in Aqueous Salt- Solutions. 



383 



Allusion has already been made to the fact that hydrogen 

 disodium phosphate is only slightly hydrolysed when dissolved 

 in water. This salt behaves rather differently from the others. 

 During the saponification the solution, which is at first 

 alkaline, becomes neutral and then acid. 



For the sake of comparison with trisodium phosphate, the 



N 

 following set of experiments was made with a ^ (mol.) solu- 

 tion of hydrogen disodium phosphate at 24 0, 2 C. The solution 

 was prepared from a sample of the salt obtained from Kahl- 

 baum, and had a slightly alkaline reaction. 



The titre of the solution during saponification was as 

 follows : — 



20 



(mol.) Hydrogen Disodium Phosphate. 



Time. 



Titre. 





min. 











006 



?%jy g.-inol. per litre (alkaline). 



2 



004 



>i >> 



5 



000 



Neutral. 



32 



0-06 



T lv g.-mol. per litre (acid). 



120 



010 



>> >» 



226 



015 



>i >> 



380 



0-17 



t> )> 



1380 



030 



♦» »» 



2815 



0-41 





4500 



0-52 



»> >> 



The titrations were made with phenol phthalein. 



At the commencement the solution contained 0*0003 gram- 

 molecule of free soda per litre. 



The case is a very complicated one, but possibly the bulk of 

 tins alkalinity is due to the formation in the solution of tri- 

 sodium phosphate ; there is still, however, a minute amount 

 of free alkali in the solution, owing to the hydrolysis of 

 hydrogen disodium phosphate. The following attempt has 

 been made to measure it, but no stress must be laid on the 

 results. I give the calculations here merely for the sake of 

 comparison with trisodium phosphate, and to show that at the 

 best there is not much free alkali in the solution. The calcu- 

 lations were made with the help of equations (6) and (7). It 

 is evident, of course, that what we measure here is the quan- 

 tity x. The initial concentrations of the salt and ester, 

 expressed in hundredths of a gram-molecule per litre, are 

 4-76 and 48-76. 



2D2 



