Spectrophotometric analysis of the CO ₂ system in aqueous solutions: A freshwater example from the Snake River, Idaho, USA

Abstract This 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 ( A C ), and total alkalinity ( A T ) later in the laboratory. All required equipment is inexpensive and portable. The paired laboratory measurements of pH and A C can be used to obtain the concentration of total inorganic carbon ( C T ). This C T 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 A C and thus the first to completely characterize C T and the carbon system of freshwaters using spectrophotometric measurements only. The concurrent measurements of A C and A T 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 HgCl 2 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.