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


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/ΔT) 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/ΔT 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/ΔT 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/ΔT. CO2SYS calculations showed a slight increase in ΔpH/ΔT with DIC on both the NBS scale and the total proton scale; and underestimated ΔpH/ΔT 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).

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The author would like to thank Dieter Wolf-Gladrow and Gernot Nehrke for the discussion and three anonymous reviewers for their insightful comments.

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Correspondence to Yubin Hu.

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Foundation item: The National Natural Science Foundation of China under contract No. 41806094; the Young Scholars Program of Shandong University under contract No. 2018WLJH43.

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Hu, Y. Temperature coefficient of seawater pH as a function of temperature, pH, DIC and salinity. Acta Oceanol. Sin. 41, 114–118 (2022).

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Key words

  • temperature coefficient
  • pH measurement
  • CO2SYS
  • sea ice
  • ocean acidification