96 HYDROGEN ION CONCENTRATION 



neutral reaction, but at the hydrion concentration corresponding to 

 that of a solution of sodium acetate, which in turn corresponds to 

 the turning point of phenolphthalein, but not to that of methyl 

 orange. If we should wish to attribute a significance to the actual 

 point of neutrality then we would have to differentiate two endpoints 

 in our titration: first, the attainment of the neutral point and 

 secondly the reaching of the point of equivalence. The number of 

 equivalents of the base which were used up in reaching the neutral 

 point may be designated as the neutralizing capacity and the number 

 of base equivalents used to reach the point of equivalence as the 

 equivalent capacity, or, the true titration capacity. It is only when a 

 strong acid is titrated against a strong base that these two conceptions 

 coincide. We must now learn to know the methods by means of 

 which these two values are determined. 



1 . The determination of neutralizing capacity 



For the estimation of this point an indicator is needed which shows 

 a definite color change at the neutral reaction. Such indicators are, 

 litmus (solution), neutral red, or phenol red. Exactly neutral water 

 is colored with litmus, and the investigated acid solution is titrated 

 until it shows the same blue-violet shade of color of its litmus as the 

 standard solution. 



Since it is very difficult to obtain really pure water, it is more prac- 

 ticable to employ instead of the water a mixture consisting of 6.9 

 volumes of M/15 Na2HP04 and 3.1 volumes of M/15 KH2PO4 solu- 

 tions. The reaction of such a mixture is exactly neutral at 18°, 

 according to S0rensen. 



The determination of the neutralizing capacity is of little practical 

 importance. 



2. The determination of the equivalent capacity 



For the discussion of this topic a . preliminary and more detailed 

 explanation of the theory of indicators is required, which follows. 



27. The theory of indicators 



The indicators are acids or bases whose shade of color is changed 

 by the hydrion concentration of the solution. Wilhelm Ostwald' 



1 W. Ostwald, Lehrb. d. allgem. Chem., 1891, p. 799 and Z. physikal. Chem 

 9, 579 (1892). 



