AND CALCIUM CARBONATE, ETC., OF WATER SOLUTIONS. 237 



EQUILIBRIUM IN AQUEOUS SOLUTIONS OF CALCIUM CARBONATE, 

 CALCIUM BICARBONATE, AND CARBONIC ACID IN CONTACT WITH 

 AN ATMOSPHERE CONTAINING CARBON DIOXIDE, AND THE SOL- 

 UBILITY OF CALCIUM CARBONATE IN WATER CONTAINING FREE 

 CARBONIC ACID. 



The most reliable and complete experimental determinations of the 

 solubility of calcium carbonate in water containing free carbonic acid were 

 made by Schloesing.* The theoretical treatment of his results from the 

 point of view of equilibrium conditions we owe to Bodlaender.' The latter's 

 work, correct in its theoretical treatment,^ shows an error of moment in 

 the calculation of the solubility constant of calcium carbonate, the constant 

 most important to us, an error due largely to an error in one of the inves- 

 tigations from which Bodlaender drew his data.* McCoy's more recent work 

 on the equilibrium in aqueous solution for sodium carbonate, sodium bicar- 

 bonate, and carbonic acid^ gives the necessary material for the correction of 

 the above error in the following pages. 



The complex conditions of equilibrium involved when water is saturated 

 with calcium carbonate and with carbon dioxide under any given partial 

 pressure may be developed as follows: For a saturated solution of calcium 

 carbonate, say in contact with the solid carbonate, we have " 



CaCOs ^ CaC03 (1)' 



Calcium carbonate in aqueous solution is very largely ionized according 



*^ CaC03±5Ca-+C03" (2) 



At a given temperature in a saturated solution of a difficultly soluble 

 salt of this nature the product of the concentrations ^ of the ions is a con- 

 stant,* which is called the solubiHty product or the ion product of the salt: 



CcaXCcO:, = KcaC03 (3) 



That the product of the ion concentrations is equal to a constant for 

 saturated solutions of difficultly soluble salts must at present be considered 

 an empirically established fact. As is well known, the law of mass-action 

 does not give constants when applied to the ionization of strong electro- 

 lytes, such as salts are. For instance, for the reversible reaction 



NaCl^Na-+Cl' (4) 



•Comptes Rendus, 74, 1552 (1872); 75, 70 (1872). 



'Zeitschrift fur Physikalische Chemie, 35, 23 (1900). 



' See below, this page, in regard to a contested question of theory. 



« McCoy, American Chemical Journal, 29, 437 (1903). 



* Ibidem. 



* Underscoring of a symbol is used to indicate that the substance is in solid form. 



' See below in regard to the conclusion usually based on this equation that at a given 

 temperature the concentration of the dissolved non-ionized calcium carbonate molecules, 

 CaCOj, or the molecular solubility, has a constant value. 



* The term concentration, for which C is used in all the equations, is taken, in accord 

 with chemical practice, to designate the number of gram molecules or moles of substance 

 in 1 liter of a solution or of a gas. 



» Ostwald, Scientific Foundations of Analytical Chemistry; Nernst, Theoretical Chem- 

 istry, p. 531 (1904); Bodlaender, loc. cit.; A. A. Noyes, Zeitschrift fur Physikalische Chemie, 

 16, 125 (1895), 26, 152 (1898), 42, 336 (1902); Report of the Congress of Arts and Science, 

 vol. IV, 322 (1904); Le Blanc, Zeitschrift fur Anorganische Chemie, 51, 181 (1906). 



