300 METHODS OF ANALYSIS [Chap. 



INDEX OF REFRACTION. 

 5 General Directions.— Tentative. 



Place the instrument in such a position that diffuse daylight or any form of arti- 

 ficial light can readily be obtained for illumination. Circulate through the prisms 

 a stream of water of constant temperature. 



Determine the index of refraction with any standard instrument, reading oils at 

 20°C. and fats at 40°C. 



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

 perature by the following formula^: 



R = R' 4- 0.55 (T' - T) in which 

 R = the reading reduced to temperature T; 

 R' = the reading at T'C. 



T' = the temperature at which reading R' is made; 

 T = the standard temperature; 

 0.55 = correction in scale divisions for 1°C. 

 With oils the factor 0.58 is substituted in the formula for 0.55, since they have a 

 higher index of refraction. 



The readings of instruments, which give the index of refraction directly, can be 

 reduced to standard temperature by substituting the factor 0.000365 for 0.55 in the 

 formula. As the temperature rises the refractive index falls. 



The instrument used may be standardized with water at 20°C., the theoretical 

 refractive index of water at that temperature being 1.3330. Any correction found 

 should be made on all readings. 



The index of refraction varies directly with the specific gravity. If the results 

 appear abnormal, compare the specific refractive power^ with the normal. Calcu- 



N - 1 

 late the specific refractive power from the formula — — , in which N equals 



the refractive index and D the specific gravity. According to Procter* the Lorenz 



N2-1 N-1 



formula gives much more satisfactory results than — — — . 



g By Means of the Abbe Refractometer.— Official. 



To charge the instrument, open the double prism by means of the screw head 

 and place a few drops of the sample on the prism or, if preferred, open the prisms 

 slightly by turning the screw head and pour a few drops of the sample into the 

 funnel-shaped aperture between the prisms. Then close the prisms firmly by 

 tightening the screw head. Allow the instrument to stand for a few minutes 

 before the reading is made, so that the temperature of the sample and the instru- 

 ment will be the same. 



The method of measurement is based upon the observation of the position of the 

 border line of total reflection in relation to the faces of a prism of flint glass. Bring 

 this border line into the field of vision of. the telescope by rotating the double prism 

 by means of the alidade in the following manner: Hold the sector firmly, move the 

 alidade backward or forward until the field of vision is divided into a light and a 

 dark portion. The line dividing these portions is the "border line". This, as a 

 rule, will not be a sharp line but a band of color which is eliminated by 

 rotating the screw head of the compensator until a sharp, colorless line is obtained. 

 The border line should now be adjusted so that it falls on the point of intersection 

 of the 2 cross hairs. Read the refractive index of the substance directly on the 

 scale of the sector. Check the correctness of the instrument, as directed under 5, 

 or by means of the quartz plate which accompanies it, using monobromnaphtha- 

 lene, and make the necessary correction in the reading. 



