76 THE CHEMICAL CONSTITUTION OF THE PROTEINS 



A 3-inch Buchner funnel is covered with a hardened filter paper of 

 such a size that it fits against the bottom and side walls ; the portion 

 of the paper against the side walls is folded into about twenty plaits 

 so that it fits snugly all round. 



The precipitate is poured into this pocket and the mother liquor re- 

 moved by suction and by pressing down the precipitate with a flattened 

 rod. 



The filtrate is returned to a beaker. 



Washing is effected with 10-12 c.c. of a solution containing 2-5 

 grammes of phosphotungstic acid arid 3*5 grammes of hydrochloric acid 

 per 100 c.c. ; this is first used to dislodge the particles remaining in the 

 flask ; it is then poured upon the precipitate which is stirred up until all 

 lumps are broken and until there is only a granular suspension. It 

 is then sucked dry as before. The washing is repeated three to four 

 times in this manner. Then the precipitate on the filter is washed 

 five to ten times with the same solution from awash bottle, commenc- 

 ing round the edges and sucking dry each time. 



It frequently happens that the later washings run through some- 

 what turbid ; these are filtered through a folded paper. All washings 

 are combined with the main filtrate. The washing must be continued 

 till the liquid is free from calcium ; I c.c. of the filtrate when tested 

 with oxalic acid in 3 per cent, sodium hydrate must give no turbidity 

 even after standing for several minutes. 



6. Treatment of the Phosphotungstate Precipitate. The precipitate 

 is removed with a spatula, as completely as possible, to a beaker of 

 over i litre capacity. The filter paper is then spread out on the bottom 

 of a basin and washed with water made just alkaline with a few drops 

 of 20 per cent, sodium hydrate. The small folded paper is treated in 

 the same way. The particles of precipitate are dissolved by the soda, 

 and any granules remaining in the original flask are dissolved in the 

 same way. The alkaline solutions and washings are poured into the 

 beaker containing the main bulk of the precipitate. The whole is then 

 carefully dissolved in soda by adding 50 percent, alkali, drop by drop, 

 with continual stirring. Phenolphthalein is added as indicator; as 

 soon as the solution becomes red, addition of the alkali must be stopped ; 

 if the colour disappears, more alkali, but only to the excess of three or 

 four drops, must be added. A red solution must finally result. 



The solution is diluted to 800 c.c. and the phosphotungstic acid is 

 removed by slowly adding, in portions of a few c.c., a 20 per cent, 

 solution of barium chloride, until a test portion gives an immediate 



