EXAMINATION OF FATS. 1415 



DETERMINATION OF INDEX OF REFRACTION. 



Determine the index of refraction with any standard instrument, 

 oils being read at 15.5 and fats at 40. 



The temperature must be controlled with great care, and in accurate 

 work the readings should be taken at standard temperature. The 

 readings of the Zeiss butyro-refractometer can be reduced to standard 

 temperature by following formula: a 



R=R'+.55 (T/-T). 

 R is the reading reduced to T. 

 R' reading at Temp. T. 

 T is standard temperature. 



To calculate to standard temperature the readings of the instruments 

 which give index of refraction directly the factor 0.000176 may be 

 used. As the temperature rises the refractive index falls. Example: 

 Refractive index of a butter fat determined at 32.4 1.4540, reduced 

 to 25, as follows: 32.4-25 = 7.4; 0.000176x7.4 = 0.0013; then 

 1.4540+0.0013=1.4553. 



The instrument used should be set with distilled water at 25, the 

 theoretical refractive index of water at that temperature being 1.3330. 

 In the determination above given the refractive index of pure water 

 measured 1.3300; hence the above numbers should be corrected for 

 theory by the addition of 0.0030, making the corrected index of the 

 butter fat mentioned at the temperature given 1.4583. 



The index of refraction varies greatly with the specific gravity, 

 increasing as it increases. In abnormal results it is often well to see if 



the specific refractive power b is different from the normal. Calculate 



$f ]_ 



the specific refractive power from the formula -jj , c in which N 



equals the refractive index and D the specific gravity. Always state 

 temperature at which the determinations were made. 



ABBE'S REFRACTOMETER. 



A later and much improved model of the Abbe instrument in which 

 arrangements are made for controlling the temperature, the weakness 

 of the older form, d is described in Benedickt. 6 



a Wiley, Prin. and Prac. of Agri. Anal., vol. 3, p. 341; Winton, Conn. Expt. Sta. 

 Kept., 1900, pt. 2, p. 142. 



b Landolt., Ber., 1882, 15, 1031; C. A. Browne, Jour. Am. Chem. Soc., 1899, 21, 991. 

 C H. R. Procter, Jour. Soc. Chem. Ind., 1898, 17, 1021-1026, has shown that the 



^2 1 N I 



Lorenz formula, (jj2 i o\j^ gives much more satisfactory results than j^ , and gives 



table for calculation. 



d For a description of the older form of the Abbe instrument see U. S. Dept. Agr., 

 Div. Chem. Bui. 46, revised, p. 49. 



e Anal, der Fette u. Wach., 3d ed., p. 105. 



