Components aqueous systems, chemical equilibrium

Medium-chain alcohols such as 2-butoxyethanol (BE) exist as microaggregates in water which in many respects resemble micellar systems. Mixed micelles can be formed between such alcohols and surfactants. The thermodynamics of the system BE-sodlum decanoate (Na-Dec)-water was studied through direct measurements of volumes (flow denslmetry), enthalpies and heat capacities (flow microcalorimetry). Data are reported as transfer functions. The observed trends are analyzed with a recently published chemical equilibrium model (J. Solution Chem. 13,1,1984). By adjusting the distribution constant and the thermodynamic property of the solute In the mixed micelle. It Is possible to fit nearly quantitatively the transfer of BE from water to aqueous NaDec. The model Is not as successful for the transfert of NaDec from water to aqueous BE at low BE concentrations Indicating self-association of NaDec Induced by BE. The model can be used to evaluate the thermodynamic properties of both components of the mixed micelle. 

DYNAMICS OF DISTRIBUTION The natural aqueous system is a complex multiphase system which contains dissolved chemicals as well as suspended solids. The metals present in such a system are likely to distribute themselves between the various components of the solid phase and the liquid phase. Such a distribution may attain (a) a true equilibrium or (b) follow a steady state condition. If an element in a system has attained a true equilibrium, the ratio of element concentrations in two phases (solid/liquid), in principle, must remain unchanged at any given temperature. The mathematical relation of metal concentrations in these two phases is governed by the Nernst distribution law (41) commonly called the partition coefficient (1 ) and is defined as = s) /a(l) where a(s) is the activity of metal ions associated with the solid phase and a( ) is the activity of metal ions associated with the liquid phase (dissolved). This behavior of element is a direct consequence of the dynamics of ionic distribution in a multiphase system. For dilute solution, which generally obeys Raoult s law (41) activity (a) of a metal ion can be substituted by its concentration, (c) moles L l or moles Kg i. This ratio (Kd) serves as a comparison for relative affinity of metal ions for various components-exchangeable, carbonate, oxide, organic-of the solid phase. Chemical potential which is a function of several variables controls the numerical values of Kd (41). 

Pankow, J.F. MacKenzie, S.W. (1991) Parameterising the equilibrium distribution of chemicals between the dissolved, solid particulate matter, and colloidal matter components in aqueous systems. Environmental Science and Technology 25, 2046-53. 

The redox status of an aqueous system is described by the concentrations of the oxidized and reduced species of all system components. Redox systems, generally not at equilibrium as the result of kinetically slow redox reactions, are poorly characterized by intensity factors (Ej or pE) alone. Capacity factors, which reflect the total concentration of relevant species, are conservative parameters that can be meaningful guides to the redox status of aqueous systems. Oxidative capacity (OXC) is defined as a conservative quantity that incorporates a comprehensive chemical analysis of the redox couples of an aqueous system into a single descriptive parameter. OXC classifies aqueous systems in terms of well-defined geochemical and microbial parameters (e.g., oxic, sulfidic). Examples of model and actual groundwater systems are discussed to illustrate the concept. A redox titration model is another tool that is useful in describing a redox system as it approaches an equilibrium state. 

A natural aqueous redox system may or may not be at total (internal) equilibrium (Q. How the redox status is characterized depends upon the state of the system relative to equilibrium. Regardless of whether the system is at equilibrium or not there is an individual, instantaneous pE for each redox couple in the system. Each pE corresponds to the relative concentrations of the oxidized and reduced species and is defined by Nemstian relationships (i). When a system of oxidants and reductants is at internal chemical equilibrium the pE of every couple is identical and this distinct value is the system pE. Only in this special case can the redox status be characterized by determining two of the following three factors pE, the total concentration of all components, and the relative concentrations of oxidants and reductants. When two of the factors are known, the third can be determined from Nemstian and mass balance relationships. Thus, only when the chemical system is at internal equilibrium and one of the redox couples of the system is electrochemically active and present in measurable concentrations can the system pE be calculated from an electrode potential. 

