208 TANNINS 



myrobalans, valonia, algarobilla, oak gall, oak wood and chest- 

 nut tannins. 



These tannins have the following characteristics : 



1. They give with ferric salts a dark blue colour 



2. They give no precipitate with bromine water. 



3. They produce on leather a " bloom " consisting of 



ellagic acid. 



(B) Pyrocatechol tannins, including all the pine barks, 

 acacias, mimosas, oak barks (but not oak wood, fruits or galls), 

 quebracho wood, cassia and mangrove barks, canaigre, cutch 

 and gambier. 



The tannins of this class are characterized by the following 

 properties : 



1. They give with iron alum a greenish-black colour, 

 though the reaction is liable to be rendered uncertain by the 

 presence of other colouring matters. 



2. When treated with bromine water, until the solution 

 smells strongly of it, they give a yellowish or brown precipi- 

 tate ; in weak solutions the precipitate may form slowly. 



3. The addition of concentrated sulphuric acid to a drop 

 of the infusion produces a dark red or crimson ring at the 

 junction of the two liquids ; on dilution the liquid turns pink. 



4. These tannins deposit no "bloom," but when boiled 

 with acids deposit red insoluble colouring matters known as 

 phlobaphenes (see p. 2 1 5*). 



Some of the tannins belonging to this group, notably 

 gambier and cutch, contain phloroglucinol as one of their 

 constituents ; this substance may be tested for by moistening 

 a pine wood shaving with a little of the infusion and then 

 adding a little concentrated hydrochloric acid ; the formation, 

 after a short time, of a bright red or purple stain indicates the 

 presence of phloroglucinol. 



TANNINS AS GLUCOSIDES. 



Although many of the tannins are substances of a gluco- 

 sidic nature and occur in the plant in combination with a 

 carbohydrate complex such as glucose (e.g. gallotannic acid, 

 p. 213) this has not as yet been established in all cases. 



To determine whether a tannin is a glucoside or not the 

 following procedure is recommended by Proctor.* 



* Procter : " Leather Industries Laboratory Book, London," 2nd ed., 1908. 



