198 TANNINS 



n. A brilliant red colour, even when the tannins are in 

 a very dilute solution, results from the addition of an aqueous 

 solution of iodine in potassium iodide mixed with a little 

 10 per cent ammonia. 



12. According to Moore,* the action of Nessler's solution 

 (a saturated solution of mercuric iodide in a solution of 

 potassium iodide and potash) varies. 



(a) A brown precipitate is formed immediately, e.g. epi- 



dermis . of primrose leaf. 



(b) A yellow colour is produced which turns reddish 



brown ; finally a brown precipitate comes down, 

 e.g. stem of Yew and Aucuba japonica. 



(c) A yellow coloration. The compound produced readily 



diffuses through the cell wall, e.g. young stem and 

 hairs of the ivy. 

 The following are microchemical tests for gallic acid : 



1. The rapidity of the reaction with potassium chromate 

 may provide a means of distinguishing gallic acid from tannic 

 acid, for in the case of the former a precipitate immediately 

 comes down, whilst in the case of tannic acid, according to 

 Drabble and Nierenstein, the reaction is either very slow or 

 entirely negative. 



2. Potassium cyanide in aqueous solution gives a pink 

 coloration with gallic acid. 



3. With Nessler's solution gallic acid gives a grey-green 

 precipitate. 



With this same reagent pyrogallol immediately yields a 

 brown precipitate ; pyrocatechol forms a deep green precipitate 

 which changes to greenish brown ; and a dirty green precipi- 

 tate is given by protocatechuic acid. 



Vinsonf recommends exposing the material to be exa- 

 mined to the vapour of nitrous ethers in order to fix and 

 stain the tannin in vegetable tissues. There is, ordinarily 

 no necessity to cut up the tissue, and as the tannin is deposit 

 in the cells in which it occurs, the method is very convenien 

 for tracing the distribution of the substances in question. The 

 resulting colour varies ; thus, the juice of unripe grapes gives 

 a dense brown precipitate ; the juice of the persimmon a wine- 



nd 



ly a 

 * \ 



; nt 



* Moore: "Journ. Linn. Soc., Lond., Bot.," 1891, 27, 527. 

 tVinson: "Bot. Gaz.," 1910, 49, 222. 





