METHODS FOR QUANTITATIVE DETERMINATIONS. 409 



taken for a liter of normal solution, because this salt contains but one atom of 

 aodium in the molecule. 



A normal salt solution may be defined as one containing in a liter the quan- 

 tity of salt resulting from the neutralization, or replacement of the hydrogen 



in a normal acid solution by metal. Thus, ^^^ Na (j^ AgNO 3 , BaC1 2 ? e x- 



L 2 



pressed in grammes, would be contained in a liter of normal solution of the 

 respective salts. 



Often normal solutions are too strong, and are diluted ten or a hundred 

 times. They are then called deci- or centi-normal solutions, respectively. 



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

 tions by ~ y centi-riorraal solutions by ^ ; solutions containing twice 

 the amount are designated as double normal, | ; half the amount 

 semi-normal, *. 



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 cannot well be deter- 

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

 the result that, by this action, an equivalent quantity 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. 



Indicators. In all cases of volumetric determination it is of the 

 greatest importance to observe accurately the completion of the reac- 



