COLOUR REACTIONS 23 



Solstieiis Reaction for Sesame Oil. — Two or three volumes 

 of oil or fat are dissolved in twice their volume of benzene 

 (b.p. 70-80°) and gently shaken with three volumes of con- 

 centrated zinc chloride saturated with hydrochloric acid, the 

 whole being kept immersed in a water bath at a tempera- 

 ture of 40° C. ; when the zinc chloride has sunk to the bottom, 

 the test tube is immersed up to the top level of the zinc chloride 

 in a water bath at 80° C. If sesame oil is present a pink colour 

 is produced. 



Halphe^is Reaction for Cotton-seed Oil. — Equal volumes 

 of oil, amyl alcohol, and a one per cent solution of sulphur 

 in carbon bisulphide are mixed in a test tube immersed to 

 half its depth in boiling brine for about ten minutes. By 

 the end of this time an orange colour should appear ; if not 

 add more carbon bisulphide and boil again. The colour is said 

 to be due to the addition of sulphur to an unsaturated bond. 



Sulphuric Acid Test. — Rape-seed oil and mustard oil when 

 shaken with sulphuric acid (sp. gr. 1-53-1 -62) produce grass- 

 green to blue-green colours. Linseed and hemp oil may also 

 give similar colorations. 



MICROCHEMICAL REACTIONS. 



1. The microscopical appearance of oil when mixed with 

 water is characteristic owing to its immiscibility with water and 

 its different refractive index. 



2. Its solubility in ether, chloroform, benzene, or other fat 

 solvents is easily noted. 



3. If oil be present in the preparation it will fairly rapidly 

 turn brown and then black when treated with a one per cent 

 solution of osmic acid. This is not absolutely conclusive since 

 osmic acid stains proteins brown. 



4. Tincture of alkannin, or a saturated solution of Scharlach 

 R in 75 per cent alcohol, colours oil globules red or pink. 



The reaction with the first-named reagent is often ill-defined 

 and frequently fails when the alkanna used has been extracted 

 from the root some time. The test is more satisfactory when 

 freshly prepared tincture is used. 



A similar reaction is given by Sudan III. 



It is important to note that these and similar reactions are 

 not conclusive of the chemical nature of the substances acted 



