Editing Tannin (section)

=== 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>