A Comparison Between Field and Laboratory pH Measurements for Seawater on the East China Sea Shelf

To evaluate the effect of sample storage on pH analysis, a comparison experiment between field and laboratory pH measurements was conducted for a total of 88 seawater samples collected on the East China Sea shelf. The results show that although pH directly measured onboard was statistically higher than the pH later measured onshore with an average residual of 0.0052, after correcting the perturbation of HgCl₂ solution addition the observed difference would be within the uncertainty in pH measurement for bottle samples. Our result, therefore, suggests that similar to TA and DIC determinations, seawater samples can be stored for pH analysis with a precision that is comparable to its own measurement uncertainty. Furthermore, for the purpose of calculating pH from TA and DIC, the internal consistency test demonstrates that the carbonate dissociation constants derived from field measurements would give the best agreement between the pH calculations and direct measurements; the next best would be the constants determined in the media of real seawater; and the worst world be the constants determined in the media of artificial seawater.