478 QUALITATIVE AND QUANTITATIVE ESTIMATION OF UREA. 



is evaporated to one-sixth of its original volume, is reduced to a temperature 

 of o and an excess of strong, pure nitric acid is added. Urea nitrate contaminated 

 with coloring-matter is precipitated. The precipitate is filtered, expressed, dis- 

 solved in a little boiling-water, mixed with animal charcoal for the removal of 

 the coloring-matter, and filtered hot. On cooling, decolorized crystals of urea 

 nitrate separate from the filtrate. These are again dissolved in hot water, and 

 barium carbonate is added so long as effervescence takes place. Barium nitrate 

 and free urea are thus formed. Evaporation to dryness is now practised, fol- 

 lowed by exhaustion with absolute alcohol, filtration and evaporation, after which 

 the urea separates in crystals. 



Combinations of Urea. Urea is capable of entering into combination with 

 acids, as nitric, oxalic or phosphoric, or with bases, or with salts, as sodium 

 chlorid, mercuric nitrate. The most important combinations are: 



1. Urea nitrate: CH 4 N 2 O.NO 3 H, whose mode of preparation from the urine 

 has just been described. The preparation of urea nitrate is employed with advant- 

 age for the microscopic demonstration of urea. If there are but a few drops 

 of watery fluid in which the presence of urea is suspected and this must be so 

 prepared that the urea present is in concentrated watery solution one drop of this 

 fluid is placed upon a glass slide, a thread is laid through the middle of the drop 

 and over both is placed a cover-slip. From the extremity of the thread a drop 

 of concentrated nitric acid is permitted to find its way beneath the cover-slip. 

 The characteristic crystals appear upon either side of the thread (Fig. 146, 3, 4, 

 5, 6). Urea nitrate is readily soluble in water, soluble with difficulty in water 

 acidulated with nitric acid. Less commonly, when crystallization takes place 

 slowly, it yields six-sided prisms. 



2. Mercuric-nitrate urea is obtained in the form of a white, cheesy precipitate, 

 when mercuric nitrate is introduced into a solution of urea. If, on the develop- 

 ment of the precipitate, the nitric acid set free is neutralized by sodium carbonate, 

 all of the urea eventually combines with the mercuric salt. When this point has 

 been reached, all excess of mercuric nitrate gives rise, on addition of sodium 

 carbonate, to the production of sodium nitrate and yellow basic mercuric carbo- 

 nate. The titration-method of J. v. Liebig for urea is based upon this reaction. 



QUALITATIVE AND QUANTITATIVE ESTIMATION OF UREA. 



The qualitative estimation of urea aims (i) at the preparation of this substance 

 directly as such. If its presence be suspected in an albuminous fluid mixed with 

 blood or pus, the following course is pursued: Three or four times its volume of 

 alcohol are added to the fluid, and filtration is practised after the lapse of several 

 hours. The filtrate is evaporated over the water-bath, and the residue is dis- 

 solved in a few drops of water. (2) This aqueous solution is employed for the 

 microchemic preparation of urea nitrate, which has important diagnostic signifi- 

 cance. (3) By means of a solution of sodium hypobromite, the urea in the fluid 

 submitted to examination is decomposed into carbon dioxid, water, and nitrogen. 

 The nitrogen rises in the mixture in the form of small bubbles. The Knop- 

 Hiibner method of quantitative estimation is based upon this reaction. (4) A 

 crystal of urea is cautiously fused in a dry test-tube, and yields an odor of am- 

 monia. On cooling, it is dissolved in a small amount of water, and sodium hy- 

 drate, together with one drop of dilute copper sulphate, is added, with the develop- 

 ment of a red-color biuret-reaction. 



Quantitative estimation of urea in the urine, according to the method of 

 Morner and Sjoqvist: 



To 2.5 cu. cm. of urine are added 2.5 cu. cm. of baryta-mixture (i volume 

 of a cold saturated solution of barium hydrate and 2 volumes of cold saturated 

 barium nitrate) and 75 cu. cm. of ether-alcohol (the alcohol must be 70 per cent.). 

 The mixture is preserved for a day sealed. It is now filtered, and the filtrate, 

 which contains of the nitrogenous substances only the urea, is evaporated at 

 a temperature of 55 C. after the addition of 0.5 gram of magnesium oxid. 

 After the addition of 10 cu. cm. of sulphuric acid it is further evaporated upon 

 a boiling water-bath, until no further reduction in volume takes place. Then 

 it is placed in a Kjeldahl boiling-flask, and the examination is continued according 

 to the Kjeldahl method. 



The method of Kjeldahl is employed for the estimation of the total amount 

 of nitrogen in the urine. It is based upon the fact that all of the nitrogen is 

 transformed into ammonia, and this is estimated quantitatively. Five cu. cm. 

 of urine of moderate concentration are measured by means of a pipet and intro- 



