Extraction equilibria, with chemical reaction

For any pure chemical species, there exists a critical temperature (Tc) and pressure (Pc) immediately below which an equilibrium exists between the liquid and vapor phases (1). Above these critical points a two-phase system coalesces into a single phase referred to as a supercritical fluid. Supercritical fluids have received a great deal of attention in a number of important scientific fields. Interest is primarily a result of the ease with which the chemical potential of a supercritical fluid can be varied simply by adjustment of the system pressure. That is, one can cover an enormous range of, for example, diffusivities, viscosities, and dielectric constants while maintaining simultaneously the inherent chemical structure of the solvent (1-6). As a consequence of their unique solvating character, supercritical fluids have been used extensively for extractions, chromatographic separations, chemical reaction processes, and enhanced oil recovery (2-6). 

Kotas [3] has drawn a distinction between the environmental state, called the dead state by Haywood [1], in which reactants and products (each at po. To) are in restricted thermal and mechanical equilibrium with the environment and the truly or completely dead state , in which they are also in chemical equilibrium, with partial pressures (/)j) the same as those of the atmosphere. Kotas defines the chemical exergy as the sum of the maximum work obtained from the reaction with components atpo. To, [—AGo], and work extraction and delivery terms. The delivery work term is Yk k kJo ln(fo/pt), where Pii is a partial pressure, and is positive. The extraction work is also Yk kRkTo n(po/Pk) but is negative. 

The initial set of experiments and the first few textbook chapters lay down a foundation for the course. The elements of scientific activity are immediately displayed, including the role of uncertainty. The atomic theory, the nature of matter in its various phases, and the mole concept are developed. Then an extended section of the course is devoted to the extraction of important chemical principles from relevant laboratory experience. The principles considered include energy, rate and equilibrium characteristics of chemical reactions, chemical periodicity, and chemical bonding in gases, liquids, and solids. The course concludes with several chapters of descriptive chemistry in which the applicability and worth of the chemical principles developed earlier are seen again and again. 

In our environment, there are many substances that, like oxygen in our atmosphere, cannot further diffuse and/or react toward more stable configurations and may be considered to be in equilibrium with the environment. Neither chemical nor nuclear reactions can transform these components into even more stable compounds. From these components, we cannot extract any useful work, and therefore an exergy value of OkJ/mol has been assigned to them. This has been done for the usual constituents of air N2,02/ C02/ H20, DzO, Ar, He, Ne, Kr, and Xe at T0 = 298.15 K and P0 = 99.31 kPa, the average atmospheric pressure [1]. Their partial pressures P in air are given in Table 7.1. 

Solvent extraction may be accompanied by a chemical reaction. The selectivity and the extraction factor can be greatly improved by carrying out the extraction with a solution of an extractant that chemically converts the solute to a form that is preferentially soluble in the extracting solvent. An additional advantage of this procedure is that the reverse extraction of solute (stripping) can often be carried out by changing the equilibrium constant of the reaction, e.g., by changing the pH or temperature. 

Vp and VL are the volumes of the extraction agent and the liquid sample, respectively, and Kle - Ce/Ce is the distribution coefficient. In practice, an extraction yield higher than 99% is usually considered to be quantitative. With the use of the same volumes of the extraction agent and the sample, this result can be obtained even in a single extraction step if Kle < 0.01. Sometimes the entire procedure can be complicated by a chemical reaction taking place, e.g., in the extractive alkylation (see p.59) or in the preparation of volatile metal chelates (see p.194), and the total yield of the extraction then involves, in addition to the interphase distribution of the initial compounds and products, also the chemical equilibrium which is attained by the reaction. If the quantitative yield of the extraction cannot be predicted on the basis of the character of the system, the extraction efficiency must be determined, otherwise the quantitative evaluation is questionable. 

This manifold has been used for the USALLE of paracetamol from suppositories [17]. Hydrolysis of the analyte prior to reaction with o-cresol in the alkaline extractant medium was also favoured by US (the entire sample plug was irradiated in EC). Hydrolysis and formation of the reaction product displaced the extraction equilibrium, thus favouring extraction into the aqueous phase. The influence of the variables related to the dynamic manifold (namely, flow rate and sample volume), chemical variables (namely, NaOH and o-cresol concentrations) and temperature was studied using the univariate method on account of their independence on the other hand, those related to US (namely, probe position, radiation amplitude and pulse duration) were the subject of a multivariate study in which the latter two exhibited an insignificant but positive effect. Positioning the probe closest to the extraction coil was found to maximize extraction efficiency. The positive effect of US on extraction and analyte hydrolysis provides the overall enhancement shown in Fig. 6.4A, which shows the results obtained in the presence and absence of US. The time required for the development of the method was significantly shorter than that required by the United States Pharmacopoeia (USP) method. In addition, the latter produces emulsions that need about 30 min for phase separation after extraction. 

To analyze the behavior of the system with the mathematical model proposed in the previous section, three parameters are needed the chemical equilibrium parameters and of the extraction and of the stripping chemical reactions, respectively, and the product, Afci, of the interfacial area of the emulsion and the kinetic constant of the forward stripping reaction. Estimation of the parameter values needs deep experimental analysis in the literature the following set of parameter values... 

