Extractants equilibrium reactions

In a liquid-liquid extraction, the analyte (or interferent) is extracted from one liquid phase into a second, immiscible liquid phase. When the analyte is involved in secondary equilibrium reactions, it is often possible to improve selectivity by carefully adjusting the composition of one or both phases. 

Separation module for a flow injection analysis using a liquid-liquid extraction (inset shows the equilibrium reaction). 

The thermodynamic or extraction equilibrium constant (K) for the reaction is given by... 

If the solute A does not undergo any reaction in the two solvents, except for the solubility caused by the solvation due to the nonspecific cohesive forces in the liquids, the distribution of the solute follows the Nernst distribution law, and the equilibrium reaction can be described either by a distribution constant or an (equilibrium) extraction constant... 

This reaction is suppressed by using a large excess of Cl ion in the stripping phase. Furthermore, the extraction equilibrium constant of the above reaction is much lower than that of reaction given by Eq. (4). The overall distribution coefficient of CPC, mg (defined as Cqp/Cp) is given by [50]... 

It was confirmed in the work at Midwest Research that propylene could be reacted with recycle alkylation acid to give a high yield of diisopropyl sulfate (DIPS), and that the DIPS could be extracted with hydrocarbon solvents, including isobutane. The isopropyl acid sulfate (IPS) is quite insoluble in hydrocarbons and only a small amount is extracted along with the DIPS. Since the reaction of propylene with sulfuric acid is an equilibrium reaction, some IPS is always present. It was found that some of the conjunct polymer is also extracted with the DIPS. It was anticipated that the water would stay in the acid phase or raffinate, and this was found to be the case. 

Approximately 90% of the used alkylation acid was converted to dipropyl sulfate and approximately 90% of the dipropyl sulfate formed was sent back to alkylation wherein the sulfuric acid was reaenerated or recovered. Overall, approximately 80% of the used alkylation acid was recovered. The reactions of sulfuric acid with propylene is an equilibrium reaction, and by the method of operation described it is not possible to convert all of the acid to extractable dialkyl sulfates some will remain as alkyl acid sulfate. In addition, with the extraction conditions used it is not possible to extract all of the dialkyl sulfates formed. 

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. 

Phase equilibria of vaporization, sublimation, melting, extraction, adsorption, etc. can also be represented by the methods of this section within the accuracy of the expressions for the chemical potentials. One simply treats the phase transition as if it were an equilibrium reaction step and enlarges the list of species so that each member has a designated phase. Thus, if Ai and A2 denote liquid and gaseous species i, respectively, the vaporization of Ai can be represented stoichiometrically as —Aj + A2 = 0 then Eq. (2.3-17) provides a vapor pressure equation for species i. The same can be done for fusion and sublimation equilibria and for solubilities in ideal solutions. 

With the assumptions mentioned above two main parameters will be necessary in the description of the separation-concentration process of one solute, that is, the membrane mass transport coefficient and the equilibrium parameter of the extraction interfacial reaction. 

Since esterification is an equilibrium reaction, the treatment was performed using an excess of alcohol. The grafting of alcohols or diols was performed in an autoclave at 150°C. For PEG, special care was taken to avoid oxidation (outgassing of the silica/PEG mix and treatment under N2 in a sealed tube at 150°Q. In each case, the excess reagent was eliminated either by heat treatment under vacuum (volatile alcohols and diols), or by solvent extraction (THF) in a Soxhlet extractor. 

As a rule, the rates of ion-exchange reactions, or rates of complex formation and destruction on the interfaces or within the IBM, are fast compared to diffusion rates. Thus, corresponding concentration gradients of the counterions are related through the extraction equilibrium constants [46, 53, 56]. The averaged sums of the IBM and LMF potentials can be experimentally realized through distribution coefficients at membrane-based solvent forward and backward extraction Ep = Mfl/Mei on the feed side and Ep = Me/M i on the strip side (see Fig. 6.2B). 

