THE PROPERTIES OF TANNIC ACID. 91 
Most metallic salts form precipitates with tannic 
acid : tin salts produce a light-colored precipitate; lead 
salts, a white precipitate, which darkens on exposure to 
air; copper salts give a dark-brown precipitate; and 
silver salts, brown precipitates. Tartar emetic causes 
no precipitate until ammonium chloride is added. 
Manganese acetate causes a white precipitate; uranium 
acetate, a crimson or dark-red color, changing to a brown 
precipitate; potassium dichromate gives a brown pre¬ 
cipitate; lime-water causes a white precipitate, turning 
blue. 
According to Berzelius, when boric acid is dissolved 
in a hot solution of tannic acid, there is formed a white 
jelly on cooling, which may be dried to a white powder. 
Tannic acid displaces carbon dioxide from carbonates 
and forms tannates; those of the alkalies rapidly turn 
dark when exposed to the air. Fehling’s solution is 
reduced by tannic acid, with formation of cuprous ox¬ 
ide ; in order, therefore, to determine the presence of 
glucose the acid must first be removed by lead acetate. 
A. H. Allen gives the following characteristic test for 
tannic acid : a dilute ammoniaeal solution of potassium 
ferricyanide, which causes a deep-red color changing to 
brown. The reaction is very delicate, and the coloi 
is destroyed by an excess of the reagent. A similar 
reaction is caused by gallic acid. 
Ammonium molybdate gives with tannic acid a rod- 
dish-yellow color, which is destroyed by oxalic acid. 
A solution of iodine in potassium iodide, when mixed 
with a small quantity of ammonia which has been pre¬ 
viously diluted with ten times its volume of water, 
produces with'tannic acid a brilliant red color. 
