CHAPTER II 



The Theory of the Quantitative Determination of Acidity 



AND Alkalinity 



summary of contents 



For the characterization of the acid nature of a solution it is necessary 

 to know: (1) The titratable acidity which may he further subdivided into 

 neutralizing capacity and equivalent capacity, (2), the hydrogen ion 

 concentration or pH value, and (3) the buffer value. 



The theory of indicators and of the titration curve is developed for the 

 elucidation of the concept of titratable acidity; the principles of the most 

 important methods for the estimation of the pH are then briefly outlined; 

 and finally the conception of buffer value is developed. 



26. Definitions and statement of problem 



The acidity of a solution is defined by a number of values, and the 

 sharp demarcation and differentiation of these values has become 

 possible with the advent of physico-chemical methods. In the older 

 anal3rtical chemistry these values were either not differentiated at all, 

 or else to an insufficient extent. It is true that in previous decades 

 excellent methods were found for the estimation of acidity and 

 alkalinity in the form of the titration, but the reason for this is due to 

 the fact that for strong acids and bases the various values which 

 define acidity practically coincide. Furthermore, for certain of the 

 weaker acids and bases some indicators had been empirically found, 

 such as phenolphthalein and methyl orange, which enabled our prede- 

 cessors to determine the equivalent content of these acids and bases. 

 Thus experiment had demonstrated that the equivalent content of an 

 acetic acid solution could be correctly determined by a titration 

 against a standard NaOH solution with phenolphthalein, that an 

 incorrect value was obtained when methyl orange was used as an 

 indicator in the same titration, and that the reverse was true for the 

 titration of ammonia against sulfuric acid. At any rate, the acidity 

 of a solution had been defined by its content of acid equivalents. 



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