PRACTICAL EXERCISES 485 



acid. One milligramme must be added, as before, to the result for 

 each 15 c.c. of the mother-liquor from which the uric acid crystals 

 were deposited. 



(b) Folin's Modification of Hopkins 1 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. concentrated Sulphuric acid, 

 and at once titrated with ^ (one-twentieth normal) potassium 

 permanganate solution, each c.c. of which corresponds to 3*75 milli- 

 grammes of uric acid. The very first pink coloration, extending 

 through the entire liquid through the addition of two drops of 

 permanganate solution, marks the end point. A correction of 3 milli- 

 grammes, owing to the solubility of ammonium urate, is added to 

 the result. 



10. Kreatinin. Qualitatively, kreatinin may be recognised 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. 



Neubauer has made the reaction of kreatinin with zinc chloride 

 the basis of a method for its quantitative estimation (Fig. 167, p. 442), 

 but the results seem to be somewhat uncertain. A more convenient 

 method is that of Folin. It depends upon the comparison of the 

 colour which kreatinin gives with picric acid in an alkaline solution 

 with that of a standard solution of potassium bichromate. Ten c.c. 

 of urine is measured into a 500 c.c. measuring - flask ; 15 c.c. of 

 a saturated picric acid solution (containing about 12 grammes 

 per litre), and 5 c.c. of a 10 per cent, solution of sodium hydroxide 

 are added. The mixture is allowed to stand for five minutes. 

 Then water is added up to the 500 c.c. mark, and' the flask shaken 

 to mix uniformly. Samples of the liquid are then at once com- 

 pared colorimetrically with a half - normal solution of potassium 

 bichromate containing 24" 55 grammes per litre. The colour of 

 the urine does not introduce a sensible error on account of the 

 great dilution. For exact work the comparison must be made 

 with a good colorimeter. It has been found experimentally that, 

 when 10 milligrammes of kreatinin are present in 500 c.c. of a 

 solution made as described, a layer of the solution 8 g i millimetres 

 in thickness has the $ame depth of tint as 8 millimetres of the 

 bichromate solution. Suppose it takes 9 millimetres of the 



