PREPARATION AND DETERMINATION OF LECITHIN 205 
Malcolm' in egg yolk. The method is an excellent one for the determination of 
phosphorus in lecithin, and consists in carrying out a modified Kjeldahl determination, 
in which, however, phosphorus is determined as magnesium pyro-phosphate, instead 
of nitrogen as ammonia. 
About i*5 gramme of lecithin is taken and boiled in a Kjedahl flask with pure 
concentrated sulphuric acid, potassium sulphate being added from time to time to 
assist in the oxidation, when a colourless fluid is obtained it is allowed to cool 
neutralized by ammonia and the phosphoric acid estimated gravimetrically as 
Mg z P,O r 
Experiment 3 
Lecithin taken 
Mg„P„0- obtained 
Percentage of 
Phosphorus 
Estimated molecular 
wt. of lecithin 
Determination 1 ... 
2752 
O-3915 
3"973 
780 
2 ... 
I-66 3 
0-2362 
3-966 
782 
l-5 + l 
0 - 2 1 70 
3-933 
788 
The molecular weight of di-stearyl-lecithin is 807, di-palmityl-lecithin is 751, di- 
oleal-lecithin is 803, and of di-linoleal-lecithin is 743. According to Cousins/ who 
separated and determined the fatty acids in lecithin of egg-yolk, this contains linoleic 
acid, 24-0 percent.; oleic acid, 33 per cent. ; palmitic acid, 28-5 percent. ; and stearic 
acid, 14*2 per cent. This composition would lead to a mean molecular weight of 772, 
which fairly closely corroborates the above results. 
Journ. of Physiol. (1901), vol. xxvii, p. 35;. 
2. Soc. Biol. 55, p. 913 
