258 EQUILIBRIUM BETWEEN CARBON DIOXIDE OF ATMOSPHERE 



to 14.8x10-5 gram molecule per liter and only 0.0046 gram of calcium car- 

 JDonate would be deposited with 1.07 grams of calcium sulphate crystalliz- 

 ing out as gypsum by the concentration, representing a contamination of 

 0.43 per cent. 



With a partial pressure of 0.003 atmosphere of carbon dioxide the 

 original solubility of calcium bicarbonate would be 10 times as great as in 

 the case just discussed, amounting therefore to 8.3 X lO"* moles. Evapora- 

 tion of the solution to one-quarter its volume would increase the solubility 

 to 14.8 X lO--* gram molecules per liter and 0.046 gram of calcium carbonate 

 would be deposited with 1.07 grams of sulphate, representing 4.3 per cent. 



The results are summarized in table 6.^ 



Table 6. 



Column 1 gives the carbon-dioxide pressure in atmospheres; column 2 

 gives the weight in grams of calcium sulphate in 1 liter; column 3 the solu- 

 bility of calcium carbonate and bicarbonate (expressed in grams of calcium 

 carbonate) ; columns 4 and 5 give the values of the two previous columns 

 in moles per liter. The last column gives the contamination of gypsum with 

 sodium carbonate (referred to dehydrated gypsum) produced by evapora- 

 tion of three-quarters of the solution. 



This table can be regarded only as a rough approximation, as it was 

 not considered desirable to make more involved calculations at the present 

 time, and certain factors modifying the results were therefore not considered, 

 such as the change in solubility of carbon dioxide, the influence of the 

 primary ionization of potassium sulphate, etc. It suffices, however, as an 

 orientation as to the effect of the presence of sulphates on the probable 

 contamination of gypsum with calcium carbonate: the decreased solu- 

 bility of gypsum in the presence of such sulphates, allowing the calcium 

 bicarbonate to dissolve more nearly as it does in the absence of gypsum, 

 would increase the tendency to contamination of the gypsum and so a 

 perfectly pure or exceedingly pure gypsum crystallizing under such condi- 

 tions would a fortiori indicate a very low partial pressure of carbon dioxide. 



' The calculations involve no greater total concentration than about 0.08 gram mole- 

 cule total sulphates per liter (p. 257). Cameron and Seidell, Journal of Physical Chemis- 

 try, 5, 650 (1901), have determined the solubility of gypsum in the presence of sodium sul- 

 phate and found that 0.1 gram molecule sodium sulphate at 22° reduces its solubility from 

 2.1 grams calcium sulphate to 1.4 grams, which is qualitatively in accordance with the 

 above calculations. No attempt was made by them or by the author to determine to what 

 extent the ion product for calcium sulphate remains a constant in these determinations. 

 In more concentrated solutions of the sodium sulphate, as Cameron and Seidell point out, 

 double salt formation increases the solubility of the gypsum. 



