50 THE CHEMICAL CONSTITUTION OF THE PROTEINS 



lead sulphate. The excess of lead is removed with hydrogen sulphide, 

 the lead sulphide washed with water containing hydrogen sulphide and 

 the liquid is evaporated in vacuo to a small volume. It is transferred 

 to a Jena glass dish, evaporated until all liquid disappears and the 

 alanine dried by placing in a vacuum desiccator over sulphuric acid 

 and caustic potash. The substance becomes discoloured if it be dried 

 on the water-bath. The product is nearly ash-free alanine, but it may 

 contain glycine, if glycine was present in the original mixture. The 

 rest of the alanine in the mixture is obtained after isolating the valine. 



The phosphotungstic acid filtrates and washings containing the 

 valine is made up to a known volume and the amount of valine esti- 

 mated by a nitrogen determination by Van Slyke's method, using 10 

 per cent, sulphuric acid instead of water in a blank determination. 

 The reagents are removed with lead acetate as described above under 

 alanine, and the solution evaporated till valine begins to crystallise ; 

 2-3 volumes of 80 per cent, acetone are added and the mixture 

 washed into a flask with 80 per cent, acetone ; the flask is stoppered 

 to prevent evaporation of the acetone and allowed to stand for twelve 

 hours. Valine crystallises out ; it is filtered off and washed with 80 

 per cent, acetone. Its yield is 80-85 P er cent, of the amount found 

 in the mixture by analysis. 



The filtrate is evaporated to dryness and the above separation re- 

 peated. It is then practically quantitative. 



