URINE 535 



heat, and excess of the sulphide is completely expelled by renewed boiling. Fil- 

 tration under suction, and thorough washing of flask and filter result in a filtrate 

 which is perfectly clear and nearly colorless. This is concentrated if necessary, 

 and made up to a convenient volume which must of course be sufficiently large to 

 retain, when cool, the uric acid hi solution. Of this an aliquot part is utilized 

 directly for the cqlorimetric determination of uric acid. In the remainder the 

 residual uric acid is destroyed and bases determined according to the regular 

 Kriiger-Schmidt procedure. This modification is recommended particularly 

 where the amount of uric acid present is minute. 



3. Welker's Modification of the Methods of Arastein and of Salkowski. 2 

 Principle. The phosphates are removed by treatment with magnesia mixture. 

 The purine bases and uric acid are then thrown down as their silver salts and the 

 nitrogen content of this precipitate determined. 



Procedure. Four hundred c.c. of urine, free from protein, are treated with 

 100 c.c. of magnesia mixture and 600 c.c. of water. This is then filtered and of the 

 clear nitrate a measured quantity (600-800 c.c.) is treated with an excess (10 c.c.) 

 of a 3 per cent silver nitrate solution. Concentrated ammonium hydroxide is 

 added in small quantities, with stirring, until all the chlorides have dissolved. 

 Allow the flocculent precipitate of the silver purine compounds to settle to the bot- 

 tom, then pass the supernatant liquid through the filter before disturbing the 

 precipitate. Finally transfer the precipitate quantitatively to the paper which 

 must be of known nitrogen content. The precipitate is washed with dilute (i per 

 cent) ammonium hydroxide. The paper with the precipitate is then transferred 

 to a Kjeldahl flask and about TOO c.c. of water and a small quantity (about o.i 

 gram) of magnesium oxide are added. The water is then boiled until all the am- 

 monia has been driven off. Test the steam with litmus paper. 



The material in the flask is then digested by means of the usual Kjeldahl method 

 (see page 504). The digestion must be watched carefully at the time the sulphuric 

 acid reaches sufficient concentration to affect the filter paper, inasmuch as the SOj 

 produced causes considerable frothing. The total nitrogen (purine base, uric acid 

 and filter-paper nitrogen) is now determined in the usual way (see Kjeldahl Method, 

 page 504). This result minus the uric acid and filter-paper nitrogen will give the 

 figure for the purine-base nitrogen. 



Allantoin 



i. Method of Wiechowski-Handovsky. 2 Principle. The urine is precipitated 

 with phosphotungstic acid and lead acetate and in the presence of chlorides with 

 silver acetate. The heavy metals are removed with hydrogen sulphide. The allan- 

 toin is then precipitated as a mecuric compound and the amount of mercury and 

 hence of allantoin in the precipitate determined by titration with ammonium 

 thiocyanate. This method, though rather tedious, is probably the most accurate 

 method for the determination of allantoin. 



Procedure. The urine is diluted to about i per cent urea. As rabbit urine 

 contains in the day's output about 2-4 grams of urea, and that of other herbivora 

 usually forms about a 4 per cent urea solution, it is usually desirable to dilute 3-4 



1 Dittman and Welker: New York Med. Jour., May- June, 1909. 

 s Handovsky: Zeit. physiol. Chem., go, 211, 1914. 

 Wiechowski: Neubauer-Huppert: Analyse des Harns, Wiesbaden, 1913, p. 1076. 



