254 



EQUILIBRIUM BETWEEN CARBON DIOXIDE OF ATMOSPHERE 



evaporation such a liter would deposit 0.059 gram of calcium carbonate as 

 a contamination of the gypsum or 0.059/2.07, or 2.85 per cent (referred to 

 the dehydrated gypsum). Briefly the amount of calcium carbonate pro- 

 duced under a tenfold increase in the partial pressure for carbon dioxide is 

 very closely proportionate to the square root of 10. 



For a solution saturated at 18° with gypsum, calcium carbonate and 

 bicarbonate under a partial pressure one-tenth as great as its present 

 average value, viz, 0.00003 atmosphere, we may put 



ChcOs = V 0- 1 X 0.0003 = 0.000095 



and Cca 0.0000475. The degree of ionization is 78.5 per cent, and the total 

 dissolved bicarbonate is 0.0000475/0.785 or 0.00006 gram molecule per liter. 



A solution saturated with gypsum, calcium carbonate and bicarbonate 

 at 18° under a pressure of 0.00003 atmosphere of carbon dioxide would 

 contain 6X 10~^ gram molecules of calcium bicarbonate per liter, and would 

 deposit by its evaporation 0.006 gram of calcium carbonate, producing a 

 contamination of 0.006/2.07 or 0.3 per cent (referred to dehydrated gypsum). 



In table 5 the results are summarized for the calculations made for 

 aqueous solutions under the conditions named. Column 1 gives the carbon 

 dioxide pressure in atmospheres; column 2 gives weight in grams of calcium 

 sulphate present in one liter; column 3 shows similarly the calcium bicar- 

 bonate present, but expressed in grams of carbonate per liter. Columns 

 4 and 5 give the values of columns 2 and 3 in terms of gram molecules per 

 liter. The last column gives the proportion of calcium carbonate which 

 would be found in gypsum separating from such an ideal solution by evapo- 

 ration, the proportion referring to dehydrated gypsum.^ 



Table 5. 



DISCUSSION. 



The significance of these results is as follows: For an ideal condition, 

 if the natural waters of the earth were supposed to contain only lime salts, 

 that is the sulphate, carbonate and bicarbonate in equilibrium with the 

 carbon dioxide of the atmosphere, then by evaporation they would deposit 

 first, as is now the case, until the solution became saturated with gypsum, 

 all the calcium carbonate in solution in excess of the amounts given in 



' Cameron and Seidell, Journal of Physical Chemistry, 5,652 (1901), got experimentally 

 0.037 g. calcium carbonate or 1.9 per cent from a solution containing 1.93 grams calcium 

 sulphate (0.0143 mole) in equilibrium with air. The partial pressure of the carbon dioxide 

 in the air is not specified (laboratory air is notably richer in carbon dioxide) and no tempera- 

 ture is mentioned, but it probably was between 22° and 25° (see pp. 643 and 650). Only 

 one determination is reported, and under the circumstances we must be content with the 

 fact that the result is of the order calculated. (Vide McCoy, American Chemical Journal, 

 29, 461 (1903), who also questions the quaHty of the air used by Cameron.) 



