7 - The chemistry of natural waters

Summary

The external aspects of geochemical cycles, the phenomena that occur at relatively low temperatures (typically from 0 to +3°C) in the ocean, the atmosphere, and in rivers, are largely governed by chemical equilibria in solution or at the water–mineral interface. The cycles themselves imply transfer controlled primarily by water–rock interaction (erosion, sedimentation, hydrothermal reactions) and by biological activity. A central role is played by the carbonate system. We will apply these concepts to the geochemistry of erosion and of the ocean, with a discussion of the impact of these cycles on climates in particular.

Basic concepts

A few important concepts that are part of college chemistry are required.

1. Acidity is the concentration [H⁺] (mol kg^–1) of protons in a solution. The exact form, H+ or H₃O⁺, in which these protons occur is of little significance. A scale of acidity is defined by the potential pH of the protons in the solution, such that pH = –log [H⁺]. At 25°C, pure water has a pH of 7. A lower pH indicates an acidic solution and a higher pH a basic solution.

2. Ion behavior is dictated by the dissociation of acids and bases. In an acid–base reaction, the acid is the proton donor and the base is the acceptor. A strong acid such as HCl or a strong base such as NaOH become completely dissociated to produce Cl^- and Na⁺ ions, which behave essentially like inert species and are of relevance only in terms of charge.

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