Figure 1.1 shows chemical equilibrium model of the natural system. Variables which determine the thermochemical feature of this system include temperature, total pressure, activities of dissolved species in aqueous solution (ions, ion pairs, complexes etc.), gaseous fugacity, activities of components in solid phases and dissolved species in aqueous solution where activity of i species, is equal to Yi mj (mi is molality of i species in aqueous solution and mole fractiOTi of i component in solid solution and yi is activity coefficient of i species in aqueous solution and of each component in solid phase). 

In an electrochemical system with phases a and p, the criterion of equilibrium should be modified due to different electric potentials cp and respectively, in phases a and p. As an example, let us consider zinc electrode of the Daniell cell, which was introduced by John Frederic Daniell in 1836. Think of a piece of zinc metal being dipped into a dilute solution of ZnS04(aq). Between the solution and metal phases, aqueous zinc ions, Zn +(aq), can be transferred. If the initial solution is extremely dilute, then the rate of transfer of ions from the metal to the solution is faster than the transfer from the solution to the metal. When Ztf+(aq) leaves the metal surface, electrons are left behind because they cannot enter the solution. This builds up a negative electric potential in the metal phase. After some time, an equilibrium state is reached between the so-called electrochemical potential of Zn +(aq) within the metal and solution phases. The electrochemical potential of the species in each phase comprises two components (1) the chemical potential of the species p, and (2) the electric... 

The redox status of an aqueous system is described by the concentrations of the oxidized and reduced chemical species of all components in the chemical system (Scott Morgan 1990). Because of the slowness of oxidation-reduction reactions, natural rock-water systems are often not in redox equilibrium and thus the concept of a system Eh or pE becomes meaningless. Intensity factors such as Eh or pE are not very useful descriptors of the redox state of the system, but capacity factors which reflect the total concentration of redox-sensitive species may be better and more conservative measures of redox state. 

With a three-component system, such as a polymer in an aqueous salt solution, preferential adsorption of one component to the polymer can affect the analysis of light-scattering data.199 Such interactions can affect the SRI. Therefore, measurements of the SRI must be made at constant chemical potential. Constant chemical potential is achieved experimentally by dialyzing the solvent and polymer solution to equilibrium through a membrane permeable to the solvent but impermeable to the polymer.199... 

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,... 

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. 

Equilibrium between simple salts and aqueous solutions is often relatively easily demonstrated in the laboratory when the composition of the solid is invariant, such as occurs in the KCI-H2O system. However, when an additional component which coprecipitates is added to the system, the solid composition is no longer invariant. Very long times may be required to reach equilibrium when the reaction path requires shifts in the composition of both the solution and solid. Equilibrium is not established until the solid composition is homogeneous and the chemical potentials of all components between solid and aqueous phases are equivalent. As a result, equilibrium is rarely demonstrated with a solid solution series. 

It is certainly more constant than that of sediments being introduced into the basin. This fact is due to the greater mobility of material in solution which tends to even out local fluctuations in concentration through the action of waves and currents. The sediment is much less subjected to such a mechanical homogenization process and tends, therefore, to attain equilibrium by localized mineral reaction. The type of thermodynamic system operative is most likely to be "open", where each point of sediment has some chemical variables fixed by their concentration in the sediment (inert components due to their low solubility in the solution) and other chemical components, which are soluble, have their concentration in the sediment a function of their activity in the aqueous solution. The bulk composition of the resulting sediment will be largely determined by the composition of the waters in which it is sedimented and the length of time it has reacted with this environment. The composition of the aqueous solution is, of course, determined to a minor extent by these reactions. 

Since the major chemical reactions take place through the agency of an aqueous fluid, the system can be considered to be saturated with respect to water. H O is always the major component of an omnipresent fluid phase during the attainment of equilibrium and it is therefore considered a component in excess. We are left with a four component system, Na-K-Al-Si where, for unspecified P-T conditions over a short range, there will be a maximum of four phases coexisting. 

