METHODS FOR QUANTITATIVE DETERMINATIONS. 255 



Normal solutions are generally designated by , deci-normal solu- 

 tions by , centi-normal solutions by ^; solutions containing 

 10 100 



2 

 twice the amount are designated as double normal, -; half the 



amount semi-normal, 



2t 



In some instances volumetric solutions are prepared which do not 

 belong to the above system of normal solutions, but are adjusted to 

 correspond to a certain unit of the special substance they are to act 

 upon. Such solutions are called empirical solutions; as an instance 

 may be mentioned Fehling's solution, used for the determination 

 of sugar. This solution is so adjusted that 1 c.c. decomposes or 

 indicates 0.005 gramme of grape-sugar. 



Different methods of volumetric determination. Of these we 

 have at least three, which may be called the direct, the indirect, and 

 the method of rest or residue. 



The direct methods are used in all cases in which the quantities of 

 volumetric solutions can be added until the reaction is complete : for 

 instance, until an alkaline substance has been neutralized by an acid, 

 or a ferrous salt has been converted into a ferric salt by potassium 

 permanganate, etc. 



In the indirect methods one substance, which canoot well be deter- 

 mined volumetrically, is made to act upon a second substance, with 

 the result that, by this action, an equivalent amount of a substance 

 is generated or liberated, which may be titrated. For instance : Per- 

 oxides, chromic and chloric acids when boiled with strong hydro- 

 chloric acid, liberate chlorine, which is not determined directly, but 

 is caused to act upon potassium iodide, from which it liberates the 

 iodine, which may be titrated with sodium thiosulphate. 



The methods of residue are based upon the fact that while it is im- 

 possible or extremely difficult to obtain complete decomposition 

 between certain substances and reagents, when equivalent quantities 

 are added to one another, such a complete decomposition is accom- 

 plished by adding an excess of the reagent, which excess is afterward 

 determined by a second volumetric solution. For instance: Car- 

 bonate of calcium, magnesium, zinc, etc., cannot well be determined 

 directly, for which reason an excess of normal acid is used for their 

 decomposition, this excess being titrated afterward by means of an 

 alkali. 



