ACIDITY 163 



Active acidity refers only to the concentration of hydrogen ions present 

 in the material. Hydrogen ions, it will be remembered, are produced by 

 ionization of an acid, for example, 



CH3COOH ^ CH3COO- + H + 



The hydrogen-ion concentration is expressed in terms of pH, which will 

 be considered later. 



Total acidity, on the other hand, refers to the total amount of acid 

 present, both ionized and nonionized. It is usually expressed as per 

 cent by weight ; for example, vinegar contains 4 to 5 per cent acetic acid. 



TOTAL ACIDITY 



Determination of total acidity or alkalinity is important when one 

 needs to know how much of some material is required to react with the 

 acid or alkali in another material. The determination usually is accom- 

 plished by titration with standard solutions, that is, by measuring the 

 volume of base or acid that is required to react with a given amount 

 of the sample. The first requisite for such an analysis, therefore, is a 

 standard solution, which is simply a solution of known concentration. 

 The concentration of standard solutions is usually expressed in terms 

 of molarity or normality. 



Molar solutions 



By definition a molar solution is of such concentration that one liter 

 of the solution contains exactly one gram molecular weight, or one 7nole, 

 of the solute. Any given fraction of a liter of such a solution, there- 

 fore, would contain an equivalent portion of a gram molecular weight. 

 Hence, withdrawal of aliquots from such a standard solution offers a 

 speedier method of obtaining a known weight of reagent than can be 

 effected by the process of weighing. Moreover, the accuracy is much 

 greater than that which can be attained by weighing minute quantities 

 of material. 



Normal solutions 



Since a mole of one compound may react with one, two, or more moles 

 of another compound, or with only a fraction — one-half, one-third, and 

 so on — of a mole of the second compound, molar solutions are seldom 

 convenient to use. This is not true of normal solutions, which are so 

 prepared that a given volume of a solution of one compound is equivalent 

 to exactly the same volume of solution of any other compound. This 

 is very clearly evident in the case of acids and bases. In displaying 



