284 TANNINS 



2. Ferric chloride produces a blue-black colour or pre- 

 cipitate according to the strength of the solution ; excess 

 of ferric salt changes the colour to green, while excess of 

 gallic acid reduces the ferric salt to ferrous and destroys the 

 colour. 



3. Iodine solution produces a transient red colour. 



4. Gallic acid does not precipitate gelatine from solution. 

 (Distinction from tannic acid.) 



5. When heated with concentrated sulphuric acid it turns 

 green and then purple, being converted into rufigallic acid, 

 C14H8O8, a substance used in dyeing. 



6. Potassium cyanide gives a pink colour which disappears 

 on standing, but returns again on shaking with air. 



7. Lime water gives a blue coloration or precipitate ; in 

 very dilute solutions a reddish colour is produced. 



GALLOYL-GALLIC ACID OR DIGALLIC ACID. Ci^HioOg. 



OH 



OH ^"A ^" °^ 

 HO< ;COOH 



HO/^ \cO . O . / ^COOH 



HOC ^CO . O -^^ / 



OH OH OH 



w-Digallic acid ^-Digallic acid 



As may be seen from the above formulae two isomeric 

 digallic acids are possible and both have been synthesized 

 by Fischer, Bergmann, and Lipschitz * in the course of their 

 researches on the synthesis of depsides. 



A long time previous to this it had been known that 



gallic acid could be converted into its anhydride digallic acid 



by heating with phosphorus oxychloride to 130° or by boiling 



with arsenic acid : — 



2C6H2(OH)3COOH = Ci4Hi„09 + H^O 

 Gallic acid Digallic acid 



This digallic acid precipitates gelatine from solution, and 

 for this reason it was regarded by Schiff f as being identical 

 with natural gallotannic acid. This view was first shown by 



* Fischer, Bergmann, and Lipschitz: " Ber. deut. chem. Gesells.," 1918, 



5». 45. 



t Schiff : id.. 1871, 4, 232, 967 ; 1879, 12, 33 ; " Annalen." 1873. 170, 



143- 



