ON THE ESTIMATION OF TANNIC ACID. 
225 
•12025 grm. cupric taunate gave on ignition -0625 grm. CuO ; 
•12025 tannin ; *0625 CuO :; 1 ; -519. 
Percentages calculated with this number correspond exceedingly well with 
the amounts obtained by the gelatine process. 
The next step to be ascertained, and which is of primary importance to 
the success of the operation, is whether the cupric tannate precipitated from 
solutions of the various tanning materials possesses in all cases the same per¬ 
centage composition as that obtained from gall nuts. I have made a number 
of experiments to elucidate this point, and find that, although the analyses 
of the tannates show a slight variation inter se, such variation is fairly within 
the limits of error, and that all the tannins which I have examined, giving a 
blue precipitate with a persalt of iron, may fairly be reckoned as gallotannic 
acid. The so-called Mimosa bark (supposed by some to be the produce of 
a cinchonaceous tree) alone appears to vary in this respect, its tannin com¬ 
bining with 19 per cent, less copper than that of the other gallotannates. In¬ 
deed, it is somewhat doubtful under what head it should be ranked, as it 
gives neither a blue nor a green colour with a ferric salt, but a dark grey 
precipitate insoluble in solution of ammonic carbonate. 1 gramme of mimosa 
tannin consumes ’2959 grm. CuO ; it is necessary, therefore, to use this num¬ 
ber instead of '489 grm. when calculating percentages of this substance. 
I append a few analyses, which are sufficient to show how the numbers were 
obtained. The various tannates were prepared in a pure state by precipitat¬ 
ing an infusion of the substance by cupric acetate, adding excess of ammonic 
carbonate, and, after boiling, washing the precipitate on a filter with hot dis¬ 
tilled water, and drying at 100 u C- 
Valonia. T147 grm. copper salt gave on ignition *0372 grm. CuO ; 
0775 tannin : -0372 CuO ;; 1 : -480. 
Pomegranate Root-bark. T223 grm. copper salt gave ’0391 grm. CuO ; 
•0832 tannin .* ‘0391 CuO [ ’ 1 ; ‘471 
Sumach. T4936 grm. copper salt gave *04986 grm. CuO; 
•0995 : -04986:: l: -501. 
Divi-divi. T249 grm. copper salt gave "0408 grm. CuO ; 
•0841: -0408:: i: -485. 
Mimosa. - 28975 grm. copper salt gave .06525 grm. CuO ; 
•2245 : -06525 :: 1 : -2906. 
Mimosa. - 3175 grm. copper salt gave ‘0735 grm. CuO ; 
•244: -0735:: i: - 3012 . 
With the exception, then, of the mimosa tannin, which seems to be anoma¬ 
lous, we see that 1 gramme of tannin derived from any substance containing 
gallotannic acid combines with a quantity of cupric oxide weighing '489 
gramme. 
This brings me to a very curious and interesting point in connection with 
the process, viz. that a tannin giving a greenish precipitate with a ferric salt 
cannot in any case be analysed by the above method, simply because the pre¬ 
cipitated tannate is more or less soluble in solution of ammonic carbonate. 
The fact was first brought under notice by the abnormally small percentages 
obtained from certain materials, and upon trial it was found that the cupric 
tannates precipitated from tormentil, rhatany, larch bark, willow bark, catechu, 
kino, eucalyptus extract, hemlock Rabies) extract, etc., were completely so¬ 
luble in caustic ammonia, and very considerably so in solution of the car¬ 
bonate. All these substances contain mimotannic acid; on the contrary, the 
tannate from a body containing gallotannic acid is totally insoluble in either 
ammonia or its carbonate. I shall allude again to this peculiarity when 
speaking of the distinctions existing between these two varieties of tannin. 
