90 
THE TANNINS . 
together with a dark color due in part to metagallic 
acid and in part, when commercial acid is used, to the 
action of the alkali on the glucose present. 
Ammonia acts differently from the fixed alkalies. 
When tannic acid is heated with aqueous ammonia in 
an atmosphere free from oxygen, which is best ac¬ 
complished by passing hydrogen into the mixture, there 
are formed gallamide and ammonium gallate, according 
to the following equation : 
c 14 h 10 o 9 + 2NH 3 =c 7 h 5 o 4 nh 2 + nh 4 c 7 h 5 o 5 - 
Fermentation causes the change of tannic into gallic 
acid, several decomposition products resulting at the 
same time. 
Ferric salts produce a blue-black color or precipitate 
with tannic acid, according to the concentration of the 
solutions. When the solution of iron salt is very di¬ 
lute, or when free acid, especially tartaric, is present, 
the color is greenish. 
Ferrous salts, when perfectly pure, added to tannic 
acid solution produce no change unless the solutions be 
concentrated, when a white gelatinous precipitate forms. 
The least trace of ferric salt is sufficient to change this 
to a dark-blue color or precipitate, according to the 
strength of the solution. Soluble vanadium salts also 
produce the blue-black color or precipitate with tannic 
acid. 
Tannic acid precipitates most alkaloids and bitter 
principles. It gives a light-colored flocculent precipi¬ 
tate with gelatin, which is rendered more insoluble by 
alum, ammonium chloride, or sodium chloride; solu¬ 
tions of starch and albumen are similarly precipitated. 
