AND CALCIUM CARBONATE, ETC., OF WATER SOLUTIONS. 251 



This means that the concentration of sulphate ions must be about 5,000 

 times as large as that of the carbonate ions in a solution saturated with 

 both salts, and consequently one or the other salt will come down first pure, 

 when precipitation from a mixture is effected, until this ratio is reached. 

 Under ordinary conditions obtaining in nature or in laboratory experiments, 

 the excess of sulphate ions is not as large as this, and so the carbonate is 

 precipitated first until the concentration of carbonate ions has fallen to 

 about one five-thousandth that of the sulphate ions. On the other hand, 

 if in such a solution, which is in equilibrium with both solid salts, the con- 

 centration of the sulphate ions is by any means increased, e.g., by addition 

 of some sodium or potassium sulphate, the solution will be oversaturated 

 with calcium sulphate [Cca X C'sOi > Kcasoj; gypsum will be precipitated, and 

 owing to the loss of calcium ions the solution will now be undersaturated in 

 regard to the carbonate [(Cca— a:) XCco3< KcacOa]; and solid calcium car- 

 bonate must dissolve until the above ratio is reached. In the same way 

 the order of precipitation may be reversed if by any means the concen- 

 tration of the carbonate ions in a solution is persistently kept below one 

 five-thousandth that of the sulphate ions during the process of precipitation 

 or crystallization, for instance, by the addition of an acid.^ 



For an aqueous solution saturated with gypsum and calcium carbonate 

 at about 18°, the concentration of calcium ions may be taken as 0.00769, 

 as practically all of the calcium ions are derived from the sulphate. The 

 concentration of carbonate ions in such a solution is, then, 



p KcaC03 1-26X10'~^ _. fi4vin-« (AVi\ 



With the aid of this value we can calculate, for varying partial pressures 

 of carbon dioxide, the maximum proportions of calcium carbonate and 

 bicarbonate which can be present in solutions saturated with gypsum and 

 calcium carbonate at approximately 18°. 



CALCIUM SULPHATE, CARBONATE, AND BICARBONATE, WITH REFERENCE TO 

 GIVEN PARTIAL PRESSURES OF CARBON DIOXIDE. 



The present average partial pressure of carbon dioxide in the atmosphere 

 is 0.0003 atmosphere. We may ask what is the maximum amount of cal- 

 cium present as carbonate and bicarbonate in 1 liter of a solution which 

 is in equilibrium with this partial pressure of carbon dioxide and which is 

 saturated both with gypsum and with calcium carbonate. The significance 

 of this quantity will be discussed presently; it can be calculated as follows, 

 with the aid of the equations developed above. We have: 



Ch X ChCOs = ^'ionization X ChsCOj (111) 



Ch X CcOa = K"ionization X CHCO3 (I V) 



ChsCOs = ^gas X CcOs (I^) 



* Analogous relations have been developed experimentally and theoretically in con- 

 nection with Guldberg and Waage's classical work on barium sulphate and carbonate. 

 Vide Nernst, Theoretical Chemistry, p. 533 (1904), and Findlay, Zeitschrift fiir Physikalische 

 Chemie, 34, 409 (1900). 



