Species, aqueous

Notice that we are now referring to reactions in which a hydrogen ion is transferred from an acid to a base without specifically involving the aqueous species H+(aq). A hydrogen ion, H+, is... 

Similar procedures can be used to relate AfH°m and AfG of aqueous species such as CC>2(aq) and H2C03(aq) so that... 

Chemistry and thermodynamics of europium and some of its simpler inorganic compounds and aqueous species. J. A. Rand, Chem. Rev., 1985, 85, 555 (343). 

B Compression affects gaseous species only. In the forward reaction, 1 mol of CO,(g) reacts to form an aqueous species. Therefore, compression favors the formation of H,C03(aq). 

Set up a concentration table for the aqueous species, using the initial concentration of thiosulfate,... 

Figure 1.93. /02-pH diagram with the stability fields of aqueous species in Na-K-H-S-Se-0 system for the conditions SS = 10 mol/kg H2O, ESe = 10 mol/kg H2O, ionic strength = 1, and temperature = 150°C. Dashed lines are the ratio ho-a i- /a i- in logarithmic units. Stability fields for native sulfur and native selenium and the boundaries between predominance regions of oxidized and reduced selenium species are omitted for clarity (Shikazono, 1978b). 

Helgeson, H.C. and Kirkham, D.H. (1976) Theoretical prediction of the thermodynamic behavior of aqueous electrolytes at high pressure and temperatures. Ill Equation of state for aqueous species at infinite dilution. Am. J. Sci., 276, 97-240. 

Reduction-oxidation is one of the most important processes controlling solubility and speciation of trace elements in soils, especially for those elements with changeable values, such as Cr, As and Se. Within normal ranges of redox potentials and pH commonly found in soils, the two most important oxidation states for Cr are Cr(III) and Cr(VI). Cr(III) is the most stable form of chromium and less soluble and nontoxic, but Cr(VI) is mobile, soluble and toxic. The main aqueous species of Cr(III) are Cr3+, Cr(OH)2+, Cr(OH)3° and Cr(OH)4" and the major aqueous species of Cr(VI)... 

A simplified series of reactions between a hafnium salt and sulfuric acid is given in Fig. 4.3. The reactions showcase important facets of thin-film synthesis (but do not address the precise identities of intermediates or complexities of aqueous hafnium chemistry.) In the first step, a hafnium oxide chloride crystal hydrate is dissolved in water to disperse small hafnium-hydroxo molecular clusters. Sulfato ligands are subsequently added in the form of sulfuric acid. Since sulfato binds more strongly than chloro, hafnium-hydroxo-sulfato aqueous species are created. Under mild heating, these species readily poly-... 

As geochemists, we frequently need to describe the chemical states of natural waters, including how dissolved mass is distributed among aqueous species, and to understand how such waters will react with minerals, gases, and fluids of the Earth s crust and hydrosphere. We can readily undertake such tasks when they involve simple chemical systems, in which the relatively few reactions likely to occur can be anticipated through experience and evaluated by hand calculation. As we encounter more complex problems, we must rely increasingly on quantitative models of solution chemistry and irreversible reaction to find solutions. 

Any number of reactants may be considered, each of which can be transferred at a positive or negative rate. Positive rates cause mass to be added to the system at negative rates it is removed. Reactants may be minerals, aqueous species (in charge-balanced combinations), oxide components, or gases. Since the role of a... 

Fixed-activity and sliding-activity paths (Sections 14.2-14.3) are analogous to their counterparts in fugacity, except that they apply to aqueous species instead of gases. Fixed-activity paths are useful for simulating, for example, a laboratory experiment controlled by a pH-stat, a device that holds pH constant. Sliding-... 

Are the equilibrium constants for the important reactions in the thermodynamic dataset sufficiently accurate The collection of thermodynamic data is subject to error in the experiment, chemical analysis, and interpretation of the experimental results. Error margins, however, are seldom reported and never seem to appear in data compilations. Compiled data, furthermore, have generally been extrapolated from the temperature of measurement to that of interest (e.g., Helgeson, 1969). The stabilities of many aqueous species have been determined only at room temperature, for example, and mineral solubilities many times are measured at high temperatures where reactions approach equilibrium most rapidly. Evaluating the stabilities and sometimes even the stoichiometries of complex species is especially difficult and prone to inaccuracy. 

Unfortunately, phases of geochemical interest are not ideal. As well, aqueous species do not occur in a pure form, since their solubilities in water are limited, so a new choice for the standard state is required. For this reason, the chemical potentials of species in solution are expressed less directly (Stumm and Morgan, 1996, and Nordstrom and Munoz, 1994, e.g., give complete discussions), although the form of the ideal solution equation (Eqn. 3.4) is retained. 

Aqueous species The chemical potential of an aqueous species Af is given by,... 

Enough aqueous species, preferably those abundantly present in solution, to... 

The aqueous species included in the basis are known as basis species, while the remaining species in solution comprise the set of secondary species. 

This choice of basis follows naturally from the steps normally taken to study a geochemical reaction by hand. An aqueous geochemist balances a reaction between two species or minerals in terms of water, the minerals that would be formed or consumed during the reaction, any gases such as O2 or CO2 that remain at known fugacity as the reaction proceeds, and, as necessary, the predominant aqueous species in solution. We will show later that formalizing our basis choice in this way provides for a simple mathematical description of equilibrium in multicomponent systems and yields equations that can be evaluated rapidly. 

