38 PROXIMATE CONSTITUENTS OF PLANTS 



consists of an aqueous solution of caffeine, tannin, 

 albuminous substances, and colouring matter. To 

 remove the tannin and albuminous substances, the 

 solution in the beaker is kept hot over a Bunsen 

 burner, and basic lead-acetate solution ^ added until 

 no further precipitation takes place. The liquid is 

 at once filtered as quickly as possible, and the 

 precipitate washed with a small quantity of hot water. 

 The excess of lead is then removed by addition of 

 sufficient dilute HgSO^, the precipitated lead sulphate 

 being removed by decantation. 



To remove the colouring matter, 4 or 5 grams of 

 animal charcoal are stirred into the liquid, which is 

 then concentrated down to about 300 c.c. The solu- 

 tion is filtered, and the treatment with animal charcoal 

 repeated if necessary. 



The caffeine is extracted from the filtrate by 

 shaking in a separating funnel with 70 c.c. chloroform. 

 This extraction should be repeated at least three times. 



The chloroform is then distilled off on the water- 

 bath, and the residue of caffeine taken up with a 

 small quantity of very hot water. The caffeine slowly 

 separates from this aqueous solution in the form of 

 white silky needles. 



Caffeine, CgHjoN^Op, is a silky crystalline substance 

 melting at 233° C. It differs from the other alkaloids 

 in being a derivative of purine, and hence related to 

 uric acid. 



22. Estimation of Nicotine in Tobacco. 



The method devised by Kissling depends upon the fact that 

 nicotine in the form of the free base is volatile in steam, and can 

 be distilled over to give an aqueous solution which is alkaline and 

 can be directly titrated with acids. 



^ Basic lead acetate can be previously prepared by boiling a 

 solution of lead acetate with excess of PbO, and then filtering. 



