THE ORGANIC CONSTITUENTS OF THE URINE. 253 



As the isolation of the individual substances from the urine in 

 amounts sufficient for purposes of study is a very complicated process, 

 and requires facilities which are not generally found in university 

 laboratories, it will suffice at this place to describe a method by 

 which they can be collectively estimated. For a consideration of 

 the chemical properties of the more important members of the group 

 and their isolation from other sources, as also of their relation to 

 uric acid and the nucleinic bases, the reader is referred to other 

 sections. 



Quantitative Estimation. The xanthin bases are best isolated 

 from the urine according to Salkowski's method, which is based 

 upon their precipitation as silver compounds, the separation of the 

 latter, and the determination of the amount of silver in combination 

 with the bases. 



600 c.c. of urine are precipitated with 200 c.c. of magnesia 

 mixture, 1 when a 3 per cent, ammoniacal solution of silver nitrate 

 is added to from 700 to 750 c.c. of the nitrate. The propor- 

 tion should be 6 c.c. for each 100 c.c. of urine. After stand- 

 ing for one hour the mixture is filtered and the precipitate 

 washed with water until all the free silver has been removed. 

 The filter is then perforated, the precipitate washed into a flask 

 with from 600 to 800 c.c. of water, acidified with hydrochloric 

 acid, and decomposed with hydrogen sulphide. The excess of hy- 

 drogen sulphide is removed by heating on a water-bath, when the 

 silver sulphide is filtered off and the filtrate evaporated to dryness. 

 The residue is treated with from 25 to 30 c.c. of dilute sulphuric 

 acid (1 : 100). This solution is brought to the boiling-point and is 

 allowed to stand over night. The uric acid which has then sepa- 

 rated out is filtered off, washed with a small amount of dilute sul- 

 phuric acid (not more than 50 c.c.), then with alcohol and ether, and 

 weighed. To the resulting weight 0.0005 gramme is added for 

 each 10 c.c. of the acid filtrate, to allow for the trace of uric acid 

 which is thus lost. 



After having filtered off the uric acid the filtrate is again treated 

 with ammonia and silver solution, and the xanthin bases thus pre- 

 cipitated. The precipitate is collected on a small filter, washed 

 with water, dried, and incinerated. The ash is dissolved in nitric 

 acid, and the silver estimated by titration with a solution of 

 potassium sulphocyanide, using ammonio-ferric alum as an indi- 

 cator. The solution of potassium sulphocyanide employed in the 

 estimation of the chlorides may be used, and is of such strength 

 that 1 c.c. corresponds to 0.00734 gramme of silver. As 1 atom 

 of silver in a mixture of the silver compounds of guanin, xanthin, 

 hypoxanthin, etc., represents 0.277 gramme of nitrogen or 0.7381 

 gramme of the alloxur bases, it is apparent that 1 c.c. of the potas- 

 sium sulphocyanide solution will represent 0.002 gramme of nitro- 



1 1 part of crystallized MgS0 4) 2 parts of NILC1, 4 parts of NH 4 OH, and 8 parts of dis- 

 tilled water. 



