BROMINE ABSORPTION. 265 



20 c.c. of iodine chloride solution. It is advisable also to increase 

 the amount of potassium iodide solution added to 20 c.c. or more. 

 Gravimetric Method of Hehner. Hehner has proved that 

 the bromine absorbed may be estimated gravimetrically. The 

 fat is weighed in a small basin, dissolved in carbon tetrachloride, 

 a solution of bromine in carbon tetrachloride added, and the 

 excess of bromine and the carbon tetrachloride evaporated on a 

 water-bath in a good draught cupboard. The residue is freed 

 from the last traces of bromine by adding several successive 

 portions of carbon tetrachloride and evaporating them, and, 

 finally, by 'drying in an air-bath maintained at a temperature 

 somewhat above 100 C. The increase in weight multiplied by 



127 



_ = 1 -5875 will give the iodine absorption. 



Ow 



Thermometric Method. Hehner and Mitchell have also 

 devised a most ingenious means of rapidly and accurately cal- 

 culating the iodine absorption, founded on the fact that when 

 1 molecule of bromine combines with 1 molecule of unsaturated 

 fat a definite amount of heat is liberated. 



One gramme of fat is weighed into a jacketed test-tube about 

 1 inch in diameter and 6 inches long, from the jacket of which 

 the air has been exhausted. 10 c.c. of chloroform are added, 

 and the temperature noted ; 1 c.c. of bromine is added, the 

 mixture stirred with the thermometer, and the highest point to 

 which the temperature rises is recorded. The difference between 

 the initial and the highest temperatures multiplied by a factor 

 will give the iodine absorption. The factor must be found 

 empirically, as it varies slightly with each apparatus, ther- 

 mometer, etc. It can be ascertained easily by submitting a 

 few fats of known and varying iodine absorption to this test, and 

 taking the mean relation between the difference of temperatures 

 noticed and the iodine absorption. Hehner and Mitchell found 

 in their experiments that the temperatures multiplied by 5 '5 

 gave the iodine absorption. The thermometer used should be a 

 good one, capable of reading to -f^ C. ; the same thermometer 

 and test-tube should always be used. When the apparatus has 

 been once standardised this method forms a rapid means of 

 estimating the iodine absorption. 



The bromine should be measured in a 1 c.c. pipette, having a 

 bulb filled with soda lime in its upper portion ; unless this is 

 done, the fumes of the bromine are apt to prove very 

 unpleasant. 



The bromine thermal test for oils and fats is modified by 

 Gill and Hatch by taking sublimed camphor as the standard 

 substance, and dividing the rise with the fat by that with the 

 camphor to a " specific temperature reaction." They find that 



