u8 THE SIMPLER NATURAL BASES 



the above methods is now evaporated to a small volume, when on 

 standing some bases, such as creatine, may crystallise out. Generally, 

 however, they are too soluble in water and must be separated by some 

 general precipitant. The most important reagent for this purpose is 

 phosphotungstic acid, introduced into physiological chemistry by 

 Drechsel. The acid is readily soluble in ether, in acetone and in 

 water. It precipitates all nitrogen bases from their aqueous solution 

 if the latter contains 5 per cent, by weight of sulphuric acid. Ammonia 

 is also precipitated and should therefore be expelled, if present in 

 quantity. It is important to employ a good preparation of phospho- 

 tungstic acid, such as that of Kahlbaum, which dissolves in water with 

 hardly any opalescence. A method for preparing the acid has been 

 given by Winterstein (" Chemiker Zeitung," 1 898, p. 539). In order to 

 obtain the bases from an aqueous solution, sulphuric acid is added to 

 the latter to make 5 per cent, and a concentrated aqueous solution of 

 phosphotungstic acid, which should also contain 5 per cent, of sulphuric 

 acid, is added until no further immediate precipitation occurs. After 

 standing for a day the precipitate is filtered off at the pump and 

 thoroughly washed with 5 per cent, sulphuric acid. Often the pre- 

 cipitate is partially or wholly soluble in acetone, and more readily in 

 a mixture of acetone and water. (Compare Wechsler, below.) By 

 pouring the solution of the precipitate into a large bulk of 5 per 

 cent, sulphuric acid, the phosphotungstates of the bases are reprecipi- 

 tated and in this way they can be purified more readily than by 

 washing at the pump. In synthetic work, and when only one or two 

 bases are present, a phosphotungstate may occasionally be crystallised 

 from a large volume of boiling water (for instance in the case of 

 iminazolyl-propionic acid). 



The bases are again liberated from their phosphotungstates by 

 means of baryta, finely powdered or dissolved in water. For this 

 purpose the phosphotungstate precipitate must be carefully suspended 

 in water in as fine a state of division as possible ; where possible it is 

 much quicker to dissolve the precipitate in dilute acetone and then 

 add an aqueous baryta solution. Wechsler [1911] recommends a 

 mixture of three volumes of acetone with four volumes of water ; this 

 dissolves arginine phosphotungstate to the extent of 120-130 per cent, 

 and of the histidine salt even 160 per cent, of its own weight, but 

 albumose phosphotungstates only to the extent of 2-7 per cent. The 

 precipitate of barium phosphotungstate and sulphate settles down 

 rapidly. Several drops of the clear supernatant fluid are sucked up 

 into a capillary pipette and tested on a glass plate. When they no 



