RAPID FIELD MEASUREMENT OF DISSOLVED INORGANIC CARBON BASED ON CO2 ANALYSIS

Dissolved inorganic carbon (DIC) is commonly measured in water and is an important parameter for understanding carbonate equilibrium, carbon cycling, and water-rock interaction. While accurate measurements can be made in the analytical laboratory, we have developed a rapid, portable technique that can be used to obtain accurate and precise data in the field as well. Sample analysis is completed by mixing 100 mL of water with 10 mL of 1 M sodium citrate buffer (pH ~ 4.5) in a closed system and then measuring the CO₂ in solution using an Anton Paar CarboQC™ carbonation meter. The final pH of the sample-buffer mixture is measured and the DIC is calculated using the measured concentration of [CO₂], the temperature-corrected dissociation constants for carbonic acid, the pH, activity coefficients for the carbonate ions, and accounting for the dilution caused by the volume of the buffer. In addition to lowering the pH of the final solution so that most of the DIC has been converted into CO₂; the buffer also establishes the same high ionic strength in the mixtures so that the activity coefficients are effectively constant over the entire natural range of DIC tested. Calculated DIC for standard sodium bicarbonate solutions was accurate within 0.5 mM C up to a concentration of 15 mM. In the field, a closed mixing system for water and buffer is accomplished by first collecting the water sample in a 140 mL syringe, after which a smaller syringe is connected to the larger syringe and the buffer injected. In this way, the increased mixed volume is accommodated by movement of the piston in the large syringe, no headspace is generated during mixing, and no CO₂ is lost prior to analysis using the carbonation meter. The method has been tested using alternative buffers (phosphate, phosphate-citrate, acetate) with similar results. The potential interaction between CO₂ and organic buffers has also been evaluated by the addition of varying amounts of methanol (0.05 to 0.5 mL) as an organic modifier. To supplement the testing of known standards, DIC was determined in samples collected from a variety of geochemical settings and compared with laboratory analysis.