402 EXCRETION 



until the carbon dioxide ceases to come off. Filter, evaporate over a water 

 bath to dryness, and dissolve the urea in 95 per cent alcohol; decant, and re- 

 crystallize by evaporating off the alcohol. 



11. Urea Determination by Doremus' Ureometer. Fill the ureo- 

 meter with hypobromite of sodium solution. Take a sample of urine in 

 the pipet which accompanies the instrument, drawing it in exactly to the 

 mark. Insert the pipet past the bend of the ureometer and slowly empty 

 the urine carefully so as not to lose any of the liberated nitrogen. The instru- 

 ment is graduated to read off the percentage of urea directly. 



12. Uric Acid. Concentrate over a water bath 500 c.c. of urine 

 to 100 c.c. and boil with 10 c.c. or more of strong hydrochloric acid. Upon 

 cooling, crystals of uric acid are formed. Decant the supernatant liquid and 

 wash the crystals with a few cubic centimeters of 10 per cent hydrochloric 

 acid. . Dissolve the crystals and test. 



The Murexide Test. Add to 2 c.c. of uric acid solution in a test tube an 

 equal quantity of nitric acid. Heat gently, a reddish ring forms at the point 

 of contact between the nitric acid and uric acid solution. Cool and add 

 ammonia carefully. The color ring deepens to a purple color. This test 

 succeeds well by evaporating a few drops of uric acid on a porcelain plate. 

 Add to the stain a drop of concentrated nitric acid and evaporate. Concen- 

 tric rings of reddish color will be formed. This color deepens to reddish- 

 purple when a drop of ammonia is added. 



13. Creatinin. Test 20 c.c. of urine in a beaker for creatinin by 

 adding a cubic centimeter of dilute solution of sodium nitroprusside and then 

 weak sodium hydrate. A ruby-red color, which quickly turns yellow, indi- 

 cates the presence of creatinin. (Weyl's reaction.) If the yellow solution 

 has an excess of acetic acid added and is then boiled, it turns first green and 

 later blue, forming ultimately a precipitate of Prussian blue. 



Urine mixed with picric acid gives a red coloration when made alkaline 

 with caustic alkali solution. 



14. Total Nitrogen in Urine. Determine the total nitrogen in 

 a sample of urine by the Kjeldahl method. This method depends upon the 

 conversion of nil the nitrogen to ammonia, the distillation of this ammonia 

 into a known quantity of sulphuric acid, and the final titration of the excess 

 of sulphuric acid when the distillation is complete. The computation is 

 made on the basis that i c.c. of a normal sulphuric acid is equivalent to 

 i c.c. normal sodium hydrate, and that in turn to i c.c. of ammonium 

 hydrate. The ammonia neutralizes a portion of the sulphuric acid in the 

 distillation. One c.c. of normal ammonium hydrate contains 0.014 gram 

 nitrogen, from which the total nitrogen in the sample used can be readily 

 computed. 



15. Pigments of Urine. The normal color of the urine is due to 

 the presence of a pigment, urobilin. Prepare urobilin by adding lead acetate 



