Activity coefficients concentrated, mixed salt solutions

With the use of thermodynamic relations and numerical procedure, the activity coefficients of the solutes in a ternary system are expressed as a function of binary data and the water activity of the ternary system. The isopiestic method was used to obtain water activity data. The systems KCl-H20-PEG-200 and KBr-H20-PEG-200 were measured. The activity coefficient of potassium chloride is higher in the mixed solvent than in pure water. The activity coefficient of potassium bromide is smaller and changes very little with the increasing nonelectrolyte concentration. PEG-200 is salted out from the system with KCl, but it is salted in in the system with KBr within a certain concentration range. 

It is not easy to quantify precisely the supersaturation levels of given species generated in mixed salt systems or in solutions in which ion association occurs. Relationships such as equations 3.67 3.69 cannot be simply applied because of the difficulty of expressing the true reference condition of equilibrium saturation. It is first necessary to identify all the possible single species, ion pairs and solid liquid phase equilibria that can occur in the system. The relevant thermodynamic association/dissociation constants (K values) must be known. The activity coefficients for the various ionic species must be calculated, e.g. by means of Debye-Hiickel type equations (section 3.6.2). Equilibrium concentrations of all the possible species present are then evaluated by iterative procedures. 

The above approach is empirical. Thermodynamic models for describing solution behavior can also be employed to determine gas solubilities, and these models are amenable to the estimation of gas solubilities in multicomponent systems from sets of single salt data. The thermodynamic approach employed is known as the Pitzer species interaction model, and it is used to determine the activity coefficient of the gas from a summation of interaction terms with anions, cations, and neutral species [3, 10, 11]. These interaction parameters are determined empirically from solubility data in a range of electrolyte solutions and have been tabulated for a wide range of salts, permitting the solubility of oxygen to be determined in mixed electrolyte solutions over a wide range of temperature and concentrations. 

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