CH. XIII.] AMMONIA AND AMINO-ACIDS. 333 



as described for Kjeldahl's method, p. 323. The result thus obtained is the 

 mgms. of ammonia-nitrogen in 5 cc. = A. 



Mgms. of ammonia-nitrogen in 100 cc. = 20 A. 

 Grams, of ammonia-nitrogen in 100 cc. = A x 0-02. 



Grams, of ammonia in i oo cc. = A x 0-02 x = A x 0-0243 (log. 2-3853). 



cc. of o-i N. acid neutralised by NH 3 of 100 cc. = 20 A x = A x 14-29 

 (log. 1-1549)- 



400. C. The estimation of ammonia and amino-acids by 

 formol titration (Cole's method). 



Principle. Neutral ammonium salts react with an excess of neutral 

 formaldehyde to give hexamethylene tetramine, the acid being liberated. 



4 NH 4 C1 + 6 CH 2 O = N 4 (CH 2 ) 6 + 6 H 2 O + 4 HC1. 



From the amount of alkali required to again make the solution neutral, 

 the amount of ammonia can be estimated. 



Neutral amino-acids also react with formol to give methylene amino-acids 

 [see p. 69 (3) ]. The result of the estimation therefore gives the sum of the 

 ammonia and the amino-acids of the urine. 



The method usually adopted is to neutralise the urine to phenol-phthalein, 

 to add neutralised formol, which makes the fluid acid, and then to determine 

 how much standard soda is again required to neutralise the mixture. The 

 great difficulty encountered is that of determining the neutral point, and 

 experience with large classes of students has revealed the fact that considerable 

 variations in results are found due to the indecision about the two end points. 



As explained on p. 215, the author has overcome this difficulty by the use 

 of the comparator shewn in fig. 27. The results obtained by untrained 

 students now agree very closely. 



Method. Use the comparator for large tubes described on 

 p. 276. 



Into tubes (2), (3) and (6) measure 20 cc. of the urine. 



Into tube (i) measure 20 cc. of buffer solution PH 8*4 (see 



Into tube (5) measure 20 cc. of buffer solution P H 8-5.* 

 Into tube (4) place about 30 cc. of water. 



To tubes (i), (3) and (5) add 10 to 20 drops of 0-5 per cent. 

 phenol phthalein, adding exactly the same*amount to each by the 

 use of a dropping pipette (fig. 5). The amount necessary varies 



* If only one buffer solution is used, as is done in the exercise on p. 215, 

 it should be P H = 8-45. 



