MAGNESIUM AND ZINC. IONIC EQUILIBRIA 453 



be 1 g. of hydrogen-ion per liter. Yet, even in the much smaller 

 concentration actually present (0.004 g. per liter), the acid taste of 

 the H + and its effect upon indicators can be distinctly recognized. 

 If, now, solid sodium acetate is dissolved in the solution, the liquid 

 no longer gives an add reaction with one of the less delicate indica- 

 tors, like methyl orange (an organic dye which turns pink in the 

 presence of a fair amount of hydrogen-ion). The explanation is 

 simple. Sodium acetate is highly ionized. It gives, therefore, 

 a large concentration of acetate-ion to a liquid formerly contain- 

 ing very little. This causes a greatly increased union of the H+ 

 ions and C 2 H 3 2 ~ ions to occur, and the former, being already 

 very few in number, disappear almost entirely. Hence the solu- 

 tion becomes, to all intents and purposes, neutral. There is no 

 less acetic acid present than before, but the concentration of 

 hydrogen-ion is very much smaller. 



Formulation and Quantitative Treatment of the Case of 



Excess of One Ion. If [H+] and [C 2 H 3 2 ~] represent the molec- 

 ular concentrations of hydrogen-ion and acetate-ion, respectively, 

 and [HC 2 H 3 O 2 ] that of the acetic acid molecules at equilibrium, 

 then: 



[H+] X [C 2 H 3 2 -] = K 

 [HC 2 H 3 2 ] 



The value of the ionization constant K is unchanged, whether 

 the concentration of the solution of acetic acid is great or small, 

 and even when another substance with a common ion is present. 

 In the latter case, [C 2 H 3 2 ~] and [H + ] stand for the whole concen- 

 trations of each of these ionic substances from both sources. 



Now, in normal acetic acid [H+] = 0.004, [C 2 H 3 2 -] = 0.004 (for 

 the number of each kind of ions is the same), and [HC 2 H 3 2 ] = 

 0.996, practically 1. Substituting in the formula: 



0004^0004 = K (= 004l6) 



