CH. IV.] TRYPTOPHANE. 91 



(ii.) Cautiously add the mercuric sulphate reagent prepared 

 as described in C (iii.) above. Add this until a slight 

 definite precipitate is obtained. Usually about 15 cc. are 

 required. Allow the mixture to stand for 10 minutes. 

 Filter. 



(iii.) To the filtrate add 80 to 100 cc. of the mercuric reagent, 

 and allow the mixture to stand over-night. 



(iv.) Filter on a Buchner funnel, wash with cold 5 per cent, 

 sulphuric acid, and then thoroughly with water. 



(v.) Suspend the precipitate and paper in 50 cc. of water. 



(vi.) Add 2 grms. of barium hydroxide dissolved in 70 cc. of 

 boiling water. 



(vii.) Decompose by SH 2 and proceed as in D (iv.) to D (ix.), 

 taking care to have only a very slight amount of free 

 sulphuric acid present. 



F. Removal of sulphuric acid. 



(i.) Heat the fluid on a boiling water bath, and add a hot solu- 

 tion of baryta to a point when no further precipitation 

 can be seen. 



(ii.) Filter a portion until a clear filtrate is obtained, and test 

 with a drop or two of the baryta solution. If no pre- 

 cipitate is obtained, test another portion of the hot 

 nitrate with a drop of dilute sulphuric acid. Should this 

 fail to give a precipitate also, the correct point is reached, 

 being that at which neither sulphuric acid nor baryta 

 gives a precipitate. Baryta or sulphuric acid must be 

 added until this condition is attained, the samples tested 

 being added to the bulk. The process is much facilitated 

 if the relative concentrations of the alkali and aeid are 

 roughly determined against one another. Filter to 

 obtain a perfectly clear nitrate. 



(iii.) Add a single drop of 10 per cent, ammonia. 



