Reference 
Forster RE, Gros G, Lin L, Ono Y, Wunder M. The effect of 4,4'diisothiocyanatostilbene2,2'disulfonate on CO2 permeability of the red blood cell membrane. Proc Natl Acad Sci U S A. 1998 Dec 22 95(26):1581520. p.15819 left column 2nd paragraphPubMed ID9861053

Comments 
"In the present case of
C18O16 exchange, the thickness assumed for the cell is the
same, D for CO2 is taken as 6.5X10^6cm^2/sec, and
the exchange velocity constant as 44.8sec^1. This value is
calculated from the Mills and Urey relationship (Eq. 1), a CA [carbonic anhydrase]
acceleration of 14,000, an uncatalyzed hydration velocity constant
of 0.18sec^1, and a pH of 7.4. The average (intracellular
[CO2])/(extracellular [CO2]) is 1, so that the rate of 18O exchange
should be the same in cells and in lysate, as Barinov et al. found
(2)." 