COLOUR REACTIONS 23 



three minutes or more until the mercury is completely dis- 

 solved. After the lapse of twenty minutes, the mixture is shaken 

 for another minute, and is then set aside and the time noted 

 which is required for the oil to solidify. 



Olive oil requires one hour. 



Arachis or earth-nut oil requires one and a half hours. 



Colza and sesame oil require three hours. 



Linseed oil gives a red pasty froth. 



Hempseed oil remains unchanged. 



The temperature of the mixture must be maintained con- 

 stant during the test, and must not exceed 25. 



Bromide Test. This test, which is chiefly used for dis- 

 tinguishing between drying and semi-drying oils, depends 

 on the fact that linolic, linolenic acids and other unsaturated 

 acids produce insoluble additive compounds with bromine, 

 containing six or eight atoms of this element. According 

 to Hehner and Mitchell* from 1-2 c.c. of oil are dissolved 

 in 4Oc.c. of ether containing a few cubic centimetres of glacial 

 acetic acid ; the mixture is then cooled to 5 and treated with 

 bromine drop by drop until no more is absorbed. After three 

 hours the precipitate is filtered off on a tared asbestos filter and 

 washed four times with 10 c.c. of ether; it is then dried in a 

 steam oven. The weight of the precipitate is directly propor- 

 tional to the amount of unsaturated acids present in the fat. 



Sulphuric Acid Test. On mixing fats of the oleic series 

 with concentrated sulphuric acid no heat is evolved, whilst with 

 fats of the linolic series the opposite is the case. 



COLOUR REACTIONS OF INDIVIDUAL FATS. 



Many of the colour reactions described for fats are of 

 doubtful value owing to the modifying influence of small 

 quantities of resins and of proteins. The following tests are, 

 however, fairly reliable : 



Badouiris Test for Sesame Oil. Twenty c.c. of sesame 

 oil are thoroughly shaken for a short time with 10 c.c. of 

 hydrochloric acid (sp. gr. 1*9) containing 0*18 gram of cane 

 sugar. A rose colour should appear immediately after the two 

 layers of oil and water have separated ; if left to stand longer 

 the sugar solution causes a brown coloration. 



* Hehner and Mitchell : " The Analyst," 1898, 23, 313. 



