Some has figured it out before us? That might be quite an underestimation.I found CO2 probes of this type already exist for blood gas analysis. They have a gas permeable membrane with a carbonate/distilled water solution between the membrane and the pH sensor, just as Tom is proposing. Looks like once again someone has figured it out before us, and probably patented the design.
This is a list of references from an article I found dealing with a probe-type detector for real-time PCO2 and PO2 in patients. Notice many of these publication dates are least 20 or 30 years old.
Bruggen van and Scott, 1962. J.T. Bruggen van and J.C. Scott , Microdetermination of carbon dioxide. Anal. Biochem. 3 (1962), pp. 464-471.
Eigen et al., 1961. M. Eigen, K. Kustin and G. Maass , Die Geschwindigkeit der Hydratation von SO2 in wässriger Lösung. Z. Physik. Chem. (N.F.) 30 (1961), pp. 130-136.
Gibbons and Edsall, 1963. B.H. Gibbons and J.T. Edsall , Rate of hydration of carbon dioxide and dehydration of carbonic acid at 25.0 °C. J. Biol. Chem. 238 (1963), pp. 3502-3507. Abstract-MEDLINE
Harned and Scholes, 1941. H.S. Harned and S.R. Scholes, Jr. , The ionization constant of HCO2−3 from 0 to 50.0 °C. J. Am. Chem. Soc. 63 (1941), pp. 1706-1709. Full Text via CrossRef
Harned and Davis, 1943. H.S. Harned and R. Davis, Jr. , The ionization constant of carbonic acid in water and the solubility of carbon dioxide in water and aqueous salt solutions from 0 to 50.0 °C. J. Am. Chem. Soc. 65 (1943), pp. 2030-2037. Full Text via CrossRef
Jones and Bradshaw, 1933. G. Jones and B.C. Bradshaw , The measurement of the conductance of electrolytes.V. A redetermination of the conductance of standard potassium chloride solutions in absolute units. J. Am. Chem. Soc. 55 (1933), pp. 1780-1800. Full Text via CrossRef
Kempen van, 1972. L.H.J. Kempen van , Estimation of free and hemoglobin-bound CO2. Thesis (1972) Nijmegen .
Kempen van et al., 1972. L.H.J. Kempen van, H. Deurenberg and F. Kreuzer , The CO2-quinhydrone electrode. A new method to measure partial CO2 pressure in gases and liquids. Respir. Physiol. 14 (1972), pp. 366-381.
Lunn and Mapleson, 1963. J.N. Lunn and W.W. Mapleson , The Severinghaus PCO2, electrode; a theoretical and experimental assessment. Brit.J. Anaesthesiol. 35 (1963), pp. 666-678.
Maffly, 1968. R.H. Maffly , A conductometric method for measuring micromolar quantities of carbon dioxide. Anal. Biochem. 23 (1968), pp. 252-262. Abstract
Murakami et al., 1965. I. Murakami, S. Takashima, K. Fujisaku, H. Sasamoto, Y. Takagi and Y. Oota , A new method for determination of PCO2, both in liquid and gas. In: Digest 6th Internal. Conf. Med. Electron. Biol. Engin. (1965), pp. 610-611.
Robinson and Stokes, 1959. R.A. Robinson and R.H. Stokes , Electrolyte Solutions. , Butterworths, London (1959).
Stow et al., 1957. R.W. Stow, R.F. Baer and B.F. Randall , Rapid measurements of the tension of carbon dioxide in blood. Arch. Phys. Med. Rehabil. 38 (1957), pp. 646-650. Abstract-MEDLINE
Taylor, 1953. G. Taylor , Dispersion of soluble matter in solvent flowing slowly through a tube. In: Proc. Roy. Soc. A 219 (1953), pp. 186-203. Full Text via CrossRef
Tsao and Vadnay, 1964. M.U. Tsao and A. Vadnay , A method for continuous measurement of blood PO2, and PCO2. J. Lab. Clin. Med. 63 (1964), pp. 1041-1053. Abstract-MEDLINE