Potentiometric measurements were made using a glass electrode that was spectrophotometrically calibrated following the methods described in Easley and Byrne (2012) and Martell-Bonet and Byrne (2020). The spectrophotometric measurements were made following the guidelines of Clayton and Byrne (1993) and Dickson et al. (2007).

Following the methodology introduced by Byrne and Kester (1974), sodium borate decahydrate (borax) was added to natural seawater at different S_P-T pairs to determine the pH_T at which the salt addition caused no change in pH_T. The pH_T at which the addition of borax causes no change is the pK_B for that specific S_P-T pair. For each S_P-T pair, approximately 15 samples were analyzed. From these, two samples were selected: (1) the sample with the lowest pH_initial that lowered upon the addition of borax, and (2) the sample with the highest pH_initial that increased upon the addition of borax. The experimental pK_B (exp-pK_B) for each S_P–T pair was calculated as the mean of the initial and final pH values of the two selected samples; specifically, as the average of pH_initial for samples (1) and (2) and the average of pH_final for samples (1) and (2).