Spectrophotometric analysis of the CO2 system in aqueous solutions: A freshwater example from the Snake River, Idaho, USA

Abstract

AbstractThis work describes a new, generally applicable method to comprehensively characterize the inorganic carbon system of aqueous solutions. The method requires only simple spectrophotometric measurements and is appropriate for not only open‐ocean seawater (where convenient assumptions and approximations may be made) but also the more challenging case of freshwaters. The overall approach is to (1) measure pH in the field at the time of sample collection and (2) measure sample pH, carbonate alkalinity (AC), and total alkalinity (AT) later in the laboratory. All required equipment is inexpensive and portable. The paired laboratory measurements of pH and AC can be used to obtain the concentration of total inorganic carbon (CT). This CT can in turn be paired with the field pH measurements to comprehensively characterize carbon‐system parameters in the sampled water body at in situ conditions. To our knowledge, this method is the first to spectrophotometrically measure AC and thus the first to completely characterize CT and the carbon system of freshwaters using spectrophotometric measurements only. The concurrent measurements of AC and AT can also be used to partition alkalinity into its carbonate and noncarbonate components. This work additionally describes how to quantitatively correct for artifacts that may arise (especially in freshwater samples) from using HgCl2 to halt respiration in sample bottles. The use of these methods is illustrated using samples collected from the Snake River (Idaho, USA) before and during the 2020 spring flow.