EDIBLE FATS AND OILS. 



131 



3. Index of Refraction. 

 (a) GENERAL DISCUSSION. PROVISIONAL. 



Determine the index of refraction with any standard instrument, oils being 

 read at 15.5 C. and fats at 40 C. 



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

 readings should be taken nt standard temperature. The readings of the 7 



butyro-refractometer can be reduced to standard 

 temperature by the following formula : 



R = R' -f 0.55 (T' T), 



in which R is the reading reduced to T, R' the 

 reading at temperature T'. T the standard tem- 

 perature, and 0.55 the correction for 1 C. in scale 

 divisions. With oils the factor 0.58 is substituted 

 in the formula for 0.55, since they have a higher 

 index of refraction. 



To calculate to standard temperature the readings 

 of the instruments which give the index 

 of refraction directly the factor 0.000365 

 should be used. As the temperature risos 

 the refractive index falls. Example : The 

 refractive index of a but- 

 ter fat determined at 

 32.4 C.= 1.4540 is reduced 

 to 25 C., as follows: 

 32.425 = 7.4; 0.000365 X 

 7.4=0.0027; it is then 

 1.4540+0.0027= 1 .45<',7. 

 The instrument used may be set with 

 distilled water at 18 C., the theoretical 

 refractive index of water at that tem- 

 perature being 1.3330. In the deter- 

 mination above given the refractive 

 index of pure water measured 1.3300; 

 hence the above numbers should be cor- 

 rected for theory by the addition of 

 0.0030, making the corrected index of 



tne butter fftt mentioned at the tem . 



perature given 1.4597. 



Fi, : . 4.-The Abbe refracUmietcr, w 

 ture-con trolled prisms. 



tempera- 



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 speciflc 

 refractive power & is different from the normal. Calculate the specific refrac- 



tive pow r er from the formula 

 D the specific gravity.*? 





in which N equals the refractive index and 



Wiley, Principles and Practice of Agricultural Analysis, 8: 341; Winton, Conn. Agr. 

 Exper. Stat. Kept, 1900, Part 2, p. 142. 



"Landolt, Ber. d. chem. Ges., 1882, 15: 1031; C. A. Browne, jr., J. Amer. Chem. Soc., 

 1899, 21: 991. 



"Procter (J. Soc. Chem. Ind., 1898, 17: 1021) has shown that the Lorenz formula 



(N 2 +2)D & ives much more satisfactory results than -jj- and gives a calculation table. 



