120 THE SIMPLER NATURAL BASES 



neutralised with sodium hydroxide, is filtered. The filtrate is then 

 made alkaline with sodium carbonate and the histidine is precipitated 

 by adding alcoholic mercuric chloride solution. Engeland has worked 

 out a method for separating the bases of meat extract in which all the 

 bases are first precipitated by the alternate addition of cold saturated 

 solutions of mercuric chloride and of sodium acetate. The precipitate 

 dissolves for the most part in hot water acidulated with hydrochloric 

 acid and is freed from mercury by means of hydrogen sulphide. After 

 evaporation of the aqueous filtrate the residue is dissolved in alcohol 

 and alcoholic mercuric chloride is added ; finally the solution is satur- 

 ated with the powdered salt. This precipitates neosine, carnitine and 

 vitiatine as mercurichlorides which are removed by filtration. Alcoholic 

 sodium acetate solution is now added and precipitates the mercury 

 salts of histidine, methyl guanidine and /3-alanine. Cf. also p. 114. 



Silver nitrate is principally used to precipitate bases containing an 

 imino-group and is of great value for their separation. As in the case 

 of mercuric chloride, the degree of acidity or alkalinity of the solution 

 is the determining factor. In the presence of (nitric) acid only purine 

 bases are precipitated as insoluble silver compounds ; in a slightly 

 alkaline solution, i.e. after the addition of a limited quantity of baryta, 

 the silver compounds of histidine and allied bases are thrown down ; 

 excess of baryta then precipitates the silver compound of arginine. 

 The separation of arginine and histidine in this manner may be 

 rendered quantitative and if silver sulphate is used instead of the 

 nitrate, the process affords a means of estimation by determination of 

 the nitrogen in the various fractions (see Plimmer's " Chemical Constitu- 

 tion of the Proteins," Part I, pp. 35-8). The practical details in the 

 application of silver nitrate may be illustrated by a description of 

 Kutscher's method for the isolation of bases from meat-extract. After 

 purification by means of tannin, as described above, and concentration 

 to a small volume, creatine and some creatinine crystallise out. Then, 

 after filtration, the solution is acidified with sulphuric acid and the 

 resulting precipitate of lead sulphate is filtered off. Now a 20 per cent, 

 silver nitrate solution is added to the filtrate and this causes the pre- 

 cipitation of the purine bases (as compounds with silver nitrate), together 

 with a little silver chloride. After standing for some time this pre- 

 cipitate is filtered off and enough silver nitrate is added to the solution 

 to enable the whole of the bases capable of forming silver compounds 

 to be precipitated as such by subsequent addition of baryta. Enough 

 silver nitrate has been added for this purpose when a drop of the 

 solution, mixed on a watch glass with cold saturated baryta water, 



