176 FIXATION OF CARBON DIOXIDE CHAP. 8 



Reactions (8.4) and (8.5) can be interpreted as additions of HOH and 0H~ respectively 

 to a C==0 double bond in CO2. 



According to Olsen and Joule (1940), the activation energy of reaction (8.4) is 

 19 kcal, and that of reaction (8.5) between 10 and 13 kcal. 



In the pH range of 8-10, the rates of the two reactions (8.4) and (8.5), are of the 

 same order of magnitude. At pH > 8, the pH-independent reaction (8.4) predomi- 

 nates, while at pH > 10, hydration and dehydration occur practically exclusively by 

 reaction (8.5). At pH > 8, and 18° C, a dissolved carbon dioxide molecule lives, on 

 the average, about one minute before it is hydrated, but remains only about 0.1 second 

 in the hydrated state. Hydration and dehydration are accelerated by the anions of 

 many weak acids, e. g. phosphate, borate, and acetate (Roughton and Booth 1938). 

 This is of importance whenever buffers are used. Particularly strong is the effect of a 

 specific enzyme, carbonic anhydrase, found in red blood corpuscles by Meldrum and 

 Roughton (1932): cf. the reviews by Roughton 1934, 1935. 



Further results on the rate of hydration of carbon dioxide were obtained by Mills 

 and Urey (1939, 1940) by the use of isotopic indicators. Their results are summarized 

 in table 8.III. 



The apparent first dissociation constant of carbonic acid was redetermined by 

 Maclnnes and Belcher (1933), who found: 



Kauko and Carlberg (1935) obtained a smaller value, 3.50 X 10"^. The true 

 dissociation constant is considerably larger: 



(8.7) gp, = ^^r^?n?'"-^ = ^'^^ ^^T + ^^ ^f^ = 1-8 X 10-^ (25° C.) 

 LJl2l>UjJ /VH2O AH2O 



The equilibrium constant of reaction (8.5) is: 



<«■'"> '^°" - [CftKOH-] ° [H^YoH-] - '■' X '"' <'5°C-' 



Thus, the standard free energies of hydration of CO 2 molecules are: 



AFhsO = + 3.7 kcal/mole (18° C.) 

 for the hydration to H2CO3 molecules, and: 



Ah/^oh = - 10.4 kcal/mole (25° C). 



for the association with hydroxyl ions to HCOj~ ions. The second dissociation constant 

 of carbonic acid (according to Maclnnes and Belcher) is: 



(8.8) Kv, = '"^HCOr]"' = ^-^^ ^ ^°~" ^^^° ^'^ 



A knowledge of the equilibrium constants Ks, Kn^o, K'di and Kdj 

 permits the calculation of the equilibrium concentrations of all the 

 molecular species in carbonic acid solutions (cf. Tables 8. IV and 8.V). 

 Table 8. IV and figure 17 show the composition of carbonic acid solutions 

 at 0° C. according to Faurholt (1924), based on the following values of 

 the dissociation constants: 



Kdi = 2.24 X 10-' and Kv, = 3.2 X IQ-" (0° C.) 



