Temperature coefficient of seawater pH as a function of temperature, pH, DIC and salinity

• Published in: Acta oceanologica Sinica Vol. 41; no. 6; pp. 114 - 118
• Main Author: Hu, Yubin
• Format: Journal Article
• Language: English
• Published: Berlin/Heidelberg Springer Berlin Heidelberg 01.06.2022; Springer Nature B.V
• Subjects: Climatology; Conversion; Dissolved inorganic carbon; Earth and Environmental Science; Earth Sciences; Ecology; Engineering Fluid Dynamics; Environmental Chemistry; Hydrogen; Hydrogen ions; Marine & Freshwater Sciences; Mathematical analysis; Oceanography; pH effects; Protons; Salinity; Salinity effects; Sea ice; Seawater; Seawater pH; Temperature; Temperature changes; ocean acidification; CO2SYS; pH measurement; temperature coefficient; sea ice
• ISSN: 0253-505X; 1869-1099
• DOI: 10.1007/s13131-021-1955-3

pH is a measure of the hydrogen ion activity in a solution, which is a function of temperature. Under normal seawater conditions, it is well constrained. Nowadays, with an increasing interest in complex environments (e.g., sea ice), a better understanding of the temperature change on pH under extreme conditions is needed. The objective of this paper was to investigate the temperature coefficient of the seawater pH (ΔpH/Δ T ) over a wide range of temperature, pH, dissolved inorganic carbon (DIC) and salinity by a method of continuous pH measurement with the temperature change, and to verify the application of CO2SYS for pH conversion under extreme conditions (on the National Bureau of Standards (NBS) scale and the total proton scale). Both experimental results and CO2SYS calculations showed that ΔpH/Δ T was slightly affected by temperature over the range of 0°C to 40°C and by pH (at 25°C) from 7.8 to 8.5. However, when pH was out of this range, ΔpH/Δ T varied greatly with pH value. According to the experimental results, changes in DIC from 1 mmol/kg to 5 mmol/kg and salinity from 20 to 105 had no significant effect on ΔpH/Δ T . CO2SYS calculations showed a slight increase in ΔpH/Δ T with DIC on both the NBS scale and the total proton scale; and underestimated ΔpH/Δ T at high salinity (i.e., beyond the oceanographic range) on the NBS scale. Nevertheless, CO2SYS is still suitable for pH conversion even under extreme conditions by simply setting the input values of DIC and salinity in CO2SYS within the oceanographic range (e.g., DIC=2 mmol/kg and S =35).