THE PROPERTIES OF TANNIC ACID. 
93 
of attention. The compounds Fe(C u H 8 0 9 )(C 14 H 9 0 9 ) 
and Fe(C u H 9 0 9 ) 3 + Fe0(C 14 H 9 0 9 ) have been described. 
All the tannates of iron are unstable, and we know 
them best in solution as ink. A compound of tannic 
acid with aluminium has been prepared by adding alu¬ 
minium hydrate to tannic acid solution ; the compound 
is soluble in excess of the acid. Zinc tannate is formed 
bv precipitating an alkali tannate with zinc sulphate, 
or by adding the acid to ammoniacal zinc sulphate. 
Copper tannate, Cu 3 C u H 4 0 9 , is formed by precipitating 
the acid with copper acetate. When alkali is added 
the precipitate redissolves, and reduction to cuprous 
oxide takes place. Wackenroder is authority for the 
statement that, when the copper tannate which has 
been prepared by an excess of the acid is dissolved in 
ammonium hydrate, the copper is not precipitated 
therefrom on the addition of hydrogen sulphide. 
Compounds of tannic acid with nearly every known 
metal are described in many of the text-books, but 
many of these salts were made by the older chemists 
before the acid had been prepared pure; consequently, 
beyond the fact that we get insoluble compounds of a 
certain color, there is very little accurate knowledge 
concerning them. 
