Determination of the rate constants for the carbon dioxide to bicarbonate inter-conversion in pH-buffered seawater systems

Experimental setups to study modes of inorganic carbon acquisition and fixation rates by marine phytoplankton commonly make use of so-called disequilibrium techniques. The chemical or isotopic disequilibrium, either caused by phytoplankton cells taking up inorganic carbon or by a small disturbance of the isotopic equilibrium in the carbonate system, requires to account for the relatively slow chemical interconversion of carbon dioxide (CO2) to bicarbonate (HCO3−) in seawater. Because in such experiments a constant pH is a prerequisite, pH buffers are generally used. However, a possible influence of such buffers on the kinetics of the carbonate system has hitherto not been investigated. Here, a model of the carbonate system in seawater is employed to show how pH buffers are operating. Furthermore, a new approach is presented to determine the rate constants, k+ and k−, for the conversion reaction of CO2 to HCO3− and vice versa, by means of membrane inlet mass spectrometry (MIMS). For the two pH buffers tested (HEPES and BICINE) it is shown that measured rate constants are in good agreement with calculated values for k+ and k− in a pH range of 7 to 8.5 and at temperatures from 10 to 25 °C.