48 PROXIMATE CONSTITUENTS OF PLANTS 



The citric acid is neutralised by chalk, and the calcium citrate 

 formed is precipitated by boiling. The calculated quantity of 

 dilute H2SO4 is then added to the calcium citrate, the calcium 

 sulphate so formed is removed, and the solution of citric acid is 

 evaporated until it crystallises. 



Three or four lemons are cut in half and the juice 

 squeezed into a small beaker. The liquid is diluted 

 with about an equal volume of water, and the solution 

 boiled for some time to coagulate albuminous substance^. 

 It is then filtered while hot. 



The volume of the liquid is carefully measured 

 by means of a graduated flask (100 c.a) and a burette, 

 an aliquot part (10 c.c.) removed, and the citric acid 



N 

 present in it estimated by titration with — NaOH 



solution, using phenolphthalein as indicator. 



The amount of standard HgSO^ equivalent to the 

 citric acid in the remaining volume of juice is then 

 calculated. This quantity of HgSO^ is that added to 

 the calcium citrate in a later stage of the preparation. 



Powdered chalk is then stirred into the liquid in 

 small quantities at a time, excessive frothing being 

 avoided, until a slight excess of the chalk has been 

 added, as rendered evident by the fact that some of the 

 chalk remains undissolved at the bottom of the vessel. 



The solution containing calcium citrate is poured 

 ofif from the undissolved chalk, and boiled vigorously 

 for a few minutes. The calcium citrate is thus 

 rendered insoluble, and falls as a thick white crystal- 

 line precipitate. 



This precipitate is filtered off at the pump on a 

 Biichner funnel, and well washed with boiling water. 



The washed calcium citrate is transferred, while 

 still wet, to a beaker, and the calculated quantity of 

 standard acid (above) run on to it from a burette. 



