294 TITRIMETRIC METHODS 



B. Saponifiable Fraction 



1. Evaporate the toluol left in the aqueous phase from step A7 

 in the desiccator, as above. Should the iodine number subsequently 

 be determined on the saponifiable fraction it would be necessary to 

 correct it for the presence of the remaining unsaponifiable lipid. 

 This would require that the iodine number of the unsaponifiable 

 fraction be determined also. It is imjiossible to obtain complete 

 separation of the two phases by pipetting. 



2. Add 2.5 fi 1.4 iV hydrochloric acid and 20 /x toluol, and seal 

 the capillary tube as before. 



3. Mix as in step ^5 and separate the layers by centrifuging 

 without heating. 



4. Remove an aliquot of the toluol phase as in step A7. If the 

 sample is to be used for titration of fatty acids, transfer the aliquot 

 to a titration vessel with the standard ground mouth (Fig. 86). If 

 the iodine number is to be determined, transfer to a paraffined 

 reaction tube. Evaporate the toluol as in step A7. 



IODINE NUMBER OF LIPIDS 



Schmidt-Nielsen ( 1944a) employed the principle of Kaufmann 

 (1926) to develop a method having a precision of about 1% for 

 the determination of the bromine-combining power of lipids in 

 samples of the order of 10 fig. Unsaturated bonds in the lipid are 

 saturated with bromine and the excess bromine is titrated iodometri- 

 cally. The iodine number can be calculated from the titration value 

 if the amount of lipid is also measured (page 291). Schmidt-Nielsen's 

 innovation of using glycol monobutyl ether (butyl Cellosolve) 

 for the fat solvent has the advantage that the low vapor pressure 

 of the liquid obviates difficulties that would result in a micro method 

 from evaporation. As the solvent is also miscible with water, it has 

 the additional advantage of enabling the titration to be performed 

 in a single liquid phase. 



Kretchmer, Holman, and Burr (1946) employed a similar method 

 for 10-100 fig. samples of lipid in chloroform solution, using 0.05 A^ 

 pyridine sulfate dibromide in glacial acetic acid as the brominating 

 agent. At least 20 min. was required for completion of the reaction. 



