PRACTICAL EXERCISES 523 



talc may be put in to prevent bumping. Instead of methyl orange, 

 ' alizarin red,' which is bright red in the presence of the slightest trace 

 of alkali, may be used. 



10. Uric Acid (i) Qualitative Test for Uric Acid Murexide Test. 

 A small quantity of uric acid or one of its salts is heated with a little 

 dilute nitric acid. The colour of the residue left by evaporation 

 becomes yellow, and then red, and on the addition of ammonia changes 

 to deep purple-red. Potassium or sodium hydroxide changes the 

 yellow to violet. In the reaction alloxantin is formed by oxidation of 

 the uric acid. When ammonia acts on alloxantin it is changed into 

 purpuric acid, and this into its ammonium purpurate, the purple-red 

 substance called murexide. Thus: 



C 8 H 6 N 4 8 + NH 3 =C 8 H 5 N 6 ? + 2H a O. 



Alloxantin. Purpuric Acid. 



The reaction is also given by theobromine (dimethylxanthin), an alkaloid 

 in cocoa, and theine or caffeine (trimethylxanthin), an alkaloid in tea 

 and coffee, which are also purin derivatives (p. 48 1')- 



(2) Quantitative Estimation Folin's Modification of Hopkins' s 

 Method. The chief reagent is a solution of 500 grammes ammonium 

 sulphate, 5 grammes uranium acetate, and 60 c.c. 10 per cent, acetic 

 acid, in 650 c.c. of water. 



One hundred and fifty c.c. of urine is measured into a tall, narrow 

 beaker or a cylinder, and 37^ c.c. of the reagent added. If enough 

 urine is available, 200 c.c. of urine and 50 c.c. of reagent are to be used. 

 Allow the mixture to stand without stirring for about half an hour. 

 The uranium precipitate has then settled, and the clear supernatant 

 liquid is removed by siphoning or decantation. One hundred and 

 twenty-five c.c. of this liquid is measured into another beaker, 5 c.c. 

 of strong ammonia added, and the mixture set aside till next day. The 

 precipitate is then filtered off, and washed with 10 per cent, ammonium 

 sulphate solution until the filtrate is quite or nearly free from chlorides. 

 The filter is then removed from the funnel, opened, and the precipitate 

 rinsed back into the beaker. Enough water to make about 100 c.c. is 

 added, and the precipitate is then dissolved by means of 15 c.c. con- 

 centrated sulphuric acid, and at once titrated with ^ 6 (one -twentieth 

 normal) potassium permanganate solution (made by dissolving 

 1-581 grammes of the permanganate in a litre of water), each c.c. of 

 which corresponds to 3-75 milligrammes of uric acid. The very first 

 pink coloration, extending through the entire liquid on the addi- 

 tion of two drops of permanganate solution, marks the end point. 

 A correction of 3 milligrammes, owing to the solubility of ammonium 

 urate, is added to the result. 



11. Creatinin. Qualitatively, creatinin may be recognized in very 

 small amounts by Weyl's test. A few drops of a dilute solution of 

 sodium nitro-prusside are added to urine, and then dilute sodium 

 hydroxide drop by drop. A ruby-red colour appears, which soon turns 

 yellow. If the urine is now strongly acidified with acetic acid and 

 heated, it becomes first greenish and then blue. Enough acid must be 

 added to more than neutralize the alkali. 



Another test which has been made the basis of a quantitative method 

 by Folin is Jaffe's test. A little urine (say 5 c.c.) is put in a test-tube, 

 and then a solution of picric acid in water. The mixture is rendered 

 alkaline by the addition of potassium or sodium hydroxide solution, 

 and a reddish colour is produced, which turns yellow on the addition of 



