CH. IV.] HISTIDINE. 85 



(viii.) Place the solution in an evaporating basin, make it 

 distinctly alkaline by the addition of caustic soda, and 

 boil for 30 to 60 minutes to remove ammonia. This is 

 tested by holding a moist litmus paper in the vapour. 

 The removal of ammonia is hastened by adding a small 

 volume of alcohol. 



(ix.) Pour the solution into about 5 litres of water contained in a 

 large vessel. 



(x.) Add a hot saturated solution of mercuric chloride in water 

 until no further precipitate is obtained, keeping the 

 solution sufficiently alkaline to ensure complete precipi- 

 tation. Usually about 100 gms. of mercuric chloride 

 are required. 



(xi.) Allow the mercury compound of histidine to settle over- 

 night. 



(xii.) Syphon off the supernatant fluid. 



(xiii.) Filter off the precipitate on a large Buchner funnel and 

 wash it with cold water. 



(xiv.) Transfer the precipitate to a porcelain dish and add hot 

 hydrochloric acid (25 per cent, by volume) as long as any 

 of the precipitate goes into solution, avoiding any large 

 excess of acid. 



(xv.) Filter from the insoluble residue of calomel and wash this 

 with cold water, adding the washings to the bulk of the 

 fluid. 



(xvi.) Dilute the fluid to about 5 litres with distilled water. 



(xvii.) Dissolve 20 grams, of mercuric chloride in water and add 

 this to the fluid. 



(xviii.) Make the fluid markedly alkaline by the addition of 

 caustic soda. 



(xix.) Allow the voluminous precipitate to settle over-night. 



(xx.) Filter on the pump, drain, and wash thoroughly by grind- 

 ing with water in a mortar and filter again. 



