Invariance of apparent equilibrium constants with pH1
The apparent dissociation constants of carbonic acid in seawater are independent of pH for the range 6.5-8.5. These constants, therefore, can be used in the study of geochemical and biochemical processes in the oceans. The arbitrary nature of the pH of seawater does not affect its value in oceanogra...
| Main Authors: | , , |
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| Format: | Text |
| Language: | English |
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| Online Access: | http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.507.88 http://www.aslo.org/lo/toc/vol_19/issue_4/0665.pdf |
| Summary: | The apparent dissociation constants of carbonic acid in seawater are independent of pH for the range 6.5-8.5. These constants, therefore, can be used in the study of geochemical and biochemical processes in the oceans. The arbitrary nature of the pH of seawater does not affect its value in oceanographic practice. Oceanographers and marine geochemists use equilibrium constants to calculate those concentrations of ions that result from the dissociation of weak acids and their salts. They have three choices for their compu-tations: thermodynamic constants deter-mined in dilute solutions, apparent con-stants measured directly in seawater, and thermodynamic constants defined for a standard state in seawater. The use of thermodynamic constants defined in dis-tilled water requires the estimate of single ion activity coefficients, a procedure of unknown accuracy. Thermodynamic con-stants defined in seawater arc reduced in practice to apparent constants because the anionic concentrations are not extrapolated to infinite dilution of the anions of interest in the standard seawater and because aII, the single ion activity of hydrogen, cannot be determined from electrode measure-ments without a nonthermodynamic as-sumption. We show in this work on both experi-mental and theoretical grounds that ap-parent equilibrium constants, specifically those for the dissociation of carbonic acid in seawater, do not depend on the pH but only on the major ion composition of sea-water within the pH range of oceano-graphic interest. Apparent constants (Buch et al. 1932; Lyman 1956; Mehrbach et al. 1973), whether defined relative to a dilute buffer or a seawater scale, are valuable because no estimates of activity coefficients are ‘This work was supported by National Science |
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