}}</ref>

=== Ferric chloride test ===
The following describes the use of [[Ferric chloride test|ferric chloride (FeCl3) test]]s for [[natural phenol|phenolics]] in general: Powdered plant leaves of the test plant (1.0&nbsp;g) are weighed into a beaker and 10&nbsp;ml of distilled water are added. The mixture is boiled for five minutes. Two drops of 5% FeCl<sub>3</sub> are then added. Production of a greenish precipitate is an indication of the presence of tannins.{{cn|date=April 2025}} Alternatively, a portion of the water extract is diluted with distilled water in a ratio of 1:4 and few drops of 10% ferric chloride solution is added. A blue or green color indicates the presence of tannins (Evans, 1989).<ref>{{cite journal |last1=Yisa |first1=J. |title=Phytochemical analysis and antimicrobial activity of ''Scoparia dulcis'' and ''Nymphaea lotus'' |journal=Australian Journal of Basic and Applied Sciences |date=2009 |volume=3 |issue=4 |pages=3975–3979 |url=http://www.insipub.com/ajbas/2009/3975-3979.pdf}}</ref>

=== Other methods ===
The hide-powder method is used in tannin analysis for [[leather]] tannin and the Stiasny method for [[wood adhesive]]s.<ref>"Tannin analysis of Acacia mearnsii bark – a comparison of the hide-powder and Stiasny methods". Zheng G.C., Lin Y.L. and Yazaki Y., ''ACIAR Proceedings Series'', 1991, No. 35, pp. 128–131 ([http://www.cabdirect.org/abstracts/19920657723.html;jsessionid=F49F74287B59E2847142A9BFED7349DC abstract] {{webarchive|url=https://web.archive.org/web/20140709035011/http://www.cabdirect.org/abstracts/19920657723.html;jsessionid=F49F74287B59E2847142A9BFED7349DC |date=9 July 2014 }})</ref><ref>Study on Fast Determination Content of Condensed Tannin Using Stiasny Method. Chen Xiangming, Chen Heru and Li Weibin, Guangdong Chemical Industry, 2006–07 ([http://en.cnki.com.cn/Article_en/CJFDTOTAL-GDHG200607012.htm abstract] {{webarchive|url=https://web.archive.org/web/20150402162931/http://en.cnki.com.cn/Article_en/CJFDTOTAL-GDHG200607012.htm |date=2 April 2015 }})</ref> Statistical analysis reveals that there is no significant relationship between the results from the hide-powder and the Stiasny methods.<ref>{{cite journal | last1 = Guangcheng | first1 = Zheng | last2 = Yunlu | first2 = Lin | last3 = Yazaki | first3 = Y. | year = 1991 | title = Bark tannin contents of Acacia mearnsii provenances and the relationship between the hide-powder and the Stiasny methods of estimation | journal = Australian Forestry | volume = 54 | issue = 4| pages = 209–211 | doi = 10.1080/00049158.1991.10674579 | bibcode = 1991AuFor..54..209G }}</ref><ref>''Leather Chemists' Pocket-Book: A Short Compendium of Analytical Methods''. [[Henry Richardson Procter]], Edmund Stiasny and Harold Brumwel, E. & F.N. Spon, Limited, 1912–223 pages ([https://archive.org/details/leatherchemists00brumgoog book at Internet Archive] {{webarchive|url=https://web.archive.org/web/20161216075444/https://books.google.com/books?id=BrtJAAAAIAAJ&oe=UTF-8 |date=16 December 2016 }})</ref>

;Hide-powder method
400&nbsp;mg of sample tannins are dissolved in 100&nbsp;ml of distilled water. 3&nbsp;g of slightly chromated hide-powder previously dried in vacuum for 24h over CaCl<sub>2</sub> are added and the mixture stirred for 1 h at ambient temperature. The suspension is filtered without vacuum through a sintered glass filter. The weight gain of the hide-powder expressed as a percentage of the weight of the starting material is equated to the percentage of tannin in the sample.

