130 



HYDROGEN ION CONCENTRATION 



These values for pk' for acetic acid were determined electro- 

 metrically in mixtures each containing 0.02 A^ acetic acid + 0.02 

 Na-acetate and in addition 0.5 A^ of a neutral salt: 



In a mixture of 0.1 A'' Na-acetate with any concentration of acetic 

 acid the value of pk' was found to be 4.616. 



In a mixture of 0.02 N acetic acid + 0.02 N Na-acetate but free 

 of any other salt, pk' = 4.665. 



dv aoes aois 0.02 0.25 ooj aof 0.05 o.oe 0.07 o.os 0.09 0.1 o.iz5 0.15 



Fig. 16 



o.z 



0.3 



OM 0.5 0.6 0.7 asojf 



For entirely salt-free solutions of acetic acid pk = 4.733 (see table, 

 page 31). 



These figures show that in the first place, pk' of acetic acid in 0.5 

 N NaCl solution is 4.482 instead of 4.733, as it is in a salt-free solution, 

 and, secondly, that the effect varies with the different neutral salts. 



For the second dissociation constant of phosphoric acid Michaelis 

 and Kruger obtained pk' values shown in figure 16. The abscissa 

 represent the neutral salt content of the solution in terms of 

 normality, the ordinates are the pk' values. The limiting pk' value 

 for infinitely small salt content, assymptotically approached by all 

 the curves, is 7.10. 



It can be seen from these experiments with phosphoric acid that 

 the value of pk' is influenced not only by the concentration of the 



