GENERAL METHODS FOR ISOLATING BASES 117 



containing proteins and bases is evaporated to a thin syrup, and this 

 is mixed with sand and then ground up under acetone. Dry acetone 

 does not dissolve the salts of most organic bases, but enough water 

 remains behind in the aqueous extract to prevent precipitation of the 

 salts by acetone. 



The preliminary purification of a tissue extract after removal of 

 coagulable protein is, however, best effected by means of lead acetate 

 or by tannin. In the former case the solution is first treated with 

 normal lead acetate and then with the basic salt ; the joint precipitate 

 of these reagents is then filtered off and the excess of lead is removed 

 from the filtrate as sulphide, sulphate, or phosphate. The tannin 

 method has been largely employed by Kutscher and his pupils ; it 

 completely removes peptones and proteoses, but bases are also carried 

 down by the bulky precipitate ; according to Krimberg the yield of 

 bases from meat extracts is much smaller after purification with tannin 

 than with lead acetate. Many bases form tannates insoluble in neutral 

 solution, so that the reaction before precipitation should be made 

 distinctly acid by adding phosphoric acid, if necessary. A 20 per 

 cent, aqueous tannic acid solution is then added until no further pre- 

 cipitation occurs ; at this stage the precipitate ceases to be milky and 

 flocculates ; a considerable excess of tannic acid must be avoided since 

 it redissolves the precipitate (it is a case of the mutual precipitation of 

 two colloids). On standing overnight the bulky precipitate shrinks to 

 the consistency of pitch and the clear supernatant solution can easily be 

 poured off. In order to remove the excess of tannin, a warm saturated 

 baryta solution is added until, after stirring, the surface of the liquid 

 shows a reddish or purple colour. The barium tannate is filtered off 

 at the pump, the filtrate is acidified with sulphuric acid, and without 

 removing the barium sulphate formed, freshly prepared lead hydroxide, 

 suspended in distilled water, is stirred in. This removes the last 

 traces of tannin and the excess of sulphuric acid, and now, after 

 filtration, the solution should contain at most only traces of lead and 

 should be alkaline to litmus. 



The last operations illustrate the general principle that as far as 

 possible no ions should be introduced into the solution which cannot 

 afterwards be removed, for the separation of bases from inorganic salts 

 is often difficult. 



Kossel and Weiss [1910] use a solution containing 70 grm. of 

 tannic acid, 100 grm. of sodium chloride and 50 c.c. of glacial acetic 

 acid per litre for the precipitation of peptones. 



The solution of bases which has been purified by one or other of 