Aj Aqueous species in the basis, the basis species Aj Other aqueous species, the secondary species... 

Here, we have represented the activities of aqueous species with the product y m of the species activity coefficients and molalities, according to Equation 3.6. The symbol IT in this equation is the product function, the analog in multiplication to the summation . Table 3.2 lists the meaning of each variable in this and following equations. 

We have considered a large number of values (including the molality of each aqueous species, the mole number of each mineral, and the mass of solvent water) to describe the equilibrium state of a geochemical system. In Equations 3.32-3.35, however, this long list has given way to a much smaller number of values that constitute the set of independent variables. Since there is only one independent variable per chemical component, and hence per equation, we have succeeded in reducing the number of unknowns in the equation set to the minimum possible. In addition,... 

The remaining aqueous species are related to the basis entries by the reaction... 

Sometimes when a mineral becomes supersaturated, there is no logical aqueous species in the basis with which to swap the mineral. Such a situation occurs when no species appear in the reaction to form the mineral. Wolery (1979) and Reed (1982) refer to such a situation as an apparent violation of the phase rule, because adding the mineral to the basis would produce more phases in the system than there are components. 

To this point we have assumed the existence of a basis of chemical components that corresponds to the system to be modeled. The basis, as discussed in the previous chapter, includes water, each mineral in the equilibrium system, each gas at known fugacity, and certain aqueous species. The basis serves two purposes each chemical reaction considered in the model is written in terms of the members of the basis set, and the system s bulk composition is expressed in terms of the components in the basis. 

The modeler first encounters basis swapping in setting up a model, when it may be necessary to swap the basis to constrain the calculation. The thermodynamic dataset contains reactions written in terms of a preset basis that includes water and certain aqueous species (Na+, Ca++, K+, Cl-, HCOJ, SO4-, H+, and so on) normally encountered in a chemical analysis. Some of the members of the original basis are likely to be appropriate for a calculation. When a mineral appears at equilibrium or a gas at known fugacity appears as a constraint, however, the modeler needs to swap the mineral or gas in question into the basis in place of one of these species. 

Each time we change the basis, we must rewrite each chemical reaction in the system in terms of the new basis. This task, which might seem daunting, is quickly accomplished on a computer, using the transformation matrix. Consider the reaction to form an aqueous species,... 

Here, we define the total dissolved solids (in mg kg-1) for early releases of the REACT program (GWB 6.0 and previous), so the software can correctly convert our input constraints from mg kg-1 to molal units, as carried internally (i.e., variables nii and m.j). The print command causes the program to list in the output all of the aqueous species, not just those in greatest concentration. Typing go triggers the model to begin calculations and write its results to the output dataset. 

The program produces in its output dataset a block of results that shows the concentration, activity coefficient, and activity calculated for each aqueous species (Table 6.4), the saturation state of each mineral that can be formed from the basis, the fugacity of each such gas, and the system s bulk composition. The extent of the system is 1 kg of solvent water and the solutes dissolved in it the solution mass is 1.0364 kg. 

Equation 8.10 is notable in that it ascribes specific energetic effects to the interactions of the aqueous species taken in pairs (the first summation) and triplets (second summation). The equation s general form is not ad hoc but suggested by statistical mechanics (Anderson and Crerar, 1993, pp. 446 -51). The values of the virial coefficients, however, are largely empirical, being deduced from chemical potentials determined from solutions of just one or two salts. 

An important consideration in constructing certain types of geochemical models, especially those applied to environmental problems, is to account for the sorption of aqueous species onto sediment surfaces (e.g., Zhu and Anderson, 2002). Because of their large surface areas and high reactivities (e.g., Davis and Kent, 1990), many components of a sediment - especially clay minerals, zeolites, metal oxides and oxyhydroxides, and organic matter - can sorb considerable masses. 

The Langmuir isotherm (or Langmuir model) provides an improvement over the K( and Freundlich approaches by maintaining a mass balance on the sorbing sites (Stumm and Morgan, 1996). The model, for this reason, does not predict that species can sorb indefinitely, since the number of sites available is limited. When the calculation carries reactions for the sorption of more than one aqueous species, furthermore, it accounts for competition such a calculation is known as a competitive Langmuir model. 

To do so, we calculate the Jacobian matrix, which is composed of the partial derivatives of the residual functions with respect to the unknown variables. Differentiating the mass action equations for aqueous species Aj (Eqn. 4.2), we note that,... 

First, we read in the dataset of complexation reactions and specify that the initial mass balance calculations should include the sorbed as well as aqueous species. We disable the ferric-ferrous redox couple (since we are not interested in ferrous iron), and specify that the system contains 1 g of sorbing mineral. 

The process is best shown by example. Suppose that we wish to balance the reaction by which calcium clinoptilolite (CaAl2Siio024 8H2O), a zeolite mineral, reacts to form muscovite [KAl3Si30io(OH)2] and quartz (Si02). We choose to write the reaction in terms of the aqueous species Ca++, K+, and OH-. 

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