48 



HYDROGEN ION CONCENTRATION 



= 0.003, Ai = 0.0033 and ki is given as = 3 X 10-^ We now 

 add to this solution an indicator, m-nitrophenol (k2 = 5 X 10~^) 

 in the concentration of A2 = 0.003 N. In reality barely one half of 

 this amount is actually needed, although, as it happens, m-nitro- 

 phenol possesses weak tinctorial power, and much more of this dye 

 is required than in the case of the other indicators. If the [H+] 

 of tlie solution before the addition of the indicator is calculated from 

 (la), then we obtain on the basis of the given data the value [H+] 

 = 3 X 10~^ After the addition of the indicator we obtain by the 

 use of (le) [H+] = 5.83 X lO-^. 



In a similar way, assuming various values for the concentration 

 of the indicator, A2, and retaining all the other data, the figures in 

 table 11 are obtained. 



TABLE 11 



In using m-nitrophenol one employs, for example, a 0.3 per cent, 

 or, 0.027 molar solution. In adding 0.3 cc. to 20 cc. of conductivity 

 water the concentration of the indicator becomes in round numbers 

 0.0004 N, and the pH error due to the indicator is —0.06, a value 

 falling well within the experimental limits of error. In reality much 

 smaller amounts of indicator suffice. In the case of all other indi- 

 cators which have greater coloring powers and which can be used 

 in much smaller concentrations this error is certainly of no sig- 

 nificance. 



15. The degree of dissociation and the dissociation-residue of acids 



The degree of dissociation of an acid in a pure aqueous solution is 

 the term given to the ratio of the dissociated portion to the total 

 amount of the acid. This definition will be extended so that it 

 may also embrace the acid-salt mixtures. Restating then, by the 

 degree of dissociation of an acid we understand in general the ratio 



