Читать книгу Geochemistry - William M. White - Страница 142

For each chemical reaction in our system, we can write one version of eqn. 3.85. This allows us to relate the equilibrium activities of the species undergoing reaction in our system to one another.

Solution of aqueous equilibria problems often hinge on the degree to which we can simplify the problem by minimizing the number of equilibrium constant expressions we must solve. For example, H₂SO₄ will be completely dissociated in all but the most acidic natural waters, so we need not deal with reactions between H⁺, SO₄^2–, HSO₄^–, and H₂SO₄, and need not consider the latter two in our list of species. Similarly, though many natural waters contain Na⁺ and Cl^–, NaCl will precipitate only from concentrated brines, so we generally need not consider reactions between NaCl, Na⁺, and Cl^–.

Carbonate is a somewhat different matter. Over the range of compositions of natural waters, H₂CO₃, , and may all be present. In most cases, however, one of these forms will dominate and the concentrations of the remaining ones will be an order of magnitude or more lower than that of the dominant one. In some cases, two of the above species may have comparable concentrations and we will have to consider equilibrium between them, but it is rarely necessary to consider equilibrium between all three. Thus, at most we will have to consider equilibrium between H₂CO₃ and , or and , and we can safely ignore the existence of the remaining species. A successful solution of problems involving carbonate equilibria often requires correctly deciding which reactions to ignore. We will discuss carbonate equilibrium in greater detail in Chapter 6.