DISSOCIATION OF ACIDS AND BASES 



143 



The curves in figure 23 below give an idea of the change in the 

 p-curve in the presence of salts. Here we have a solution of an 

 ampholyte with ka = 10~^ and kb = 10"^^ Its p-curve may be 

 thought of as being the composite of the p-curves of an acid with 



ka = 10-^ (marked pa on the curve) and of a base with kb = TTZTi — 



10~^ (marked pb on the curve), pmax corresponds to pH 5. Let us 

 suppose that changes in pH are brought about by the addition of 

 buffer mixtures which do not form true salts with the ampholyte. 

 Now let us add to our solution a salt whose anion will combine with 

 the ampholyte to form a salt with the constant ki = 0.1, and whose 

 cation will likewise form a salt with the constant ka = 0.01. This 

 will result in the displacement of the pa curve to p'a and of the pb- 

 curve to p'b- The total new curve can be approximately pieced 

 together from the curves p'a and p'b and by rounding out the angle 



Fig. 23 



at which these two meet but leaving the maximum above the same 

 point on the abscissa. In this way the lowest curve of the figure 

 (drawn with the — . — . — line) is obtained. The maxima of the 

 original and of this latter curve are indicated by small circles. We 

 see that the maximum has been changed in a twofold way by the 

 addition of the salt. In the first place, its ordinate value became 

 smaller, and this is because the p'a and p'b-curves are situated closer 

 to each other than the pa- and Pb-curves, secondly, the maximum 

 was displaced horizontally towards the right, and this is because the 

 values of ki and kn are different. In the case where ki = kn 

 only the first ordinate displacement would occur. But in this case 

 the p maximum is in the first place diminished in its absolute value, 

 and, secondly, it is displaced to a different pH value. 



On superficial consideration it might be supposed that these 

 maxima represent the two isoelectric points, and consequently that 

 the isoelectric points were shifted by the addition of the salt. But 

 this is not the case as we shall at once see. 



