Speciation calculation computer

More commonly, s are computed on the fly by computer programs as part of an equilibrium speciation calculation. These programs employ various numerical approaches for estimating K, such as the one shown in Eq. 5.11. In this case, the... 

Although computer programs are now used to perform speciation calculations, examining how these calculations are performed provides important insights into the limitations of the model predictions. Thus, we will step through a small part of the calculation used to generate the results presented in Figure 5.4, which represents the iron... 

Various munerical techniques are used to indirectly obtain solutions to large systems of equations with too many imknowns to solve explicitly. One approach is to solve the equations iteratively. This is done by first assuming that all of the anions are unbound and, hence, their free ion concentrations are equal to their total (stoichiometric) concentrations. By substituting these assumed anion concentrations into the cation mass balance equations, an initial estimate is obtained for the free cation concentrations. These cation concentrations are substituted into the anion mass balance equations to obtain a first estimate of the free anion concentrations. These free anion concentrations are then used to recompute the free cation concentrations. The recalculations are continued imtil the resulting free ion concentrations exhibit little change with further iterations. The computer programs used to perform speciation calculations perform these iterations in a matter of seconds. 

Once a choice of molecular model for adsorbed species activity coefficients has been made and its parameters are measured, an equilibrium speciation calculation can be performed by exact analogy with that for aqueous species (see Table 9.3). Computationally, adsorbent species and adsorbed species (SR M OH H in... 

The formal similarity between adsorption and complexation reactions can be exploited to incorporate adsorbed species into the equilibrium speciation calculations described in Sections 2.4 and 3.1. To do this, a choice of adsorbent species components (SR r in Eq. 4.3) must be made and equilibrium constants for reactions with aqueous ions must be available. A model for computing adsorbed species activity coefficients must also be selected.8 Once these choices are made and the thermodynamic data are compiled, a speciation calculation proceeds by adding adsorbent species and adsorbed species (SR Mp(OH)yHxLq in Eq. 4.3) to the mole-balance equations for metals and ligands, and then following the steps described in Section 2.4 for aqueous species. For compatibility of the units of concentration, njw) in Eq. 4.2 is converted to an aqueous-phase concentration through division by the volume of aqueous solution. 

Table X. Speciation Calculations for Tailings Pore Water, Maybell, Colorado. Computed by PHREEQE (20)...

A significant recent development is the incorporation of instantaneous equilibria to kinetic analyses. Careful combination of numerical integration computations alongside the Newton-Raphson speciation calculations have made this possible (Maeder, Neuhold et al. 2002). This development has made the modelling of significantly more complex and realistic... 

When represents a metal cation not in the background electrolyte, the intrinsic constants are determined by fitting the triple layer model to adsorption edge data. This fitting entails a surface speciation calculation with previously measured values of the intrinsic constants in Eq. 5.61, the capacitance parameters Ci and C2, and the parameter Af. The computation includes Eqs. 5.58, 5.59, and 5.69, as well as surface charge and mole balance equations imposed as constraints. " ... 

Parkhurst DL, Appelo CAJ (1999) User s guide to PHREEQC (version 2) - a computer program for speciation, batchreaction, one-dimensional transport, and inverse geochemical calculations U.S. Geological Survey Water-Resources Investigations Report 99-4259, 312 pp... 

Wolery, T. J., 1983, Eq3nr, a computer program for geochemical aqueous speciat-ion-solubility calculations user s guide and documentation. Lawrence Livermore National Laboratory Report UCRL-53414. 

Parkhurst, D.L. Appelo, C.A.J. 1999. User s Guide to PHREEQC (Version 2.15.0)-A Computer Program for Speciation, Batch-Reaction, One-Dimensional Transport and Inverse Geochemical Calculations. U.S. Geological Survey, Water Resources Investigation Report 99-4259. 

Wolery T. I (1983). EQ3NR. A Computer Program for Geochemical Aqueous Speciation-Solubility Calculations User s Guide and Documentation Lawrence Livermore Laboratory, Livermore, Cal., UCRL-53414. 

PARKHURST, D. L. 1995. Users Guide to PHREEQC -A Computer Program for Speciation, Reaction-Path, Advective-Transport, and Inverse Geochemical Calculations. US Geological Survey Water Resources Investigations Report 95-4227, Reston. VA. 

One other serious criticism regarding the data on Cu speciation is the neglect of the cysteine present in blood plasma. Cu11 and cysteine undergo a facile redox reaction (Chapter 20.2). Since the reaction is irreversible, no quantitative thermodynamic quotient is available for use in the computer calculations. Another assumption often made is that the overwhelming concentration of other amino acids may prevent cysteine coordination and, as a result, stabilize the Cu11 state. Recent studies show that this assumption is totally unjustified48 and so the dilemma still has to be resolved. 

Guy, R.D. and Ross Kean, A., 1980. Algae as a chemical speciation monitor - I. A comparison of algal growth and computer calculated chemical speciation. Water Res., 19 891-899. 

A number of studies have been reported in the literature in which theoretical aqueous speciation of key radionuclides has been determined from available thermodynamic data (4.9.21-25). In general, where comparisons are possible, one finds reasonably close agreement in the calculated dominant dissolved species, the solids controlling solubility, and computed total solubilities for this study and those listed above. 

Keizer, M.G. (1991) ECO SAT A Computer Program for the Calculation of Speciation in Soil-Water Systems. Department of Soil Science and Plant Nutrition, Agricultural University, Wageningen, The Netherlands. 

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