174 



FIXATION OF CARBON DIOXIDE 



CHAP. 8 



carbon dioxide is not identical with Ks in (8.1), because in its determination all the 

 molecular species in solution are lumped together as "dissolved carbon dioxide." How- 

 ever, a' is not appreciably different from Ks in acid solutions (pH <4.5), where the 

 concentrations [HCOr] and [CO3 ] are negligible (c/. Table 8.III). 



Table 8.1 shows the solubihty of carbon dioxide in water at different temperatures. 

 In addition to Ostwald's distribution coefficients, a', the table contains the Bunsen 

 solubihty coefficients, a — the volumes of gas, reduced to 0° C, dissolved in unit volume 

 of water at t° C. The relation between the two constants is: a = a' (1 + 0.00367 t). 



Table 8.1 

 Carbon Dioxide Solubility ln Water (Bohr 1899) 



AHs = - 4.2 kcal/mole (25° C.) 



Using the values Ks = 0.827, DhjO (dissociation constant of water) = 1.04 X 10~" 

 and Kt>i = 4.54 X 10"' for the calculation of [H+] and [HCO3-], and neglecting the 

 species HjCOj and COj — because of the small values of the constants KhjO and K^i 

 (see below), one obtains the compositions of carbon dioxide solutions (at 25° C.) given 

 in table 8.II. 



Table 8.II 



Distribution of Carbon Dioxide between Air and Distilled Water at 25° C. 

 (calculated with Ks = 0.827 and Kt>^ = 4.54 X 10"^) 



" Assuming ideal gas laws. 



>• Cf. Quinn and Jones (1936), p. 119. 



' Normal carbon dioxide content of free atmosphere. 



