40 PUBLIC HEALTH CHEMISTRY 



Kjeldahl's Process, in which the organic nitrogen is 

 converted into ammonia and estimated by distillation 

 along with the free and saline ammonia. This method is 

 very useful in highly polluted waters and sewage effluents, 

 where the estimation of the albuminoid ammonia is tedious 

 and difficult. It is much used to determine the total 

 nitrogen in food-stuffs, from w r hich the total proteins is 

 got by multiplying by a factor which for meat foods is 

 6-25, and varies for other foods. 



Free and Saline Ammonia. 



By Wanklyn's Method. Solutions required : (i) Nessler's 

 reagent. This is a saturated solution of mercuric-potassic- 

 iodide in ammonia-free distilled water, the whole being 

 rendered strongly alkaline with caustic soda or potash. 

 It may be made thus : Dissolve 35 grm. of potassium 

 iodide in 200 c.c. of ammonia-free distilled water and 

 12*5 grm. of corrosive sublimate in 300 c.c. of ammonia-free 

 distilled water. Add the iodide solution to the sublimate 

 one, when a yellow to scarlet precipitate is obtained, which 

 re-dissolves in the excess of potassium iodide present. 

 (Mercuric iodide is almost insoluble in water.) 



HgCl 2 -f 2KI = HgI 2 + 2KC1. 



HgI 2 + 2KI = HgI 1 .aKI. 



Now add carefully a cold saturated solution of corrosive 

 sublimate, stirring all the time, until a slight red precipitate 

 remains permanent. In this way excess of potassium 

 iodide, above that required to keep the mercuric iodide 

 in solution, is used up. 120 grm. of caustic soda in stick 

 are now added to the mixture and allowed to dissolve 

 and cool. If the red precipitate has disappeared, add 

 again a little of the saturated solution of corrosive sublimate, 

 until a slight permanent red precipitate appears. Make 

 up bulk to i litre with ammonia-free distilled water. The 

 solution is now ready for use. 



Nessler's solution gives a yellowish tinge with the 

 faintest trace of ammonia, and if much ammonia is present 

 a yellow-brown precipitate forms of di-mercuric-ammonium- 

 iodide : 



