90 



PROPERTIES OF CERTAIN AMINO-ACIDS. [CH. IV. 



(iii.) Test the reaction of the solution by means of litmus paper. 

 If it is still acid add a hot solution of baryta until 

 alkaline. 



(iv.) Pass in a stream of sulphuretted hydrogen gas, shaking at 

 intervals, until the mixture is fully saturated. 



(v.) Heat on the water bath to about 50 C., shake well, and 

 pass in more of the SH 2 if the odour of the gas is not 

 perceptible. 



(vi.) Filter from the mixture of mercuric sulphide and barium 

 sulphate. 



(vii.) Wash the precipitate with hot water, and squeeze the 

 paper in a piece of muslin. Filter these washings, etc., 

 through another small paper, and add them to the bulk 

 of the fluid obtained in (vi.). 



viii.) Add a few drops of 5 per cent, sul- 

 phuric acid. If a white precipitate 

 of barium sulphate is obtained, it 

 indicates that an excess of baryta 

 had been added, and that barium 

 sulphide is present. Continue to 

 add the dilute sulphuric acid until no 

 further precipitate is obtained. Filter 

 off the barium sulphate. If the addi- 

 tion of the dilute sulphuric acid 

 does not cause a precipitate, proceed 

 directly to : 



Remove the sulphuretted hydrogen by 

 a strong current of air, using the 

 apparatus shown in fig. 12. (It is 

 preferable to remove the SHg by dis- 

 tillation in vacuo at 45 C.) 



Fig. 12. Apparatus for 

 removal of SH2 by 

 means of an air 

 current. 



E. Removal of cystine and reprecipitation of the tryptophane. 



(i.) Measure the fluid and add 14 cc. of 50 per cent, sulphuric 

 acid for every 86 cc. 