Preceding chapters have dealt largely with pure substances or with constant-composition mixtures. e.g., air. However, composition changes are the desired outcome, not only of chemical reactions, but of a number of industrially important mass-transfer operations. Thus composition becomes a primary variable in the remaining chapters of tliis text. Processes such as distillation, absorption, and extraction bring phases of different composition into contact, and when tlie phases are not in equilibriimi, mass transfer between the phases alters their compositions. Botli tlie extent of change and tlie rate of transfer depend on the departure of the system from equilibrium. Thus, for quantitative treatment of mass transfer the equilibrium T, P, and phase compositions must be known. 

Surface complexation models of the solid-solution interface share at least six common assumptions (1) surfaces can be described as planes of constant electrical potential with a specific surface site density (2) equations can be written to describe reactions between solution species and the surface sites (3) the reactants and products in these equations are at local equilibrium and their relative concentrations can be described using mass law equations (4) variable charge at the mineral surface is a direct result of chemical reactions at the surface (5) the effect of surface charge on measured equilibrium constants can be calculated and (6) the intrinsic (i.e., charge and potential independent) equilibrium constants can then be extracted from experimental measurements (Dzombak and Morel, 1990 Koretsky, 2000). 

The sample residence time in the flow manifold, often associated with the expression sample incubation time, is an important parameter in flow-based analytical procedures involving relatively slow chemical reactions and/or physicochemical processes, e.g., dialysis, gas diffusion or liquid—liquid extraction. This parameter may become a limiting factor in the system design, especially if sensitivity is critical. Moreover, the susceptibility to biased results is less pronounced when the chemical reactions and /or the involved physico-chemical processes tend towards completion. Fig. 1.4 refers to a hypothetical situation where biased results are obtained when chemical equilibrium is not reached. 

The relatively large valons found for the extraction equilibrium constant of copper with Kelex 100 (3 and 90) indicate (hat shipping of copper from (his rcngenl should he difficult. It is fonnd, however, that copper does strip reedily into sulfuric acid solutions because Kalex 100 reacts with sulhiric acid in preference to copper. Fitting the extinction of sulforic acid by Kelex 100 by a chemical-reaction equilibrium constant. 

Amine-extraction equilibria can also be modeled by chemical-reaction equilibrium constants. Figure 8.3-3 indicates that cations such as iron(IIl), zinc, cobelt(ll) and coppeifU) exhibit high distribution coefficients with chloride solutions, wherese nickel. iron(II), and manganese are not extracted to any great extent. The besis for the differences in distribution coefficients lies mainly in the tendency for the former group of cations to fonn chloride complexes. Stability constants for these complexes are available in the literature,11 and they can be used to develop quantitative phase-equilibrium models. 

Unfortunately, few of the published studies of extraction equilibria heve provided complete quantitative models that are useful for extrapolation of data or for predicting multiple metal distribution equilibria from single metal data. The chemical-reaction equilibrium formulation provides a framework for constructing such models. One of the drawbacks of purely empirical correlations of distribution coefficients is that pH has often been chosen as an independent variable. Such a choice is suggested by the form of Pigs. 8-3-5 and 8.3-8. Although pH is readily measured and contmlled on a laboratory scale, it is really a dependent variable, which is detenmined by mass belances and simultaneous reaction equilibria. An appropriate phare-equilibrium model should be able to predict equilibrium pH, at least within a moderate activity coefficient correction, concurrently with other species concemrations. 

Phase equilibria can be modeled in terms of equilibrium constants for the relevant reactions. Because of low mutual solubilities of the phases, extraction reactions appear to be heterogeneoue. Some reactions eshibil slow chemical kinetics, with die reaction step constituting a resistance to extraction in series with intraphase mass transfer. 

To present the attempts made to characterize the physicochemical interactions between both the solvent (extractant and additives) and the solid support and the solvent and the metal ions To present the attempts made to describe the extraction process in terms of chemical reactions and equilibrium constants To present the efforts made to describe the kinetics of the extraction process with the purpose of identifying the nature of the process and the rate-determining step and determining the kinetic parameters of the systems... 

To separate and purify the radionuclide of interest in the sample, the analyst can depend on the similar behavior of the stable element and its radioisotopes. Chemical reactions involving the radionuclide will proceed with essentially the equilibrium and rate constants known for the stable element in the same chemical form. Slight differences result from small differences between the isotopic mass of the radionuclide and the atomic mass (i.e., the weighted average of the stable isotopic masses) of the stable element. Because of this similarity in chemical behavior, many ra-dioanalytical chemistry procedures were adapted from classical quantitative and qualitative analysis. For the same reason, new methods published for separating chemical substances by processes such as precipitation, ion-exchange, solvent extraction, or distillation are adapted for and applied to radionuclides. One exception occurs when the radionuclides to be separated are two or more isotopes of the same element. Here, effective separation can be accomplished by mass spectrometer (see Chapter 17). 

In membrane extraction of metals, the mass transport of solute from one phase to another occurs by diffusion. It is controlled by phase equilibrium and the resistances of boundary layers in two phases and the membrane material. Both types of materials are used for membrane extraction and stripping, hydrophilic and hydrophobic, and composite hydro-philic/hydrophobic barriers are also developed to avoid the membrane solubilization [122,123]. To enhance separation, the reactive liquids that induce chemical reaction with one of the separated species can be used. In membrane SX of metals, extracting agents, such as tri- -octylphosphine oxide (TOPO), di(2-ethylhexyl)phosphoric acid (D2EHPA), and n-octyl(phenyl)-A,A-diisobutylcarbamoylmethylphosphine oxide (CMPO), and commercial reagents like CYANEX 301, CYANEX 923, LIX622, and LIX622N are applied. 

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