334 ANALYSIS OF URINE. [CH. XIII. 



with the appearance of the urine, more being required for deeply 

 pigmented urines. 



Titrate with standard soda, which may be o-i to 0-2 N., as 

 described in Ex. 322, until the colour as seen through Y is inter- 

 mediate between that seen through X and Z. During the course 

 of this titration, the standard soda is added to (2) and (6) and 

 distilled water to (i) and (5), as described in Ex. 322. Usually a 

 considerable precipitate of earthy phosphates appears in the three 

 tubes that contain urine. The contents of these tubes must be well 

 mixed by rotation or otherwise immediately before an observation 

 is made. 



Measure 5 cc. of commercial formaldehyde (40 per cent.) into a 

 test-tube. Add one-third the number of drops of phenol phthalein 

 added to the urine and then the standard soda, drop by drop, until a 

 faint pink tinge is obtained. Add the whole of this solution to 

 tube (3). Note that the pink tinge and the precipitate of earthy 

 phosphates disappear, owing to the acidity developed. To tubes 

 (2) and (6) add 5 cc. of water, to dilute the urinary pigment to the 

 same degree as that in tube (3). 



Read the burette containing the standard soda. 



Titrate the contents of tube (3) with the soda, until the appear- 

 ance at Y approaches that seen at X. To tubes (i), (2), (5) and (6) 

 add the same volume of distilled water as the soda added in this last 

 operation. Mix the contents carefully and complete the titration, 

 so that the appearance at Y is intermediate between that seen at 

 XandZ. 



Calculation. If (a) cc. of soda of normality (n) are required to neutralise 

 20 cc. after the addition of the formol, then 20 cc. urine contain (a) x (n) x (14) 

 mgms. of Nitrogen of ammonia and amino-acids. So 100 cc. urine contain 

 (a) x (n) x (70) mgms. of (ammonia + ammo-acid) Nitrogen. This amount, 

 less 20 A (the mgms. of ammonia-Nitrogen determined in the previous exercise) 

 is the mgms. of amino-acid Nitrogen in 100 cc. urine. 



D. The Estimation of Urea. 



The use of the enzyme urease (see p. 287) has rendered obsolete a large 

 number of methods that had been devised for the estimation of urea in urine. 

 The time required for D. Van Slyke's method is not much greater than that 

 for the old hypobromite method, and the results obtained are accurate, 

 whereas with hypobromite they are most unreliable. 



