10 GENERAL METHODS 



ACIDIMETRY AND ALKALIMETRY l 



This covers the analysis of acids and bases. For the quantita- 

 tive determination of an acid solution it is necessary to start with 

 an alkali of known strength and vice versa. In acidimetry and 

 alkalimetry use is made of what is known as normal solutions. 



Normal Solutions 



A normal solution is one which contains in a liter the quantity 

 of active ingredient which will replace, combine with or oxidize 

 1 gm. of hydrogen. Or again a normal solution is one which 

 contains one " gram-equivalent " of the active reagent dissolved 

 in a liter of solution. By " gram-equivalent " is meant the 

 amount of substance corresponding to one gram-atom (1.008 

 gms.) of hydrogen. For convenience in computation the con- 

 centration of solutions used for volumetric purposes are expressed 

 in terms of their normality, i.e., a solution is 2 normal, \ normal, 

 3^ normal, etc. The letter N is used as an abbreviation for normal. 



The essential point in the employment of normal solutions is 

 that one normal solution is equivalent to any other normal solu- 

 tion. Thus 1 c.c. ^ N HC1 is equivalent to 1 c.c. iV N KOH is 

 equivalent to 1 c.c. iV N sulphuric acid, because each is equivalent 

 to an identical quantity of hydrogen. It follows, therefore, that 

 if one has a normal solution of either an acid or an alkali unknown 

 strengths of other acids and alkalies may be determined with ease. 



Since the molecular weight of sodium hydroxide (NaOH) is 

 40 and that of hydrochloric acid (HC1) is 36.46, it follows that 

 40 gms. of the former contain the same number of molecules as 

 36.46 gms. of the latter. If 40 gms. of sodium hydroxide and 36.46 

 gms. of hydrochloric acid are each dissolved in pure water sufficient 

 to make one liter of solution, each liter will contain the same 

 number of dissolved molecules. 



Mixing equal volumes of two such solutions is therefore the 

 same as bringing together practically the same number of the 

 two kinds of molecules, and the result is the instantaneous and 

 essentially complete transformation into sodium chloride (and 

 water). 



X NaOH+X HC1 = X NaCl+X H 2 O. 



1 Adapted from Folin: A Laboratory Manual of Biological Chemistry, 1919. 



