22 PROVISIONAL METHODS FOE ANALYSIS OF FOODS. 



(2) Determination. 



Weight of fat at the temperature of boiling water. The flask is rinsed with alcohol and 

 ether, and dried for a few minutes at the temperature of boiling water. It is filled 

 with the dry, hot, fresh-filtered fat, which should be entirely free from air bubbles, 

 replaced in the water bath, and kept for thirty minutes at the temperature of boil- 

 ing water. The stopper, previously heated to 100 C, is inserted, the flask removed, 

 wiped dry, placed in the balance after it has nearly cooled to room temperature, and 

 weighed when the balance temperature is reached. The weight of fat having been 

 determined, the specific gravity is obtained by dividing it by the weight of water 

 previously found. 



Example: Grams. 



Weight of flask, dry 10. 0197 



Weight of flask, plus water 37. 3412 



Weight of water 27.3215 



Weight of flask, plus fat 34. 6111 



Weight of fat 24. 5914 



Specific gravity=24.5914H-27.3215=0.90008. 



The weight of the flask dry and empty may be used constantly if great care be 

 taken in handling and cleaning the apparatus, but the weight of water at boiling 

 temperature must be determined under the barometric conditions prevailing at the 

 time the determination is made. 



Example: Grams. 



Weight of flask, dry and empty 10. 0028 



Weight of flask after three weeks' use 10. 0030 



3. DETERMINATION OF INDEX OF REFRACTION.* 



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 at standard temperature. The readings of the Zeiss butyro- 

 refractometer can be reduced to standard temperature by following formula: 1 * 



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



in which R is the reading reduced to T, R' the reading at Temp. T, T the standard 

 temperature, and .55 the correction for 1 C. in scale divisions. With oils the factor 

 .58 is substituted in the formula for .55, since they have a higher index of refraction. 



To calculate to standard temperature the readings of the instruments which give 

 index of refraction directly the factor 0.000365 may be used. As the temperature 

 rises the refractive index falls. Example: The refractive index of a butter fat deter- 

 mined at 32.4C.=1. 4540 is reduced to 25 C., as follows: 32.4 25=7.4; 0.000365X7.4= 

 0.0027; it is then 1.4540+0.0027=1.4567. 



The instrument used should be set with distilled water at 18C., 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 cor- 

 rected index of the butter fat mentioned at the temperature given 1.4597. 



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 



See appendix, p. 150. 



fcWiley, Prin. and Prac. of Agri. Anal., vol. 3, p. 341. Winton, Conn. Expt. Sta. Kept., 1900, pt. 2, 

 p. 142. 



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



