GENERAL METHODS FOR ISOLATING BASES 119 



longer give a precipitate with baryta, but precipitate both with sul- 

 phuric acid and with sodium carbonate solutions, enough baryta has 

 been added to liberate the bases. The barium phosphotungstate is 

 then filtered off on the pump and washed out thoroughly with hot 

 water until the washings no longer give a precipitate with a phos- 

 photungstic-sulphuric acid solution. The excess of barium is at once 

 removed from the filtrate and washings by passing carbon dioxide 

 through them ; on filtration and evaporation the organic bases are 

 obtained either in the free state or as carbonates. 



Should it be necessary to remove the excess of phosphotungstic 

 acid from the filtrate, after precipitation of bases as phosphotungstates, 

 this can be done either by precipitation with excess of baryta, or, ac- 

 cording to Jacobs [1912], by extracting the acid solution with amyl- 

 alcohol, which may be conveniently mixed with up to four parts of 

 ether. This method may also be used for decomposing the phos- 

 photungstates of bases if they are soluble in hot water. 



Mercuric chloride is next in importance to phosphotungstic acid 

 as a precipitant of bases. It is not so universal a precipitant and 

 is most frequently used after phosphotungstic acid to separate the re- 

 covered bases into several fractions. With suitable precautions mer- 

 curic chloride may, however, often replace phosphotungstic acid 

 altogether. It was first used extensively by Brieger for isolating 

 putrefaction bases, before phosphotungstic acid had come into general 

 use. 



Mercuric chloride is generally used in saturated alcoholic solution 

 which is added to an alcoholic or sometimes to an aqueous solution 

 of the bases to be precipitated. Some bases are precipitated from 

 neutral solution, but others only after the solution has been made 

 slightly alkaline. In aqueous solution sodium carbonate is used, in 

 alcoholic solution fused sodium acetate, dissolved in alcohol, is added, 

 or the solution is saturated with powdered sodium acetate. If such a 

 solution is afterwards also saturated with powdered mercuric chloride, 

 very few bases escape precipitation. Generally the mercuric chlorides 

 are much more soluble in hot water than in alcohol ; Brieger extracted 

 the precipitate formed in alcoholic solution with boiling water, when 

 the mercuric chloride compounds of peptones remained undissolved. 

 On filtration and cooling choline mercurichloride crystallised out. 

 Another exampje of the use of mercuric chloride is the preparation of 

 histidine from blood, by Frankel's method. After the blood (or haemo- 

 globin) has been hydrolysed by boiling with concentrated hydrochloric 

 acid, .'most of the acid is distilled off and the residue, after being nearly 



