244 THE URINE. 



of the urine, and as the resulting amount of nitrogen is not con- 

 stant, a further error is incurred. In scientific research we are 

 hence forced to resort to some other procedure, such as that 

 proposed by Folin. 



METHOD OF FOLIN. This is based upon the following considera- 

 tions : crystallized magnesium chloride, MgCl 2 .6H 2 O boils in its 

 water of crystallization at a temperature of about 160 C. Urea 

 is quantitatively decomposed in such a solution into ammonia and 

 carbon dioxide within one-half hour. If the process is carried out 

 in acid solution, the ammonia can subsequently be distilled off after 

 rendering the mixture alkaline, and is then titrated. The cor- 

 responding amount of urea is ascertained by calculation. At the 

 same time, however, the preformed ammonia is obtained, and it is 

 hence necessary to eliminate this source of error by a separate 

 estimation of this form. This is conveniently done according to 

 the method which has likewise been suggested by Folin (see below). 



Method. Three c.c. of urine are placed in an Erlenmeyer flask 

 of 200 c.c. capacity, together with 20 grammes of magnesium 

 chloride and 2 c.c. of concentrated hydrochloric acid. It is neces- 

 sary to measure the urine very accurately with a pipette graduated 

 in one-twentieths c.c. (The magnesium chloride usually contains 

 a small amount of ammonia, which must be separately determined.) 

 The flask is closed with a perforated stopper through which a 

 specially constructed safety tube passes. 1 The mixture is now 

 boiled until the drops flowing back through the tube produce a 

 hissing sound on coming in contact with the solution. After this 

 point has been reached, the boiling is continued more moderately 

 for about forty-five minutes. In order to obviate immoderate foam- 

 ing a piece of paraffin about twice as large as a coffee bean is 

 added. The solution while still hot is carefully diluted to about 

 500 c.c. at first by allowing the water to flow drop by drop 

 through the tube : it is then transferred to a 1000 c.c. retort, 

 treated with about 7 or 8 c.c. of a 20 per cent, solution of sodium 

 hydrate, and the ammonia distilled off into a measured amount 

 of a decinormal solution of sulphuric acid. The distillation may 

 be interrupted when about 350 c.c. have passed over (viz., after 

 about sixty minutes). The distillate is boiled for a moment to 

 remove any carbon dioxide which may be present in solution, and 

 on cooling is titrated to determine the excess of acid. Each cubic 

 centimeter of the decinormal ammonia present in the distillate cor- 

 responds to 0.003 gramme, viz., to 0.1 per cent of urea. 



From this result the amount of preformed ammonia and that 

 present in the 20 grammes of magnesium chloride must be deducted. 



If desired, the estimation can also be made with the urea-con- 

 taining filtrate obtained with Morner and Sjoquist's method, but 

 Folin states that the previous isolation of the urea in such manner 

 is probably not necessary. 



Estimation of the Preformed Ammonia (according to Folin). 



> The tube can be procured from Elmer & Amend's, of New York. 



