130 METHODS OF ANALYSIS. 



(b) DETERMINATION AT 100 C. OFFICIAL. 



(1) STANDARDIZATION OF FLASKS. 



(a) Method I. Use a small specific-gravity flask of from 25 to 30 cc capacity. 

 Wash the flask thoroughly with hot water, alcohol, and ether, and then dry it. 

 After cooling in a desiccator accurately determine the weight of the flask and 

 stopper. Fill the flask with freshly boiled, hot, distilled water. Keep the 

 water of the bath in brisk ebullition for thirty minutes, any evaporation from 

 the flask being replaced by the addition of boiling distilled water. Then insert 

 the stopper, previously heated to 100 C., remove the flask, wipe it dry, and after 

 it has nearly cooled to room temperature place it in the balance and weigh 

 when the balance temperature is reached. 



(&) Method II. The following formula may be used for calculating the 

 weight of water (WT) which a given flask will hold at T (weighed in air with 

 brass weights at the temperature of the room) from the weight of water (W l ) 

 (weighed in air with brass weights at the temperature of the room) contained 

 therein at t : 



d x =the density of water at T. 



d* =the density of water at t. 



y =the coefficient of cubical expansion of glass.** 



(2) DETERMINATION. 



Rinse the flask with alcohol and ether and dry for a few minutes at the tem- 

 perature of boiling water. Fill the flask with the dry, hot, fresh-filtered fat, 

 which should be entirely free from air bubbles ; replace it in the water bath, and 

 keep for thirty minutes at the temperature of boiling water. Insert the stopper, 

 previously heated to 100 C., remove the flask, wipe dry, place in the balance 

 after it has nearly cooled to room temperature, and weigh when the balance 

 temperature is reached. The weight of fat having been determined, obtain the 

 specific gravity by dividing the weight of fat 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 _____________________________ 1 _____________________ 27.3215 



Weight of flask, plus fat ____________________________________________ 34.6111 



Weight of fat ______________________________________________________ 24.5914 



Specific gravity=24.5914-r-27.3215=0.90008. 



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

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

 ing temperature must be determined under the barometric conditions prevailing 



at the time the determination is made. 



_ _ ^ 



This factor is commonly given as 0.00026, but It varies considerably. Schulze (Zts. 

 anal. Chem., 1882, 21: 167) found that the glass he used varied from 0.0000288 to 

 0.0000305 an average of 0.0000296. Ewell has used 0.000028 In his work (U. S. Dept 

 Agr., Division of Chemistry, Bui. 62, p. 121). 



