LAWS OF ELECTROLYTIC DISSOCL\TION 



51 



The curve is distinguished by the following characteristics: 



1 . This curve can be divided into three parts : The first and third 

 portions run asymptotically to the X-axis, the first at the height 

 0, the third at the height 1. 



2. Between these two portions lies an almost straight-line steep 

 portion which passes into the as^nnptotes at sharp angles of in- 

 flection. 



3. Strictly speaking this middle portion is not a straight line, but 

 is slightly curved or S-shaped, and it has a point of inflection at the 

 ordinate height of |. The projection of this point on the abscissa 

 gives the negative logarithm of the dissociation constant k. If we 



0.5 





*» 5 6 7 8 9 10 If 



Fig. 3. Dissociation curve of an acid whose constant is k = 1 X 10 ~^. 

 Abscissa-pH, ordinate-a. The scale of the ordinates is enlarged X 5 with 

 respect to the scale of the abscissa. 



At the point marked x a = f, and the projection of the ordinate on the 

 abscissa gives the value of pK (the negative logarithm of the dissociation 

 constant). 



state, analogously with the previously used method, — log k = pK, 

 then it may be said that the parameter of this function is pk, 

 and that geometrically it means that it denotes that value on the 

 X-axis (pH-axis) for which the ordinate = \. If similar disso- 

 ciation curves be drawn for various values of pK (therefore for acids 

 of various strengths) then all such curves will be parallel. They 

 may be graphically derived from the given curve by simply moving 

 it horizontally on its abscissa until the projection of its ordinate 

 value of \ coincides with the value of pK of the particular acid. 



4. Finally, this graphic representation has the special advantage 

 insofar as the dissociation-residue curve is an exact mirror image 

 of the dissociation curve. 



