The amount converted, in moles, is related to the quantity of electricity by Faraday’s constant (96 489+2 coulombsmol-1). The coulometric titration of CO2 invloves the electrolytic generation of a strong base to titrate the weak acid formed by the reaction of CO2 and ethanolamine. Thus, CO2, extracted from seawater is quantitatively converted to hydroxyethylcarbamic acid and titrated with OH-ions electrogenerated by the reduction of H2O at a platinum cathode. The equivalence point is detected photometrically with thymolphthalein as indicator, and the complete sequence includes neutralization, redox, and complexation reactions.
To be quantitative, the reduction of H2O must occur without the involvement of other chemical species and each faraday of electricity must bring about a chemical change corresponding to one equivalent of the analyte. If true, 100% current efficiency is maintained even though the neutralization reaction is secondary to the electrode reaction, i.e., CO2 does not directly participate in the electron transfer process at the electode. Descriptively, the cell solution is blue at the equivalence point due to thymolphthalein (blue at pH 10.5 and colorless at pH 9.3 in aqueous solutions). CO2 drives down the pH and raises the transmittance (%T) which initiates the above sequence by passing current at a low (1-5 mA) or high (50-100mA) level depending upon the magnitude of %T. The increasing pH indicated by the decreasing %T, as acid is titrated, causes the titration current to pass from high to low to zero as the equivalence point is approached and sensed by the optical detector. During titration, the coulometer passes a current through a very precise and stable calibration teflon shell. After the analysis, 5-9 min per replicate, the drain stopcock (B4) is opened to exhaust the spent sample. Within broad limits, the carrier gas flow rate is not critical. Rates of 100-200 ml mins-1 were used, with the bulk of the analyses being at rate of 150 ml min-1.