182 MILK 



lows, according to the Association of Official Agricultural Chem- 

 ists: " About 5 grams of milk are placed in a Kjeldahl digestion 

 flask, 10 grams powdered potassium sulphate and 15 to 25 c.c. 

 (average 20 c.c.) sulphuric acid added. The digestion is started 

 with a temperature below the boiling-point and the heat gradually 

 increased until frothing ceases. The mixture is digested for some 

 time after it has become colorless or until oxidation is complete. 

 A small piece of paraffin is added to prevent bumping. After 

 cooling the mixture is diluted with about 200 c.c. water, a few 

 pieces of granulated zinc or pumice stone added, and enough soda 

 solution to make the fluid strongly alkaline. The soda solution 

 must be poured down the side of the flask so that it does not mix 

 at once with the acid solution. The flask is then connected with 

 a condenser, the contents mixed by shaking, and heated until all 

 ammonia has passed over. The ammonia is received in a definite 

 amount of standard acid. This operation usually requires from 

 forty minutes to one hour and a half. The distillate is then 

 titrated with standard alkali. The percentage of nitrogen ob- 

 tained is multiplied by 6.38 to obtain the percentage of total pro- 

 tein. This method is not applicable when nitrates are present. 

 This, however, occurs rarely with milk. 



" The Reagents Used. Potassium sulphate should be pulver- 

 ized before using. 



''Standard sulphuric acid for receiving the ammonia should 

 be half normal or one-tenth normal when very small amounts of 

 nitrogen are present. 



"Standard alkali solution should be one-tenth normal. 



"Concentrated sulphuric acid. 



"Sodium hydrate solution should be saturated. 



"Cochineal is used as indicator." 



Determination of Total Nitrogen by the Ritthausen Method. 

 Ten grams of milk are measured into a beaker and diluted with 

 water to about 100 c.c. Richmond modifies the process by neu- 

 tralizing the milk to phenolphthalein before proceeding. To the 

 diluted milk are added 5 c.c. copper sulphate solution prepared by 

 dissolving 34.64 grams copper sulphate in 500 c.c. water and the 

 mixture stirred. Then a "solution of sodium hydroxid (2.5 per 

 cent.) is slowly added till the mixture is nearly neutral. Excess 

 of alkali is to be avoided, since this prevents complete precipita- 

 tion of the proteins. After the precipitate has settled the super- 

 natant fluid is poured through a weighed and dried filter. The 

 precipitate is washed several times by decantation and then 

 transferred to the filter. It is washed with water, drained, and 

 washed with strong alcohol, dried, extracted with ether in a Soxh- 

 let extraction apparatus, and then transferred on the filter to an 



