Aqueous solutions composition

For each pair of runs nearly identical solid-aqueous solution compositions were observed after periods of three to four weeks, which compared favorably to compositions previously reported (6,7). Table I summarizes the original data (8). 

Although nearly identical solid-aqueous solution compositions are observed in recrystallization from two directions under conditions of total constant composition, this alone is insufficient proof of the establishment of equilibrium. In order to test for equilibrium, the solid solution activity coefficients must be determined and used to compare observed solid and aqueous solution compositions with the appropriate values expected at equilibrium. 

The final solid solution-aqueous solution compositions of Table I will fall into one of three catagories (1) they will either be at equilibrium, (2) at stoichiometric saturation, or (3) correspond to some non-equilibrium state. 

This permits provisional calculation of the compositional dependence of the equilibrium constant and determination of provisional values of the solid phase activity coefficients (discussed below). The equilibrium constant and activity coefficients are termed provisional because it is not possible to determine if stoichiometric saturation has been established without independent knowledge of the compositional dependence of the equilibrium constant, such as would be provided from independent thermodynamic measurements. Using the provisional activity coefficient data we may compare the observed solid solution-aqueous solution compositions with those calculated at equilibrium. Agreement between the calculated and observed values confirms, within the experimental data uncertainties, the establishment of equilibrium. The true solid solution thermodynamic properties are then defined to be equal to the provisional values. 

Bradbury Baeyens (2002a, b) provided sorption data bases for Opalinus Clay and MX-80 bentonite, respectively, based on numerous experiments with varying aqueous solution compositions. Since sorption can be influenced by the complexa-tion of radionuclides with various ligands, corrections had to be made to account for the differences in speciation between the experimental solutions and the Opalinus Clay and bentonite porewaters. For this purpose, the Nagra/PSI TDB 01 /01 was used to model the speciation of all safety-relevant radionuclides in the experimental solutions and in the Opalinus Clay and bentonite porewaters. 

Alternatively to the keyword SOLUTION, SOLUTIONSPREAD can be used for the input of solution. The input is transposed compared to the input for SOLUTION, i.e. the rows of input for SOLUTION become the columns of input for SOLUTION SPREAD. It is especially convenient to define more than one aqueous solution composition using this tab-limited format. Data obtained e g. from a laboratory spreadsheet format can be copied directly into the PHREEQC input file. SOLUTION SPREAD is compatible with the format of many spreadsheet programs, as e.g. Excel. The column headings are element names, element valence state names or isotope names. One subheading can be used to define speciation (e.g. as S04 , or as N03 ), specify element specific units, redox couples, phase names and saturation indices. All succeeding lines are the data values for each solution, with one solution defined on each line. 

To completely describe thermodynamic equilibrium, a second relation must be derived from equations 1 and 2 (2 Glynn and Reardon, Am. J. Sci.> in press). The "solutus" equation expresses EITeq as a f unction of aqueous solution composition ... 

The second hypothesis considers the solid as a multi-component solid-solution, capable of adjusting its composition in response to the aqueous solution composition, given the long equilibration period, the relatively high solubility of the solid and relatively low solid to solution ratio. In this case, the assumption of thermodynamic equilibrium may apply. If the equilibration period is too short for thermodynamic equilibrium to have been achieved, but if an outer surface layer of the solid has been able to recrystallise (because of the high solubility of the solid), the concept of primary saturation may apply. 

The composition of a SSAS system at primary saturation or at stoichiometric saturation will be generally independent of the initial solid to aqueous-solution ratio, but will depend on the initial aqueous-solution composition existing prior to the dissolution of the solid. In contrast, the final thermodynamic equilibrium state of a SSAS system attained after a dissolution or recrystallisation process will generally depend not only on the initial composition of the system but also on the initial solid to aqueous-solution ratio (Glynn et al, submitted to Gcochim, Cosmochim. Acta). 

Usually when reaction paths are simulated, the irreversible reactant is an unstable mineral or a suite of unstable minerals that is, the stoichiometry of the irreversible reaction is fixed. Evaporation poses a special problem in reaction path simulation because the stoichiometry of the irreversible reaction (defined by the aqueous solution composition) continually changes as other minerals precipitate (or dissolve). In the second problem (above) evaporation of seawater was simulated by irreversible addition of "sea salt", that is, a hypothetical solid containing calcium, magnesium, sodium, potassium, chloride, sulfate and carbon in stoichiometric proportion to seawater. The approach used was valid as long as intermediate details of the reaction path are not required. The reaction path during evaporation could be solved in PHRQPITZ by changing the stoichiometry of the irreversible reactant (altered "sea salt") incrementally between phase boundaries, but this method would be extremely laborious. 

Novel composite cross-linked PDMS/PSf PV membrane was reported by Liang et al. (2005) to remove 1,2-dichloroethane (1,2-DCE) from aqueous solution. Composite PV membranes were prepared by the cross-linking of H-terminated oli-gosilylstyrene (oligo-SiHj) and vinyl-terminated PDMS (vinyl-PDMS) on the surface of PSf UF membrane. The membrane showed the characteristic of preferential selection to 1,2-DCE and the separation factor was 600-2300. In another article, Liang et al. (2006) studied the mass transfer of dilute 1,2-dimethoxyethane (1,2-DME) aqueous solution during PV on vinyl-PDMS/PSf membrane. The effect of a boundary layer on the mass transfer of dilute 1,2-DME aqueous solution was observed during the PV of the composite membranes constructed from cross-linked PDMS as the top layer and PSf UF membrane as the substrate. 

Aqueous Solution Composition Bueche Analysis Graessley Analysis ... 

Fig. 6 Volta-potential as a ftmction of pH of an aqueous solution. Composition of reaction mixture 20 mM tris-HCl + 1 mM NADH + 10 M 2-JV-methylamino-l,4-naphtoquinone + 5 g/ml chlorophyll [48]...

During the computer simulation, the equations for the chemical reactions occurring at each node are solved separately, on the assumption that the characteristic times of these reactions are much shorter than those of the mass transport or other corrosion processes. At the end of each time step, the resulting aqueous solution composition at each node is solved to equilibrium by a call to an equilibrium solver that searches for minima in Gibbs energy. The model was tested by... 

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