COMPOSITION OF TANNIC ACID. 
97 
By heating cligallic acid as well as a purified natu¬ 
ral tannic acid with acetic anhydride, Schiff obtained 
pentacetyl-tannin, C 14 H 5 (C 2 H 3 0) 5 0 9 , consisting of warty 
crystals melting at 137°. This compound is insoluble 
in water and cold alcohol, but moderately soluble in 
hot absolute alcohol, and gives no color reaction with 
salts of iron. 
Two years later, in 1873, Schiff published a review 
of his former work, with the maturer conclusions which 
experience gave him. He offered the following as the 
reaction taking place between gallic acid and phos¬ 
phorus oxychloride : 
. 4C 7 H 6 O s + POCl 3 = 2C 14 H 10 O 9 + PH0 3 + 3HC1. 
The tannic acid thus obtained gave a blue-black precip¬ 
itate with ferric salts, yielded pyrogallic acid on heat¬ 
ing, and was converted into gallic acid by the action of 
dilute acids, without the formation of a trace of glucose. 
Phosphorus pentachloride and trichloride also con¬ 
verted gallic into tannic acid, but some intermediate 
and decomposition products resulted. 
Schiff also further investigated the action of arsenic 
acid on gallic acid, and determined that when they are 
heated together in aqueous solution the change into 
tannic acid is complete without the arsenic acid under¬ 
going any change. He considered the reaction to be as 
follows: 
(1) C 7 H 5 0 4 -OH + HAsO s = C 7 H 5 0 4 -0. As0 2 + h 2 o ; 
(2) C? A A} 0 +°>ha.oh =%1$} o+ asho 3 . 
Pure digallic acid was recommended to be prepared 
9 * 
