G. Williams 203 



A bulky precipitate (unlike lead sulphate) difficult to filter, separated 

 out. The excess of lead was now removed by means of hydrogen 

 sulphide. The precipitate came down in a very fine state of division, 

 and absolutely refused to filter clear. The hquid now became quite 

 blue in colour. This must have been due to an insufficiency of lead 

 acetate having been added leaving some phosphotungstic acid still 

 remaining in the liquid. In order to remove the blue colour and any 

 remaining phosphotungstic acid, baryta water was added until the 

 solution became alkaline, the precipitate filtered off and the excess 

 of barium removed quantitatively by means of sulphuric acid. 



Levene and Van Slyke recommend the lead method of removing 

 the phosphotungstic and sulphuric acids because less occlusion takes 

 place than when barium is used. In this case both lead and barium 

 had to be used. The result was that when the final filtrate was 

 evaporated down to dryness and dried in a vacuum desiccator over 

 sulphuric acid and caustic potash, it weighed only 3-7 grms. — a yield 

 of less than 40 per cent. 



Determination and isolation of the valine. 



The decanted filtrates and washings from the alanine phospho- 

 tungstate were diluted down to 2000 c.c. and the amino nitrogen 

 determined in 10 c.c. 



A blank determination with 10 c.c. of 10 per cent, sulphuric acid 

 gave 0-9 c.c. N ; 10 c.c. of solution gave 16-45 c.c. N at 16° C. and 764 mm. 



- 15-34 c.c. at 0°C. and 760 mm. 



= 7-67 c.c. from amino groups. 

 2000 c.c. would give 15-35 c.c. 



— 1-93 grains N. 

 1-93 grms. of nitrogen correspond to 16-13 grms. of vahne. 



The weight of nitrogen in the 25-23 grms. of mixture taken for this 

 separation was 25-23 x 0-142 grm. = 3-58 grms. The nitrogen in the 

 alanine and vahne liquids as determined by Van Slyke's method was 

 1-93 + 1-53 grms. = 3-46 grms. A loss of 0-12 grm. nitrogen had taken 

 place during the manipulations. 



The total weight of alanine and vahne indicated as present is 

 9-73+ 16-13 grms. = 25-86 grms., a weight 0-63 grm. more than the 

 mixture originally taken. This discrepancy is due to the fact that some 

 alanine is left with the valine in solution. It is recommended to carry 

 out a second separation to remove this residual alanine. 



Journ. of Agric. Sci. viii l^t 



