SECTION III. 
THE DETECTION AND ESTIMATION OF THE TANNINS. 
The simple detection of the tannins is comparatively 
easy. A few tests applied to an infusion of the plant 
or substance serve to decide their presence or absence. 
Ferric acetate or chloride is usually first employed, 
when a green or blue color and precipitate indicate 
tannin. Ferrous salts, when free from ferric, cause 
no change with tannin solutions unless the latter be 
concentrated, when a white gelatinous precipitate is 
formed, rapidly becoming dark on exposure to air. 
Potassium dichromate forms brown precipitates with 
nearly all tannins. Ammoniacal copper sulphate forms 
precipitates of different shades of color; but, according 
to Procter, the tannin of Hungarian larch is not pre¬ 
cipitated by this reagent, and it does form insoluble 
compounds with some substances other than the tan¬ 
nins. Allen recommends an ammoniacal solution of 
potassium ferricyanide, which produces with the tan¬ 
nins a deep-red color changing to brown, even in very 
dilute solutions. Most of the alkaloids, when not in 
too dilute solution, produce whitish precipitates with 
the tannins. Calcium hydrate produces a whitish pre¬ 
cipitate rapidly turning blue with gallotannic, but red 
with some other varieties, notably that from mangrove. 
An aqueous solution of iodine in potassium iodide, 
mixed with a small quantity of ammonia previously 
31 
