178 



FIXATION OF CARBON DIOXIDE 



CHAP. 8 



Such exhaustion effects — caused by the slow diffusion of carbon 

 dioxide through water — can falsify the results of kinetic studies; the use 

 of carbonate buffer solutions prevents these errors. Warburg's buffer 

 No. 6 (Table 8.V) contains, for example, 5800 HCOs" ions (and an 

 equal number of CO3 ions) for each CO2 molecule. One can withdraw 

 10~3 moles of CO2 from a liter of this solution, that is, 120 times more 

 than its initial content in CO2 molecules — and the concentration of CO2 

 molecules wdll not change by more than 12% (from the initial 8.7 X 10~^ 

 moles/1, to 7.8 X 10-« moles/1.). 



Table 8.V 

 Carbonate-Bicarbonate Buffer Solutions" 



" Warburg (1919), recalculated by Smith (1937), using the dissociation constants of Maclnnes and 

 Belcher. 



* This buflfer is closest to the concentration of carbon dioxide in pure water equiHbrated with the 

 free atmosphere. 



Other carbonate-bicarbonate buffer mixtures are listed in Kolthoff's book (1937), 

 p. 259. 



Warburg's buffers are strongly alkaline (pH 8.5 to 11) and therefore 

 "unphysiological," which calls for a certain caution in their use (c/. 

 Vol. II, Chapter 27). Pure bicarbonate solutions have, at 25° C, in the 

 concentration range of 0.001 to 0.1 mole per liter, an approximately con- 

 stant pH of 8.37 (Kolthoff 1937, p. 21), and therefore also an approxi- 

 mately constant ratio [HC03~]/[C02] = 90. 



The solubility of carbon dioxide in water is enhanced by the presence of solid 

 alkaline earth carbonates. Investigators have usually been concerned with another 

 aspect of this phenomenon — the dissolving action of carbonated water on sohd carbonates 

 — because this effect has great practical importance. According to the equations: 



