[Vol. 9 



354 ANNALS OF THE MISSOURI BOTANICAL GARDEN 



by the ionic theory of Arrhenius (II, *13)\ According to this 

 theory when a strong electrolyte dissolves in water in a dilute so- 



hly dissociated 



Hydrochlo 



ric acid, for example, breaks up into positive -f- H ions and nega- 

 tive — CI ions, and sodium hydroxide breaks up into -f~ Na 

 ions and — OH ions. When a solution of hydrochloric acid is 

 mixed with a solution of sodium hydroxide the hydrogen ions 

 combine with hydroxyl ions to form water and in this union neu- 

 tralization takes place. Pure water itself dissociates into -f- H 

 and — OH ions, and the dissociation yields equal amounts of 

 -j- H and — OH ions. A neutral solution may therefore be de- 

 fined as "one in which the concentration of hydroxyl ions is 

 equal to the concentration of the hydrogen ions. From this it 

 follows that an acid solution is one in which the concentration of 

 hydrogen ions exceeds that of OH ions, while an alkaline solution 

 is one in which the concentration of hydroxyl ions exceeds that 

 of hydrogen ions." Strong acids and alkalis like hydrochloric 

 acid and sodium hydroxide are almost completely dissociated at 

 certain concentrations, while weak acids like acetic acid and weak 

 alkalis like ammonium hydroxide are only slightly dissociated. 



The extent to which substances are dissociated into their ions 

 may be determined by the hydrogen electrode and by other 

 methods, and this determination, in which the hydrogen- and hy- 

 droxyl-ion concentration is definitely determined, becomes a mea- 

 sure of the acidity or alkalinity of the solution. 



Normal solutions of hydrochloric acid and of acetic acid will 

 neutralize the same amount of sodium hydroxide, but since hy- 

 drochloric acid is a strong acid it is almost completely dissociated, 

 while the weak acetic acid is but weakly dissociated. The result 

 is that normal hydrochloric acid, according to Michaelis (II, '14, 

 p. 23), contains about 0.8 gram of hydrogen per liter, while nor- 

 mal acetic acid contains only 0.0043 gram of hydrogen per liter. 



f ron i 



between the 2 acids may 



As has already been indicated 



€ 



d 



-j- H ions and — OH ions. Sorensen determined 



at 22° C. contains a concentration of 1/10,000,- 



n, that is, that it contains 0.0000001 grain of H 



and the same amount of OH 



word 



'This author, in a comparatively simple account, has summarized in English, the 

 theories of solutions. The book is easy to read and is not burdened with mathe- 



on sy 

 matieal formulae. 



