Rapid determination of CO2 production rates by a coulometric method

Abstract

Dissolved inorganic carbon (DIC) is one of fundamental parameters affecting physicochemical conditions of aquatic environments. Normally, for fresh water samples, acidbase titration with Gran plot is used to determine DIC. The procedure, however, requires more than tens of milliliters of solution volume, and CO2 in the ambient air may interfere with the titration. A monitoring system composed of a flow-through device and a CO2 coulometer was developed to measure slow CO2 production rate in an aqueous system. In this method, sample solutions are directly injected into a flow-through chamber in which a stream of CO2-free carrier gas continuously removes dissolved CO2 from the injected solution. Degassed CO2 is delivered to the coulometer in which integrated amounts of CO2 are measured. Detection limit of the method was found to be lower than 10×10−12 molesCO2/s, which depends on factors such as flow rates, total CO2 concentrations as well as baseline drift of the coulometer. As a demonstration, the method was applied to the determination of CO2 production rates via degradation of coal organic matter. The method can be used for monitoring of slow DIC production involved in various geochemical processes including the weathering of silicates, dissolution of carbonates and the oxidation of sedimentary organic carbon.