PHYSICAL PROPERTIES OF FATTY ACIDS 69 



saturated acids. This reaction may be employed quite satisfactorily for 

 the determination of the so-called polybromide number. Although the re- 

 action of the various acids with i)romine is not quantitative, certain em- 

 pirical formulas are a\'ailable from which the type or amount of the spe- 

 cific unsaturated acid may be deduced. 



However, the bi()mo-derivati\'es vary considerably in solubility and they 

 can be separated from each other by choosing the appropriate solvent. 

 The solubility of hexabromostearic acid in 28 different solvents has re- 

 cently been reported by Seidell. ^-^ 



The monoethenoid acids such as oleic, erucic, and ricinoleic, form di- 

 bromo-acids on bromination which are soluble in most of the fat solvents. 

 The tetrabromide of linoleic acid is soluble in ethyl ether, but is largely in- 

 soluble in petroleum ether. The hexabromide formed from linolenic acid 

 is insoluble in diethyl ether and in a number of the other fat solvents. 

 The octabromide of arachidonic acid is also insoluble in cold ethyl ether 

 and benzene, while the polybromide prepared from clupanodonic acid is 

 relatively insoluble in all organic solvents. 



e. Variations in Solubility of Metallic Salts (Soaps) of Fatty Acids in 

 Different Solvents. Both saturated and unsaturated fatty acids form 

 combinations with a variety of metallic cations. Considerable data are 

 available on the solubilitj', in a number of solvents, of the salts of the alkali' 

 metals (lithium, potassium, and sodium), of the alkali earth metals (bar- 

 ium, calcium, magnesium, and strontium), and of the hea\'y metals (co- 

 balt, copper, gold, iron, lead, mercury, nickel, and sih^er). The variation 

 in solubilit}' of the salts of different acids in specific soh'ents has been an 

 important basis for separation. 



(o) Lead Salts in Ether and Alcohol. One of the oldest and most widely 

 used methods for the separation of saturated and unsaturated fatty acids 

 is based upon the observation first reported by Gusserow,^-^ that the lead 

 salts of the liquid (unsaturated) acids are soluble in ether, while the lead 

 salts of those fatty acids which are solid (saturated) are insoluble in ether. 

 The procedure has been modified by T\^^tchell-^^ for the determination of 

 saturated and unsaturated fatty acids by the use of 95% ethanol in place 

 of diethyl ether. The Twitchell method is the basis of the official proce- 

 dure adopted by the Association of Official Agricultural Chemists-'-^ and 



-^ A. Seidell, Solubilities of Onjanic Coinpouruls, Vol. II, 3rd ed., Van Xostrand, New 

 York, 1941. 



"6 C. A. Gusserovv, Arch. Pharm., 27, 153-244 (1828). Cited by K. S. Markley, 

 Fatty Acids, Interscience, New York, 1947, p. 596. A. \V. Ralston, Fatty Acids and Their 

 Derivatives, Wiley, New York, 1948, p. 282. 



"7 E. Twitchell, J. hid. Eng. Chem., 13, 806-807 (1921). 



-^ Official and Tentative Methods of Analysis, 5th ed., Assoc. Official Agr. Chem., 

 \\as:hington, D. C, 1940, p. 434-436. 



"^-^ Official and Tcnlativr .Methods of Analysis, 2nd ed., Am. Oil Chem. Soc, 1946, \'. O. 

 Mehlenhadier, ed., Cd6-38. 



