412 ANALYTICAL CHEMISTRY. 



weighed substance until the reaction is completed. We also 

 speak of the standard or titer of a volumetric test-solution, when 

 we refer to its strength per volume (per liter or per cubic centi- 

 meter). 



Of the principal processes of titration, or of volumetric meth- 

 ods used, may be mentioned those based upon neutralization (acid- 

 imetrv and alkalimetry), oxidation and reduction (permanganates 

 and chromates as oxidizing, oxalic acid and ferrous salts as 

 reducing agents) precipitation (silver nitrate by sodium chloride), 

 and finally those which depend on the action of iodine and thio- 

 sulphate (iodimetry). 



The substance to be titrated should be diluted with pure water to a 

 volume of about 75 c.c. The relationship between any two volumetric 

 solutions, for example, acid and alkali, should also be determined in 

 the same volume. Any convenient quantity, as 10 or 20 c.c., of one 

 solution is drawn from a burette into a beaker and diluted to about 

 75 c.c. before titrating with the other solution. If the comparison is 

 made in a much smaller or much greater volume, a somewhat different 

 relationship will be found. In general, the titer of a volumetric solu- 

 tion should be determined in a volume corresponding approximately to 

 that in which the titration of a substance is to be carried out. This 

 is usually the volume stated above, but sometimes, for certain reasons, 

 it may be much greater. 



Acidimetry and alkalimetry. Preparing the volumetric test- 

 solutions is often more difficult than to make a volumetric deter- 

 mination. Whenever the reagents employed can be obtained in a 

 chemically pure condition it is an easy task to prepare the solution, 

 because a definite weight of the reagent is dissolved in a definite 

 volume of water. In many instances, however, the reagent cannot 

 be obtained absolutely pure, and in such cases a solution is made and 

 its standard adjusted afterward by methods which will be spoken of 

 later. 



Neither the common mineral acids, such as sulphuric, hydro- 

 chloric, and nitric acids, nor the alkaline substances, such as sodium 

 hydroxide or ammonium hydroxide, are sufficiently pure to permit of 

 being used directly for volumetric solutions, because these substances 

 contain water, and an absolutely correct determination of the amount 

 of this water is an operation which involves a knowledge of gravi- 

 metric methods. 



