Electrolyte activity complexation model

Pitzer s formulation offers a satisfactory and desirable way to model strong electrolyte activity coefficients in concentrated and complex mixtures. When sufficient experimental data are available, one can make calculations which are considerably more accurate than those presented in this paper. Attaining high accuracy requires not only experimentally-based parameters but also that one employ third virial coefficients and additional mixing terms and include explicit temperature dependencies for the various parameters. 

The property packages available in HYSYS allow you to predict properties of mixtures ranging from well defined light hydrocarbon systems to complex oil mixtures and highly nonideal (non-electrolyte) chemical systems. HYSYS provides enhanced equations of state (PR and PRSV) for rigorous treatment of hydrocarbon systems semiempirical and vapor pressure models for the heavier hydrocarbon systems steam correlations for accmate steam property predictions and activity coefficient models for chemical systems. All of these equations have their own inherent limitations and you are encouraged to become more familiar with the application of each equation. 

UNIFAC A semi-empirical thermodynamic model used to predict the behaviour of components in complex mixtures, which uses structural groups to estimate component interactions. It is an abbreviation for UNIQUAC Functional-group Activity Coefficients and is used to predict non-electrolyte activity in non-ideal mixtures. It is used to predlctthe activity coefficients as a function of composition and temperature. It is useful when experimental data is not available. 

The constant-capacitance model (Goldberg, 1992) assigns all adsorbed ions to inner-sphere surface complexes. Since this model also employs the constant ionic medium reference state for activity coefficients, the background electrolyte is not considered and, therefore, no diffuse-ion swarm appears in the model structure. Activity coefficients of surface species are assumed to sub-divide, as in the triplelayer model, but the charge-dependent part is a function of the overall valence of the surface complex (Zk in Table 9.8) and an inner potential at the colloid surface exp(Z F l,s// 7). Physical closure in the model is achieved with the surface charge-potential relation ... 

The local composition model (LCM) is an excess Gibbs energy model for electrolyte systems from which activity coefficients can be derived. Chen and co-workers (17-19) presented the original LCM activity coefficient equations for binary and multicomponent systems. The LCM equations were subsequently modified (1, 2) and used in the ASPEN process simulator (Aspen Technology Inc.) as a means of handling chemical processes with electrolytes. The LCM activity coefficient equations are explicit functions, and require computational methods. Due to length and complexity, only the salient features of the LCM equations will be reviewed in this paper. The Aspen Plus Electrolyte Manual (1) and Taylor (21) present the final form of the LCM binary and multicomponent equations used in this work. 

Pitzer and co-workers (1973, 1974) have proposed a more detailed, but at the same time more complex, approach. Whitfield (1973, 1975) has applied these equations to seawater and has shown that this model gives good agreement with available experimental data for the osmotic coefficient and for the mean ion activity coefficient of the major electrolyte components. The results obtained yield numerical results similar to the predictions of the ion association model (see Table A6.2). 

In the preceding section, the remarkable salt concentration effect on the acid dissociation equilibria of weak polyelectrolytes has been interpreted in a unified manner. In this treatment, the p/( ,pp values determined experimentally are believed to reflect directly the electrostatic and/or hydrophobic nature of polyelectrolyte solutions at a particular condition. It has been proposed that the nonideality term (Ap/Q corresponds to the activity ratio of H+ between the poly electrolyte phase and the bulk solution phase, and that the ion distribution equilibria between the two phases follow Donnan s law. In this section, the Gibbs-Donnan approach is extended to the equilibrium analysis of metal complexation of both weak acidic and weak basic polyelectrolytes, i.e., the ratio of the free metal ion activity or concentration in the vicinity of polyion molecules to that of bulk solution phase is expressed by the ApAT term. In Section III.A, a generalized analytical treatment of the equilibria based on the phase separation model is presented, which gives information on the intrinsic complexation equilibria at a molecular level. In Secs. B and C, which follow, two representative examples of the equilibrium analyses with weak acidic (PAA) and weak basic (PVIm) functionalities have been presented separately, in order to validate the present approach. The effect of polymer conformation on the apparent complexation equilibria has been described in Sec. III.D by exemplifying PMA. 

Analysis of this model of the electrolyte solution takes the above quantities and by carrying out statistical mechanical calculations or Monte Carlo calculations leads eventually to a calculation of activity coefficients for a range of ionic strengths. The details of the procedures are complex. Results can be compared with experiment, or with the results of other theoretical treatments and indicate that this is a very fiuitfid line of enquiry. The extension to allow for the molecular nature of the solvent is a very necessary step in leading to an eventual fuU description of the behaviour of electrolyte solutions. 

Hydration theory was first applied to aqueous electrolytes by Stokes and Robinson (1) in 1948. The approach is to correct for the fact that the actual solute species do not exist as bare ions as they are normally written, but as hydrated aqueous complexes. In the case of a solution of an aqueous nonelectrolyte, this reduces to the assumption that the solution is ideal, or nearly so, if one writes the formula of the solute to include the waters of hydration and computes its concentration and that of the solvent on this basis. In the case of an aqueous electrolyte, it reduces to the assumption that the Debye-Hiickel model becomes adequate (or at least more adequate) to describe the activity coefficients if this treatment is applied. 

The kinetics of formation and dissociation of Ni(SCN)2(aq) have been studied in water and in several organic solvents, using the pressure-jump and shock wave relaxation technique at 20°C. The concentration of Ni(SCN)2(aq) ranged between 0.001 and 0.1 M. In water, only the formation of the monothiocyanato complex was observed. No background electrolyte was used, and the activity coefficients were calculated by an extended Debye-Hiickel expression. Although, this activity model is not compatible with the SIT, the ionic strengths were low. Therefore, the reported result was corrected to 25°C and the resulting value was accepted with an increased uncertainty (log, = (1.79 0.10)). 

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