OUANIN. 385 



It should be noted here that in the decomposition of the mixed 

 silver compounds with hydrogen sulphid or ammonium sulphid 

 (Schindler) the guanin often only in part passes into solution with 

 adenin and hypoxanthin, and the remainder is held back in the silver 

 sulphid precipitate. The latter should, therefore, be boiled with 

 dilute hydrochloric acid, and on saturating the filtrate with ammonia 

 the guanin after a while separates out. That portion of the guanin 

 which does pass into solution with the other two bases is separated 

 from them by digestion with ammonia on a water-bath. The two 

 portions are then combined, tranferred to a filter, previously dried 

 at 110°, and weighed, washed well with ammonia, then dried and 

 weighed. 



Owing to the slight solubility of guanin picrate it has been pro- 

 posed by Wulff as a means for the estimation of guanin. For this 

 purpose the neutral or acid guanin solution is precipitated while 

 warm, with a sufficient amount of cold saturated picric acid solution. 

 After standing twenty-four hours the solution is filtered through a 

 hardened filter, and the precipitate well drained. It is then washed 

 with one per cent, picric acid and allowed to drain, after which it is 

 placed between two watch-glasses, and dried by slowly raising the 

 temperature, finally for one and a half hours at 110°. A deduction 

 is made for the free picric acid by determining the amount of water 

 in the precipitate from the difiference in the weights before and after 

 drying. Allowance should be made for one molecule of water of 

 crystallization that is driven off. A further correction for the solu- 

 bility of the guanin salt should be made by adding 0.0035 for each 

 100 c.c. of the combined filtrate and wash-water. The results thus 

 obtained are quite satisfactory. Bruhns has employed the picrate of 

 adenin in the estimation of adenin. Xanthin and hypoxanthin were 

 supposed to yield soluble compounds with picric acid, so that either 

 adenin or guanin, or both, could be separated from these bases in this 

 way. Wulff, however, has shown that when guanin is precipitated 

 by picric acid in the presence of hypoxanthin some of the latter is 

 also precipitated, so that it is not possible to separate the two bases 

 in this way. The same is true of adenin and hypoxanthin if the 

 picrate is not filtered off until after some hours. 



In the separation of adenin and hypoxanthin from guanin by 

 heating with ammonium hydrate, some guanin is dissolved, so that 

 the filtrate cannot be used with accuracy for the separation of adenin 

 from hypoxanthin by Bruhns' method. Wulff has endeavored to 

 replace the ammonia with metaphosphoric acid. Guanin is precipi- 

 tated from feebly acid solutions by metaphosphoric acid almost com- 

 pletely. The precipitate filtered off, washed with cold water, dried 

 at 110°, and weighed as C^HjNp.HPOg + JHjO, gives usually 

 slightly low results, owing to the difficulty of washing the precipi- 

 tate, and the fact that the amount of water retained in drying varies 

 25 



