TOTAL NITROGEN 189 



estimated, either from the difference between the total nitrogen 

 and that accounted for by the casein, or by precipitation from 

 milk after removal of the casein. The lacto-globulin, being present 

 in such very small amounts, may be neglected in ordinary 

 laboratory practice. 



183. Determination of the Total Nitrogen by 

 KjeldahPs Process. 



About 10 grams of milk are weighed out from a 

 small beaker into a Kjeldahl flask, and 15 to 20 c.c. 

 of concentrated sulphuric acid added. A few grams 

 of potassium sulphate are added to raise the boiling- 

 point of the acid, and a small crystal of copper sulphate 

 is dropped in to hasten the reaction. 



The flask is then heated gently over the naked 

 flame until most of the water has evaporated, after 

 which the mixture is allowed to boil until complete 

 oxidation of all carbonaceous particles has taken place, 

 and the liquid is perfectly clear and transparent. 



The flask is then cooled, the contents washed into an 



ammonia distillation apparatus (p. 12), made alkaline 



with caustic soda, and the ammonia distilled over into a 



/ N\ 



known volume of standard sulphuric acid (50 c.c — j 



in the usual way. The excess of acid is then titrated, 

 and the percentage of nitrogen in the milk calculated. 



As casein contains 15-65 per cent, nitrogen and 

 lacto-albumin 15-77 per cent, the percentage of total 

 protein may be obtained by multiplying the percentage 



of total nitrogen by 6-38 (= — — ^j.^ 



The total protein determined thus is always subject to 

 a slight error, owing to the presence of small quantities 

 of nitrogenous substances other than proteins in milk. 



^ Total protein = 85 per cent, casein, and 15 per cent, lacto- 

 albumin, hence 85 per cent, of 15-65 + 15 per cent, of 1577 = 15-66. 



