Seawater proton concentration is a master variable that controls the air-sea gas exchange of CO₂, the ability of organisms to produce calcium carbonate shells, and that tracks the production and respiration of organic carbon as CO₂ is removed or added to water by biological processes.  The proton concentration in seawater [H⁺] (mol kg-seawater^-1) is typically reported as the pH = -log₁₀ [H⁺].  The in situ proton concentration ranges from about 3 to 30 nmol kg-seawater^-1 (7.5<pH<8.5).

Seawater pH is measured from profiling floats using a Deep-Sea DuraFET pH sensor that is manufactured at MBARI (Johnson et al., 2016) or at Sea-Bird Scientific (Float pH).  The core of each sensor is an Ion Sensitive Field Effect Transistor (ISFET), produced by Honeywell, that responds to proton activity and a Ag/AgCl reference electrode that responds to chloride ion activity.  When properly calibrated, the sensor measures the activity of HCl, which is the product of the activities of the two ions a_H+ × a_Cl- (Martz et al., 2010).