250 EQUILIBRIUM BETWEEN CARBON DIOXIDE OF ATMOSPHERE 



EQUILIBRIUM BETWEEN CALCIUM CARBONATE AND GYPSUM. 

 THE SOLUBILITY PRODUCT OF CALCIUM SULPHATE. 



For a saturated solution of gypsum, ionized according to 



CaSO,^Ca"+SO/ (34) 



we may put, as we did for calcium carbonate, 



Cca X CsOi = KcaSOi (35) 



The value of this solubility product constant can be determined from 

 the solubility of calcium sulphate in water and its degree of ionization in 

 the saturated solution, since the salt dissolves without any hydrolytic de- 

 composition. Kohlrausch and Rose^ give the solubility of gypsum at 18° 

 as 2.07 g. (anhydrous calcium sulphate) per liter of water. This represents 

 a concentration of 2.07/136.1 or 0.0152 gram molecule and 0.0304 gram 

 equivalent of calcium sulphate. The degree of ionization is best ascertained 

 from its conductivity in the saturated solution; the specific conductivity 

 of the solution is given ^ as 0.001891 reciprocal ohms at 18°; its equivalent 

 conductivity is, then 



, 0.001891X1,000 __^ 

 ^^^so^ ^ 0:0304 ^ ^^-^ 



The conductivity of calcium sulphate at extreme dilution^ is 123 and its 

 degree of ionization in the saturated solution therefore 62.2/123 or 50.6 per 



cent. Then Cca = Cso4 = 0.0152X0.506 = 0.00769 



and 



Kcaso, = Cca X Cgo^ = 0.00769- = 5.92 X 10-^ (36) 



The solubiUty product of calcium sulphate at 18° is therefore 5.92 X 10~^. 



CALCIUM SULPHATE AND CALCIUM CARBONATE. ^ 



If we have a solution saturated both with calcium sulphate and with 

 calcium carbonate, for instance in contact with both solid salts, we have 

 in the saturated solution simultaneously 



Cca X CcOs = KcaCOs (37) 



and 



Cca X CsOi = KcaSOi (38) 



The value for Cca is the same now in both equations, representing as it 

 does the total concentration of calcium ions, irrespective of their source 

 from sulphate, carbonate, or bicarbonate. Then, dividing equation (38) 

 by equation (37), we have for a solution in equilibrium with both salts at 



16° to 18° ^ j^^^^^ 5.92X10-^ 



CcOa KcaCOa 1.26X10" 



= 4,700 (39)* 



'Zeitschrift fur Physikalische Chemie, 12, 241 (1893). 

 ' Kohlrausch and Holborn, loc. cit., p. 77. 

 » Jbid., p. 200, table 86. 



* The value for KcaCOs determined at 16° is used ; no correction is made for the differ- 

 ence of 2°. 



