DETERMINATION OF ACIDITY AND ALKALINITY 95 



Therefore, according to this definition a normal hydrochloric acid 

 and a normal acetic acid solution were of exactly the same acidity. 

 On the other hand, it could not be entirely ignored that these two 

 equivalent acid solutions were in some respects quite different, and a 

 stronger acidic property had to be conceded to the hydrochloric acid 

 than to the acetic acid. There were obviously two entirely different 

 properties which were designated as acidity, and only one of these 

 properties could be quantitatively estimated by means of the titra- 

 tion. For the other property there were only qualitative approxima- 

 tions. Thus it was known, for instance, that normal hydrochloric 

 acid attacked and dissolved zinc better than normal acetic acid, that 

 the former changed congo-red paper to a bluer tint, that it had a more 

 corrosively acid taste, that it inverted cane sugar more rapidly, etc. 



By applying the principles of the ionic theory we may now define 

 acidity quantitatively in three ways, which do away with the above 

 described uncertainties. Furthermore, we shall learn of a fourth 

 value or factor which is necessary for the complete characterization 

 of the acid properties of a solution. 



One of our new definitions, "titratable acidity," corresponds some- 

 what to the older definition. It is the number of equivalents of free 

 acid in the solution. But as soon as we wish to define the methods 

 by which these equivalents are determined, we find that the older 

 conception is inadequate. For, if we were to ask a chemist of a few 

 decades ago to what endpoint was an acid solution to be titrated he 

 would say : until the neutral reaction was reached ; and as a means of 

 recognizing this point he would probably suggest the change of color 

 of phenolphthalein. We now know that such a reply contains two 

 errors. In the first place, a solution consisting of equivalent parts of 

 acetic acid and NaOH, or of sodium acetate, is not neutral, but, 

 because of the ensuing hydrolysis, it is somewhat alkaline (see table, 

 page 80). Secondly, the turning point of phenolphthalein is not at 

 the neutral, but at an alkahne reaction. Therefore, our analyst 

 will in the end be practically quite correct, in spite of his erroneous 

 explanation. But he could not have explained why he should not 

 have just as well used methyl orange in his titration. At best he 

 would say that "methyl orange is not sensitive enough to acetic 

 acid" without being able to explain why it was more sensitive to 

 hydrochloric acid. 



In reahty the endpoint of the titration of acetic acid is not at the 



