UKEA AND CHLOKIDES IN UKINE 205 



The alcohol and ether are then driven off at a temperature of 55 C., and 

 evaporation is continued at this temperature until the volume of the residue 

 is 10-15 c.c. The nitrogen in this is estimated by Kjeldahl's method. The 

 nitrogen found is multiplied by 2*143, and the result is the amount of urea. 



PREPARATION OF UREA FROM URINE 



(1) Evaporate the urine to a small bulk. Add strong pure nitric acid in 

 excess, keeping the mixture cool during the addition of the acid. Pour off 

 the excess of fluid from the crystals of urea nitrate which are formed ; strain 

 through muslin and press between filter paper. Add to the dry product 

 barium carbonate in large excess. This forms barium nitrate and sets the 

 urea free. Mix thoroughly with sufficient methylated spirit to form a paste. 

 Dry on a water-bath and extract with alcohol ; filter ; evaporate the filtrate 

 on a water-bath and set aside. The urea crystallises out, and may be de- 

 colourised by animal charcoal and purified by re crystallisation. 



(2) The following method is well adapted for the preparation of micro- 

 scopic specimens of urea and urea nitrate : Take 20 c.c. of urine ; add 

 baryta mixture (see footnote 3, p. 5) until no further precipitate is produced ; 

 filter, evaporate the filtrate to a thick syrup on the water-bath, and extract 

 with alcohol ; pour off and filter the alcoholic extract ; evaporate it to dry- 

 ness on the water-bath and take up the residue with water. Place a drop 

 of the aqueous solution on a slide and allow it to crystallise ; crystals of 

 urea separate out. Place another drop on another slide and add a drop of 

 nitric acid ; crystals of urea nitrate separate out. 



ESTIMATION OF CHLORIDES 



The chlorides in the urine consist of those of sodium and potassium, the 

 latter only in small quantities. The method adopted for the determination 

 of the total chlorides consists in their precipitation by a standard solution 

 of silver nitrate (Mohr's method). 



The following solutions must be prepared : 



Standard silver nitrate solution. Dissolve 29'075 grammes of fused 

 nitrate of silver in a litre (1,000 c.c.) of distilled water : 1 c.c. = 0'01 gramme 

 of sodium chloride. 



(a) Saturated solution of neutral potassium chromate. 



Analysis. Take 10 c.c. of urine ; dilute with 100 c.c. of distilled water. 



Add to this a few drops of the potassium chromate solution. 



Drop into this mixture from a burette the standard silver nitrate solu- 

 tion ; the chlorine combines with the silver to form silver chloride, a white 

 precipitate. When all the chlorides are so precipitated, silver chromate (red 

 in colour) goes down, but not while any chloride remains in solution. The 

 silver nitrate must therefore be added until the precipitate has a pink tinge. 



From the amount of standard solution used, the quantity of sodium 

 chloride in 10 c.c. of urine, and thence the percentage, may be calculated. 



Sources of Error and Corrections. A high-coloured urine may give rise 

 to difficulty in seeing the pink tinge of the silver chromate : this is overcome 

 by diluting the urine more than stated in the preceding paragraph. 



One c.c. should always be subtracted from the total number of c.c. of the 



