GENERAL METHODS FOR ISOLATING BASES 123 



purine bases) can be readily crystallised from water and are particu- 

 larly little soluble in dilute nitric acid. 



Much more frequently picrates are prepared. The picric acid is 

 added in aqueous and also in alcoholic solution ; the precipitated 

 picrate is recrystallised from water, from dilute or from strong alcohol. 

 Often, on cooling a hot solution, it separates first in oily drops which 

 only become definitely crystalline on standing. Ammonium salts, 

 when present, may sometimes lead to confusion owing to the forma- 

 tion of ammonium picrate, which is not very soluble in water and 

 forms long thin pale yellow needles ; these have no proper melting 

 point, but decompose suddenly on heating. When a base is insol- 

 uble in ether (as is the case with most of the simpler natural bases) it 

 can be readily recovered from its picrate by dissolving the latter in 

 hot dilute hydrochloric acid and, after cooling, extracting the picric 

 acid with ether or with benzene. On the large scale most of the 

 picric acid generally separates and can be filtered off. The estimation 

 of picric acid in picrates can be carried out very conveniently and 

 with enough accuracy by means of the "nitron" reagent of Busch 

 [1905]. This process has the further advantage over a combustion 

 that the base is recovered unchanged. 



Picrolonates are much less soluble than picrates and generally 

 crystallise well, but to some extent this advantage is neutralised by 

 the slight solubility in water of picrolonic acid itself. An alcoholic 

 solution of the acid is generally added to an aqueous solution of the 

 base. The precipitate is at first often amorphous, but readily crystal- 

 lises from hot water in some cases. The high molecular weight of 

 picrolonic acid renders the melting points and analyses of picrolonates 

 of less significance than those of picrates. 



Platinic chloride is used in concentrated aqueous or (more frequently) 

 alcoholic solution. The platinichlorides of the simplest bases are 

 often readily soluble in water, but not in alcohol, and may be crystal- 

 lised from dilute alcohol. 



Gold chloride is generally used in a 30 per cent, aqueous solution. 

 Aurichlorides sometimes partially decompose on recrystallisation, gold 

 being set free. In order to avoid this and obtain a gold salt of normal 

 composition, the salt should be recrystallised from - I per cent, 

 hydrochloric acid to which a little gold chloride has been added. 



In special cases zinc chloride or cadmium chloride are used for 

 forming double salts in alcoholic solution, or the base is isolated 

 as chromate^ perchlorate or metaphosphate. 



