Ion Pair or Complex Effects

Therefore, the total calcium concentration [Ca. ] resulting from gypsum s solubility is the sum of all the calcium species in solution  

Quantitatively Important Species Known to Exist in Minor Quantities 

Source Selected and adapted from L. G, Sillen and A. E. Martel 1, Stability Constants of Metal-Ion Complexes, Special Publication No. 17, The Chemical Society, London, 1964. 

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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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