94 PUBLIC HEALTH CHEMISTRY 



PROCESS. Weigh 2 grm. of sample into a 200 c.c. 

 flask, add 25 c.c., approximately, N/2 (seminormal) 

 alcoholic solution of KOH. A like amount of KOH 

 solution is run into an empty flask for a blank experiment. 

 The flasks are fitted with corks carrying vertical tubes 

 4 ft. long to act as condensers. Both flasks are then 

 heated on the water-bath for not less than thirty minutes, 

 with frequent agitation. Thereafter a few drops of phth. 

 are added to each flask, and both are titrated with exactly 

 N/2 HC1 solution, i c.c. of which = 0-02805 grn*- KOH; 

 therefore the difference between the two titrations, 

 multiplied by this factor, gives the amount of KOH taken 

 up by the oil, and from this the percentage is easily 

 calculated, and the Sn value and the Sn equivalent. The 

 Sn value shows the number of milligrammes of KOH 

 required to saponify I grm. of the oil, since 55-7 per cent 

 equals 557 per 1000, or 557 mgr. per I grm. 



Iodine absorption of butter fat ranges from 23 to 38 per 

 cent, and for margarine from 40 to 55 per cent, but is not 

 of much value in determining the amount of foreign fat 

 in butter. The fats of the oleic series readily unite with 

 a definite quantity of I, Br, or Cl, the others being 

 indifferent. 



Sesame Oil. To 10 c.c. of fat sample add 10 c.c. strong 

 HC1 containing o-i grm. of cane sugar. Shake thoroughly 

 and allow to stand, when in the presence of even 2 per cent 

 of sesame oil, the aqueous liquid becomes crimson coloured. 



Cotton-seed Oil. Mix equal volumes of the fat and a 

 saturated solution of Pb acetate, add AmOH and stir 

 quickly. On standing, the surface layer turns an orange- 

 red colour. 



Starch. Melt sample in a small beaker or tube, pour off 

 fat, and add KI solution to water and curd. Normal 

 butter gives a reddish coloration only, while a very small 

 trace of starch will give a blue. Examine curd 

 microscopically. 



Polariscope. Pure butter (using gaslight) on rotation, 

 so that the two Nicol prisms are at right angles, gives the 

 whole field equally dark. If 20 per cent or over of 

 margarine is present, it is impossible to darken the whole 

 field, no matter how the prisms be placed, but a cloudy 



