CARBON DIOXIDE-WATER EQUILIBRIUM 



175 



The table shows that, in very dilute carbon dioxide solutions, the concentration of 

 bicarbonate ions is close to that of carbon dioxide molecules; in distilled water equi- 

 librated with the atmosphere the ratio [CO2] : [HCOj"] is approximately 5:1. 

 The constant of the hydration equilibrium: 



(8.2) Kmo = CH2CO,]/[C02] 



is not known precisely, but can be calculated approximately from the rate constants of 

 hydration (kn) and dehydration (k'n)' 



(8.3) KH20 = knik'n 



McBain noticed, in 1912, that the hydration of carbon dioxide is a comparatively slow 

 process. Thiel and Strohecker (1914) and Strohecker (1916) measured its rate in the 

 alkaline region. More recently, Faurholt (1924), Stadie and O'Brien (1933), and 

 Brinkman, Margaria and Roughton (1933) determined A;h ^^^ ^'h over a wide range 

 of hydrogen-ion concentrations. 



Table 8.III 

 Hydration and Dehydration of Carbon Dioxide" 



" F— Faurholt (1924); BMR— Brinkman, Margaria and Roughton (1933); SO— Stadie and O'Brien 

 (1933); RB— Roughton and Booth (1938); R— Roughton (1940); MU— MiUs and Urey (1940). 

 * Buffered solutions! 



The hydration occurs, in addition to the reaction: 



(8.4) CO2 + H2O ^ HsCOj (equilibrium constant, Kmo) 

 also through the bicarbonate ion: 



(8.5) CO2 + OH- . HCOr (equilibrium constant, Kqb.) 