;Stiasny's method
100&nbsp;mg of sample tannins are dissolved in 10&nbsp;ml distilled water. 1&nbsp;ml of 10M HCl and 2&nbsp;ml of 37% [[formaldehyde]] are added and the mixture heated under reflux for 30 min. The reaction mixture is filtered while hot through a sintered glass filter. The precipitate is washed with hot water (5× 10&nbsp;ml) and dried over CaCl<sub>2</sub>. The yield of tannin is expressed as a percentage of the weight of the starting material.

=== Reaction with phenolic rings ===
The bark tannins of ''[[Commiphora angolensis]]'' have been revealed by the usual color and precipitation reactions and by quantitative determination by the methods of Löwenthal-Procter and of Deijs<ref name="Cardoso">Chemical study of bark from Commiphora angolensis Engl. Cardoso Do Vale, J., Bol Escola Farm Univ Coimbra Edicao Cient, 1962, volume 3, page 128 ([http://eurekamag.com/research/024/318/chemical-study-bark-commiphora-angolensis-engl.php abstract] {{webarchive|url=https://web.archive.org/web/20140607001810/http://eurekamag.com/research/024/318/chemical-study-bark-commiphora-angolensis-engl.php |date=7 June 2014 }})</ref> ([[formalin]]-[[hydrochloric acid]] method).<ref>{{cite journal |last1=Deijs |first1=W. B. |title=Catechins isolated from tea leaves |journal=Recueil des Travaux Chimiques des Pays-Bas |date=1939|volume=58 |issue=9 |pages=805–830 |doi=10.1002/recl.19390580907}}</ref>

Colorimetric methods have existed such as the Neubauer-Löwenthal method which uses [[potassium permanganate]] as an oxidizing agent and [[Indigo dye|indigo]] [[sulfate]] as an indicator, originally proposed by Löwenthal in 1877.<ref>{{cite journal |last1=Löwenthal |first1=J. |title=Ueber die Bestimmung des Gerbstoffs |journal=Zeitschrift für Analytische Chemie |date=December 1877 |volume=16 |issue=1 |pages=33–48 |doi=10.1007/BF01355993|s2cid=95511307 |language=de|url=https://zenodo.org/record/2179227 }}</ref> The difficulty is that the establishing of a titer for tannin is not always convenient since it is extremely difficult to obtain the pure tannin. Neubauer proposed to remove this difficulty by establishing the titer not with regard to the tannin but with regard to crystallised [[oxalic acid]], whereby he found that 83&nbsp;g oxalic acid correspond to 41.20&nbsp;g tannin. Löwenthal's method has been criticized. For instance, the amount of indigo used is not sufficient to retard noticeably the oxidation of the non-tannins substances. The results obtained by this method are therefore only comparative.<ref>{{cite journal |last1=Spiers |first1=C. W. |title=The Estimation of Tannin in Cider |journal=The Journal of Agricultural Science |date=January 1914 |volume=6 |issue=1 |pages=77–83 |doi=10.1017/S0021859600002173|s2cid=85362459 |url=https://zenodo.org/record/2366998 }}</ref><ref>{{cite journal |last1=Snyder |first1=Harry |title=Notes on Löwenthal's method for the determination of tanin |journal=Journal of the American Chemical Society |date=October 1893 |volume=15 |issue=10 |pages=560–563 |doi=10.1021/ja02120a004|bibcode=1893JAChS..15..560S |url=https://zenodo.org/record/1428956 }}</ref> A modified method, proposed in 1903 for the quantification of tannins in wine, Feldmann's method, is making use of [[calcium hypochlorite]], instead of potassium permanganate, and indigo sulfate.<ref>{{cite journal |title=Nouvelle methode de dosage du tannin |journal=Schweizerische Wochenschrift für Chemie und Pharmacie |url=http://digisrv-1.biblio.etc.tu-bs.de/dfg-files/00041057/DWL/00000346.pdf |language=fr|archive-url=https://web.archive.org/web/20140808065055/http://digisrv-1.biblio.etc.tu-bs.de/dfg-files/00041057/DWL/00000346.pdf|archive-date=8 August 2014 }}</ref>