24 PROXIMATE CONSTITUENTS OF PLANTS 



delivered from a pipette into the chloroform solution : 

 the resulting mixture should give a clear solution on 

 shaking. If the solution be not clear, a further small 

 quantity of the chloroform should be added. After 

 standing for two hours, the solution should still possess 

 a deep brown colour, indicative of the presence of excess 

 of free iodine. If the colour has been discharged, an 

 additional 2 or 3 c.c. of the iodine solution should be 

 run in, and the mixture allowed to stand for another 

 two hours. Twenty c.c. of a 10 per cent, solution of 

 KI are then added, and about 300 c.c. distilled water. 

 The excess of iodine is then estimated by direct 

 titration with decinormal sodium thiosulphate solu- 

 tion, with constant agitation. When the colour is 

 almost discharged, a few drops of starch solution are 

 added, and the titration carried to an end. 



For an improved method for the determination of 

 the " Iodine Value," especially of butter fat, see Wij's 

 method, p. 204. 



The- iodine values of the more important oils are as follows : — 



14. Qualitative Examination of Cottonseed Oil. 



Cotton-seed oil, obtained from the seeds of the various kinds of 

 cotton tree, is of a reddish or greenish brown colour when crude, 

 but after purification is pale yellow. Being inexpensive and of a 

 pleasant taste, it is employed largely for the adulteration of olive 

 oil (salad oil), lard, margarine, etc., and for this reason the 

 working out of methods for the rapid detection of the presence of 

 cotton-seed oil by various colour tests has received a deal of 

 attention. 



The following tests should be applied to (i.) pure cotton-seed 

 oil ; (ii.) pure olive oil, and (iii.) the latter containing 5, 25, and 50 

 per cent, of added cotton-seed oil. 