The thermodynamic treatment of systems in which at least one component is an electrolyte needs special comment. Such systems present the first case where we must choose between treating the system in terms of components or in terms of species. No decision can be based on thermodynamics alone. If we choose to work in terms of components, any effect of the presence of new species that are different from the components, would appear in the excess chemical potentials. No error would be involved, and the thermodynamic properties of the system expressed in terms of the excess chemical potentials and based on the components would be valid. It is only when we wish to explain the observed behavior of a system, to treat the system on the basis of some theoretical concept or, possibly, to obtain additional information concerning the molecular properties of the system, that we turn to the concept of species. For example, we can study the equilibrium between a dilute aqueous solution of sodium chloride and ice in terms of the components water and sodium chloride. However, we know that the observed effect of the lowering of the freezing point of water is approximately twice that expected for a nondissociable solute. This effect is explained in terms of the ionization. In any given case the choice of the species is dictated largely by our knowledge of the system obtained outside of the field of thermodynamics and, indeed, may be quite arbitrary. 

Thorstenson and Plummer (1977), in an elegant theoretical discussion (see section on The Fundamental Problems), discussed the equilibrium criteria applicable to a system composed of a two-component solid that is a member of a binary solid solution and an aqueous phase, depending on whether the solid reacts with fixed or variable composition. Because of kinetic restrictions, a solid may react with a fixed composition, even though it is a member of a continuous solid solution. Thorstenson and Plummer refer to equilibrium between such a solid and an aqueous phase as stoichiometric saturation. Because the solid reacts with fixed composition (reacts congruently), the chemical potentials of individual components cannot be equated between phases the solid reacts thermodynamically as a one-component phase. The variance of the system is reduced from two to one and, according to Thorstenson and Plummer, the only equilibrium constraint is IAP g. calcite = Keq(x>- where Keq(x) is the equilibrium constant for the solid, a function of... 

There are several other physical and chemical variables that affect the adsorption rate and the adsorption equilibrium of an adsorption system involving the separation of a solute from aqueous onto an adsorbent. These include the total surface area of an adsorbent, concentration of adsorbent, concentration of adsorbate, nature of adsorbent, nature of adsorbate, nature of the mixture of solutes (such as dissolved soUds content), hydrogen ion concentrations of the system, and the temperature of the system. In a multi-component bubble separation system, several adsorption mechanisms are involved. True adsorption phenomena cannot be clear until laboratory experiments are conducted. 

Since most TPH contamination involves a complex mixture of hydrocarbons, it is unlikely that aqueous readings beyond the NAPL zone will be near the limits of solubility (based on assumptions of a pure hydrocarbon type in equilibrium with water). If concentrations are near or above solubility limits, NAPL was probably present in the sample. TPH materials are relatively insoluble in water, with only the BTEX chemicals or some short-chain aliphatic hydrocarbons showing any appreciable potential for water solubility. When they are part of complex mixtures, the individual components never reach the concentrations predicted from their solubility constants as individual chemicals. For example, chemicals like benzene or toluene, which may constitute a small percentage within an initial bulk product like gasoline, jet fuel, or diesel fuel, have a much greater tendency to stay dissolved in the NAPL system than to become integrated into the water-based system beyond the NAPL boundary. Therefore, the effective solubility of these chemicals as part of a complex mixture is less than it would be in a release of the pure chemical. 

The splitting of redox reaetions into two half cell reactions by introducing the symbol e is highly useful. It should be noted that the e notation does not in any way refer to solvated electrons. When calculating the equilibrium composition of a chemical system, both e , and can be chosen as components and they can be treated numerically in a similar way equilibrium constants, mass balances, etc. may be defined for both. However, while represents the hydrated proton in aqueous solution, the above equations use only the activity of e , and never the concentration of e . Concentration to activity conversions (or activity coefficients) are never needed for the electron cf. Appendix B, Example B.3). 

The EQ3/6 software package consists of several principal components. These are the EQ3NR and EQ6 codes, the EQLIB library, and the thermodynamic ta base. The EQLIB library and the thermodynamic data base support both of the main modeling codes. EQLIB contains math routines, routines that perform various computer system functions, and routines that evaluate scientific submodels, such as for activity coefficients of aqueous species, that are common to both EQ3NR and EQ6. The data base covers a wide range of chemical elements and nominally allows calculations in the temperature range 0-3(X)°C at a constant pressure of 1.013 bar from 0-l()0°C and the steam-liquid water equilibrium pressure from 1(X)-3(X)°C. 

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