ACIDITY 



165 



of solutions is indicated by a number followed by M, and normality 

 by a number followed by A''. 



Standardization of solntions 



It is not always possible to prepare standard solutions by weighing 

 out the amount of reagent theoretically required, because many substances 

 are not obtainable in sufficient purity, and others take up water or 

 carbon dioxide when exposed to the air during the time required for weigh- 

 ing. Solutions of such substances can, however, be "standardized" by 

 titration against a solution of known concentration prepared from a 

 "primary standard," that is, a substance that can be obtained in a high 

 state of purity and conveniently weighed. Frequently, normal solutions 

 are too concentrated for accurate measurement of the limited amount 

 of acid or base in the substance that is being analyzed. In general prac- 

 tice, 0.1 normal solutions are quite satisfactory. 



Titration is a process of measuring the volume of one solution that is 

 required to react exactly with a definite amount of a second solution. 

 In titrating acids and bases the point at which the reaction is completed 

 is revealed by the color of an "indicator," which is added before the 

 titration is started. The selection of a suitable indicator is explained 

 below. In 'practical work equal volumes of solutions of the same 

 normality are considered to react exactly with one another. If the two 

 solutions do not have the same normality, it takes proportionately more 



