METHODS FOR QUANTITATIVE DETERMINATIONS. 257 



or liter of a volumetric test-solution, when we refer to its strength 

 per volume (per liter or per cubic centimeter). 



Of the principal processes of titration, or of volumetric methods 

 used, may be mentioned those based upon neutralization (acidimetry 

 and alkalimetry), oxidation and reduction (permanganates and chro- 

 mates 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 hyposulphite (iodimetry). 



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. 



It is for this reason that the basis in preparing a volumetric normal 

 acid solution is oxalic acid, a substance which can be readily obtained 

 in a pure crystallized condition. 



Normal acid solution. Crystallized oxalic acid has the com- 

 position H 2 C 2 O 4 .2H 2 O and a molecular weight of 125.7. Being 

 dibasic, only half of its weight is taken for the normal solution, 

 which is made by placing 62.85 grammes of pure crystallized oxalic 

 acid in a liter flask, dissolving it in pure water, filling up to the 

 mark at the temperature of 15 C. (59 F.) and mixing thoroughly. 



Normal solutions of sulphuric or hydrochloric acid are, for various 

 reasons, often preferred to oxalic acid. These solutions are best 

 made by diluting approximately the acids named, titrating the solu- 

 tion with normal sodium hydroxide, using phenol-phtalein as an 

 indicator, and adding water until equal volumes saturate one another. 

 For instance, if it should be found that 10 c.c. normal alkali solution 



17 



