Activity, Speciation, and Equilibrium Models

Chemical species may not always behave as expected based on their concentrations. In fact, ions commonly appear to be more or less concentrated than they really are because of 

All cations and anions are included in the summation term of this equation. The typical /for river water is 0.0021 m compared to 0.7 m in seawater (Libes, 1992). 

Modified from Morel and Hering (1993) and Stumm and Morgan (1996). 

As in equation 4.5, we can write an equilibrium constant using the following equation  

Since equilibrium constants are defined by ion activities, which are defined by their concentrations and coefficients (see equation 4.15), they do not include ion pairing or complexation effects. In a multi-ion and multiligand solution, where ion pairing is common, it is necessary to use thermodynamic equilibrium constants to convert the ion-pair concentrations to concentrations of free ions. This equilibrium constant (Kc) is defined by concentrations, making it useful to compute ion speciation. The thermodynamic equilibrium constant (K q) used in calculating Kc is based on the following conditions I = 0 m, 25°C and 1 atm. Thus, Kc is defined by the following equation  

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