Chemistry. — 'The second dissociation constant of sul [)hoacetic 

 and a-sidphojiropionic acids." By Prof. H. J. Backer. (Com- 

 municated hy Prof. F. AI. Jakgeh;. 



rCommunicated at the meeting of January 27, 1923). 



The determination of the second dissociation constant of a dibasic 

 acid H,A from the concentration of the hydrogen ions in the solution 

 of an acid salt readily suggests itself. 



However, A. A. Notes ') has shown, that generally these data 

 will not suffice. 



Suppose that the ionisation of the acid sodium salt (reaction I) is 

 nearly complete, and that the concentration of the HA' ions, which 

 according to reaction II are partly split further, may be ideutitied 

 with the concentration of the acid salt dissolved, we must nevertheless 

 remember that the number of hydrogen ions will decrease by 

 combination with the ions HA' (reaction 111). 



NaHA-^Na+HA' (I HA':;^H-+ A" (H) HA' + H^i^^H^A (111) 



This last reaction will be especially noticeable, when the acid 

 is weak, which is indeed the case with all organic acids examined 

 in this respect. 



The sulpliocarboxylic acids, however, are examples of strong 

 dibasic acids, which at small dilutions are already well ionised. 

 Therefore, we may expect, that the consumption of hydrogen ions 

 for formation of the free acid will only have a small influence, so 

 that from the concentration of the hydrogen ions the degree of 

 dissociation of reaction 11 may be determined, and further the 

 dissociation constants. 



NoTKS has given the following general formula for the acid salts 

 of dibasic acids: 



^ {k,+c + H)H^ 

 k, ic-H) ■ 



k, and k, are the first and second dissociation constants of the 

 acid, c is the original concentration of the acid salt in gram 

 molecules per litre) and H is the concentration of hydrogen ions 

 (ill gram ions per litre). 



1) Z. f. physik. Chemie 11, 495 (1893). 



