CONSTANTS FOR IDENTIFICATION OF FATS AND OILS 239 



(3) Refractive Index 



The refractive index may be defined as the ratio of the sine of the angle 

 of incidence to the sine of the angle of refraction. It is usually referred to 

 by an italicized letter n, with the temperature at which the observation is 

 made indicated. The standard temperature for reference in the case of 

 oils is 25°C., while for fats the temperature is 40°C. A correction factor of 

 0.000365 (or 0.00038*") may be used to calculate the values at the reference 

 temperature; however, the determination should be made as near as pos- 

 sible to the reference point. The refractive indices increase with a lower- 

 ing of the temperature. 



The refractive index of a fat is a measurement which can be made with 

 considerable precision. Only an extremely small amount of sample is re- 

 ([uired. Thus, the determination can readily be made even when the quan- 

 tity of a sample is limited. In general, the refractive index varies with the 

 specific gravity. Highest values are also obtained with increasing degrees 

 of unsaturation, as well as with larger molecular weights. Butter fat is an 

 exception to this pattern, since it has a lower refractive index than other 

 animal fats, although its specific gravity is higher. This discrepancy is ap- 

 parently related to the high proportion of fatty acids of low molecular 

 weight contained in this product. 



The index of refraction is closely related to the iodine number of the 

 fat. This is not surprising, since it is proportional to the unsaturation of 

 the fat, the degree of which is measured by the iodine number. The rela- 

 tion of refractive index to iodine number is so constant that a simple re- 

 fractometer graduated in iodine values is now available for field work. It 

 is stated that the precision is such as to make it accurate to one-half of an 

 iodine value unit. According to Peter and Kron,*^^ the iodine number of 

 butter fat may be calculated from the refractive index by the following 

 equation : 



Iodine Number = (n^" - 1.45268)5700 + 26 



The index of refraction is also related directly to the melting point of the 

 fat. These relationships are illustrated in Figures 4 and 5, which are based 

 on the data of Sudborough et al.^'^^ 



The refractive index is also related to the proportion of short-chain acids. 

 In general, it has been found that the higher the Reichert-Meissl value the 

 lower the refractive index. However, the correlation in this case is quite 

 poor. Soybean oil has the highest refractive index (1.475) of the common 

 fats, and this is followed closely by poppyseed, corn, and mustard oils. 



»" H. D. Richmond, Analyst, 32, 44-40 (1907). 

 '2^ A. Peter and S. Kion, Milchw. Forsch., 14, 378-386 (1932). 



'^ J. J. Sudborougli, H. E. Watson, and U. Y. Athawale, J. Indian Inst. Sci., 5, 47-69 

 (1922). 



