Weak Complexing at Moderate Concentrations The Sea Water Problem

Weak Complexing at Moderate Concentrations The Sea Water Problem 

As an important practical case of multiple, homogeneous equilibria, consider sea water. The analytical concentrations of major ions are given in Table 9-5. Ionic strength is about 0.7 M, pH 8.15 

Experimental determination of the formation constants of possible complexes among all the pairs (16) in Table 9-5 at ionic strength 0.7 is difficult. The set of constants used by Garrels and Christ is given in Table 9-6. Association by chloride seems negligible. Since the other anions have such low concentrations, not much of the cations can be associated even if most of the anions are tied up with them. This simplifies the calculation For a fair approximation we may assume that the concentrations of the cations are as given in Table 9-5. The 

We can write four equilibrium conditions for sulfate and one material balance to obtain five equations in five unknowns. Let x be the equilibrium sulfate concentration (unbound), and a, b, c, and d the concentrations of the sulfate complexes of Na, K, Ca , and Using theX s and activity coefficients, we obtain 

Assuming the subtractive terms in the denominators to be small for a first approximation allows easy substitution of x for all the other unknowns to obtain x = 0.0144. Returning this to the other equations gives us, 

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