CDTech uses catalytic distillation to convert isobutene and methanol to MTBE, where the simultaneous reaction and fractionation of MTBE reactants and products takes place [51], A block diagram of this process is shown in Figure 3.31. The C4 feed from catalytic crackers undergoes fractionation to extract deleterious nitrogen compounds. It is then mixed with methanol in a BP reactor where 90% of the equilibrium reaction takes place. The reactor effluent is fed to the catalytic distillation (CD) tower where an overall isobutene conversion of 97% is achieved. The catalyst used is a conventional ion-exchange resin. This process selectively removes MTBE from the product to overcome the chemical equilibrium limitation of the reversible reaction. The MTBE product stream is further fiactionated to remove pentanes, which are sent to gasoline blending, whereas the raffinate from the catalytic distillation tower is washed with water and then fractionated to recover the methanol. 

Asai et al. (1994) have developed a reaction model for the oxidation of benzyl alcohol using hypochlorite ion in the presence of a PT catalyst. Based on the film theory, they develop analytic expressions for the mass-transfer rate of QY across the interface and for the inter-facial concentration of QY. Recently, Bhattacharya (1996) has developed a simple and general framework for modeling PTC reactions in liquid-liquid systems. The uniqueness of this approach stems from the fact that it can model complex multistep reactions in both aqueous and organic phases, and thus could model both normal and inverse PTC reactions. The model does not resort to the commonly made pseudo-steady-state assumption, nor does it assume extractive equilibrium. This unified framework was validated with experimental data from a number of previous articles for both PTC and IPTC systems. 

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. 

The complex compounds formed by the metal cations in solvent extraction systems are illustrated first by the formation of complex compounds between cations and neutral molecules in aqueous solution. An example of an equilibrium reaction involving such formation is [P2]... 

When the distribution equilibrium reaction involves hydrogen ions, changing the hydrogen ion concentration will have a strong effect on the distribution coefficient. An example of this is the extraction of metal complexes of acetyl acetone (HAa) and other weakly acid complexing agents by benzene. The equilibrium reaction for extraction of thorium by this reagent is... 

The earliest method of esterifying amino acids was developed by Fischer (5). By continuously passing hydrogen chloride gas into ethanolic suspensions of amino acids, the amino acids dissolve and the solution becomes homogeneous. After adding base to the reaction mixture, the resulting ester hydrochlorides can be isolated and extracted with ether. In principle, this is not a very efficient method since it is an equilibrium reaction. However, it can be made useful by... 

Equilibrium reaction with separation (distillation, extraction)... 

The absorption and IR spectra in both the aqueous and the organic phase were consistent with the extraction equilibrium (A.67). There is no quantitative information on the equilibrium constant for Reaction (A.67) or for the complex formation reactions in the aqueous phase. 

Wang and Wu [70] analyzed the extraction equilibrium of the effects of catalyst, solvent, NaOH/organic substrate ratio, and temperature on the consecutive reaction between 2,2,2-trifluoroethanol with hexachlorocyclotriphosphazene in the presence of aqueous NaOH. Wu and Meng [69] reported the reaction between phenol with hexachlorocyclotriphosphazene. They first obtained the intrinsic reaction-rate constant and overall mass transfer coefficient simultaneously, and reported that the mass transfer resistance of QX from the organic to aqueous phase is larger than that of QY from the aqueous to organic phase. The intrinsic reaction-rate constant and overall mass transfer coefficients were obtained in three ways. 

The selection of an appropriate method (marker residues, analytical range) therefore depends in part on the regulatory application, given the different regulatory limits applied in different jurisdictions. Since the epimerization of CTC, OTC, and TC is an equilibrium reaction, the epimers are often present to a greater or lesser extent in analytical standards and can occur throughout the extraction and clean-up procedure. Analytical method development is further complicated by the need to separate and quantify parent and epimer, even when the local legislation does not demand it. 

Three methods which do not require solution of the nonlinear partial differential equation are presented for estimating extractor performance. The choice of method depends on the value of the dimensionless outlet solute concentration, oj. If (oj + 1)/oJ is close to 1, the reaction is effectively irreversible and the pseudosteady-state solution of the advancing front model satisfactorily predicts performance after normalization to include solute solubility in the globule. If (oj + 1)/oJ is not close to 1, the advancing front results will still apply, provided that the amount of solute extracted by reaction is small and membrane solubility controls. When oj is small enough so that (oj + 1) is close to 1, then the reversible reaction model can be reduced to a linear equation with an analytical solution. Otherwise, for oj values when neither (oj + 1) nor (oj + 1)/o is nearly 1, a reasonable first approximation is made by adjusting the actual concentration of internal reagent to an effective concentration which equals the amount consumed to reach equilibrium. 

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