AND CALCIUM CARBONATE, ETC., OF WATER SOLUTIONS. 241 



would not have equal values, but they are dependent on each other to the 

 extent that the product of the two concentrations has a constant value for 

 the saturated solutions. For instance, if to a saturated solution of the 

 carbonate any acid is added, either a strong one like hydrochloric acid or 

 a weak one like carbonic acid, the hydrogen ions of the acid must imme- 

 diately combine to some extent with part of the carbonate ions, CO3", to 

 form more or less bicarbonate ions, HCO3': as the CO3" ions disappear, more 

 molecular calcium carbonate must ionize (equation 2) to re-establish the 

 equilibrium, leaving the solution undersaturated with molecular carbonate, 

 and the solid carbonate, if present, must dissolve. All this can be briefly 

 expressed in the equation 



CcaX(Cco3"-a:)<Kcaco3 (9) 



The solution is undersaturated when the product of the concentrations 

 of the calcium and carbonate ions is smaller than the solubility constant; 

 similarly oversaturated (precipitation resulting) when the product is greater 

 than the constant; and just saturated when the product equals the constant. 



When the solubility of calcium carbonate is thus increased by the addi- 

 tion of an acid (say carbonic acid) owing to the formation of bicarbonate 

 ions, HCO3', calcium bicarbonate Ca(HC03)2 is formed; it remains, as most 

 salts do in dilute solutions, very largely ionized. In equations (3), etc., 

 Cca means, of course, the total concentration of calcium ions, irrespective 

 of their origin from calcium carbonate or bicarbonate. 



These relations which we have been discussing in a qualitative sense 

 may be developed quantitatively as follows: 



The formation of calcium bicarbonate by the action of carbonic acid on 

 calcium carbonate is a reversible reaction: 



Ca-+C03" + H-+HC03' ^ Ca-+2HC03' (10) 



and 



Ca- +2HCO3' ^ Ca(HC03)2 (11) 



The calcium ions appearing on both sides of equation (10) in equal 



quantities evidently do not affect the equilibrium, and we have more 



simply 



C03"H-H+HC03'^2HC03" (12) 



and consequently: 



Cco3XCHXCHco3 = fcXC2Hco, (13) 



Canceling ChcOs on both sides, we have 



CcOa X Ch 



ChcOs 



"• Ionization V ■'•'*/ 



The study of these equations shows that, except for the formation of some 

 non-ionized calcium bicarbonate, the formation of bicarbonate and the 

 equilibrium between bicarbonate and carbonate are largely independent of 

 the nature of the metal ion present, particularly since all salts (like calcium 

 bicarbonate) are largely ionized in dilute solutions and all similar salts are 

 about equally ionized in equivalent solutions. The above conclusion has 



