428 PEACTICAL PHYSIOLOGY 



it be desired to determine the uric acid and the bases separately, a slight modifi- 

 cation of the process is necessary. 



Solutions necessary. 1. Magnesia mixture. This consists of 1 part crystallised 

 magnesium chloride, 2 parts chloride of ammonium, dissolved in 8 parts of water 

 and made strongly alkaline with 4 parts of ammonia. If the mixture be not quite 

 clear (from the presence of magnesium hydrate) more ammonium chloride should 

 be added. 



2. Ammoniacal silver nitrate. Dissolve 26 gr. silver nitrate in about 300 c.c. 

 water, add ammonia to this until the precipitate of silver oxide, which first forms, 

 redissolves. Dilute the solution to one litre. 



3. Kjeldahl's apparatus and solutions (see. p. 410). 



Determination. 240 c.c. protein free urine are mixed with 30 c.c. magnesia 

 mixture, and the solution is made up to 300 c.c. by the addition of a 20 per cent, 

 ammonia solution. This process is best done in a measuring cylinder. After the 

 precipitate has settled, which it does in a few minutes, it is filtered through a 

 dry folded filter and two portions of the filtrate are taken amounting to 125 c.c. 

 each. Each of these corresponds to 100 c.c. of the original urine. They are both 

 treated in exactly the same way, and should yield similar results. Each is mixed 

 with 10 c.c. ammoniacal silver nitrate, and the mixture, after the precipitate has 

 settled somewhat, filtered through an ash-free filter paper (of 10 c.m. diameter). 

 The last traces of the precipitate are removed from the beaker by means of weak 

 ammonia water. The next stage consists in washing the precipitate with distilled 

 water until it is free from ammonia, as the presence of this would vitiate the 

 determination of the nitrogen. In order to do this, the precipitate should be 

 allowed to stand exposed to the air over night so that it may become partially 

 dried, in which state the washing with water 'is much easier than when the 

 precipitate is moist, for then it forms a gummy mass. The washing must be 

 continued until the washings no longer react alkaline to litmus. In order to 

 remove the last traces of ammonia, the filter paper, with the precipitate on it, is 

 carefully removed to a Kjeldahl's combustion flask ; about 50 c.c. of water are 

 added, and then a little magnesium oxide. The mixture is then boiled, whereon 

 the magnesia expels the ammonia. The boiling is continued until only about 10 

 c.c. of fluid remain, and then sulphuric acid, etc., are added, and the nitrogen 

 determined. 



To Determine the Bases and Acid separately. Various methods are 



recommended. The simplest is probably to precipitate the bases by the Camerer 

 method in the filtrate from which uric acid has been removed, as ammonium urate> 

 as described under Folin's method. 



Hippuric Acid. In herbivorous animals a large amount of hippuric 

 acid is excreted, but in man on an ordinary diet and in the carnivora 

 only a small quantity. Hippuric acid may readily be obtained from 

 the urine of a herbivorous animal by the following procedure. The 

 urine is boiled for a few minutes with excess of milk of lime, filtered 

 hot, concentrated on the water-bath, cooled and acidified with hydro- 

 chloric acid. Crystals of hippuric acid separate on standing, which are 

 filtered off and dried. They may be freed from benzoic acid by extrac- 

 tion with petroleum ether, in which hippuric acid is insoluble, and 

 recrystallised from hot water, using animal charcoal to decolourise if 



