PHYSICAL CHEMISTRY 25 



Titratable Acidity. It often is desirable to know the total 

 amount of an acid or alkali present in a solution, regardless of 

 the extent to which it is dissociated. The standard used in dis- 

 cussing this value is the normal solution. A normal solution 

 of an acid is one which contains per liter one gram equivalent 

 (1.008 grams) of replaceable hydrogen. This hydrogen need not 

 be ionized, but only ionizable. If 36.458 grams of hydrochloric 

 acid is dissolved in water and made up to 1 liter, this will contain 

 1.008 grams of ionizable hydrogen, and will be a normal solu- 

 tion. Any decimal fraction of this strength also may be pre- 



N 

 pared as (0.1 N), etc. If the acid is sulphuric, one-half 



the molecular weight will be used, as it will furnish 1.008 grams 

 of replaceable hydrogen. This is more fully discussed, and 

 directions are given for making up normal solutions in the 

 laboratory section. A normal solution of an alkali is one which 

 will neutralize volume for volume a normal solution of an acid. 

 If the acid were completely ionized, it would be easy to com- 



N 

 pute the pH of any normality. Thus, in a-y/r- acid there is 0.1 



gram ions of hydrogen per liter. The C H is 10' 1 the pH is 1. 



N 

 To calculate the pH of a-y^r alkali one falls back on the water 



+ 10- 14 



constant H X OH = 10' 14 . If H = 10' 1 then OH = ^ = 10' 13 . 



N 



The amount of hydrogen ions in a -r^r sodium hydroxide solu- 

 tion is thus 0.000,000,000,000,1 or 10' 13 and the pH is 13. This 



is evident, for if the equation H X OH = 10' 14 holds true for 



water solutions, an increase in either H or OH will of course 



N 

 cause a decrease in the other (the figures given for acid 



and alkali assume complete dissociation, which really does not 



