74 F. B. GUTHRIE AND A. A. RAMSAY. 



be diluted with water, turmeric gives an 'acid reaction and 

 about *5 cc. N/10 NaOH are required to neutralize it. This 

 may be due to ionization of the solution on dilution, but it 

 is strange that other indicators do not show the same 

 phenomenon. 



It will be seen from the above that none of the methods 

 recommended are quite satisfactory, and that extraction 

 with ether, subsequently converting into an aqueous solu- 

 tion and titration with methyl orange (Herzfelder's method) 

 though the best of those recommended gives only approxi- 

 mate results in the case of phosphoric acid, while the acid 

 phosphates are not neutral towards it. 



By far the sharpest indicator is the sodium salt of alizarin 

 sulphonate which is prepared by acting upon alizarin with 

 fuming sulphuric acid, and converting the alizarin sulphonic 

 acid into a sodium salt by neutralizing with sodium car- 

 bonate. This indicator gives extremely sharp end-reactions 

 with free phosphoric acid, either in watery or ethereal 

 solution, and is almost neutral towards the acid sodium 

 and calcium salts. 



A modification of Herzfelder's method, in which ether is 

 used to dissolve out the free phosphoric acid, the ethereal 

 solution converted into an aqueous one, and the free acid 

 titrated with standard alkali using sodium alizarin sul- 

 phonate as an indicator, will probably prove to be the best 

 method for determining the amount of free phosphoric acid 

 in superphosphates. Further experiments will, however, 

 have to be conducted before this method can be safely 

 recommended. 



