Phase transfer factor

When the analyte transfer from sample to concentrate is incomplete, i.e., the phase transfer factor P is less than unity. 

The transfer of analyte from one phase to another in liquid-liquid extraction is achieved at the interface between the two immiscible phases. With appropriate agitation of the phases, mass transport within the two phases is of minor importance for the transfer efficiency. Maximum exposure of the phase interface is therefore pursued in batch extractions by vigorous shaking, resulting in the formation of highly dispersed droplets of one phase in the other. However, such conditions are not achievable in continuous flow systems such as FIA. Nevertheless, in most cases phase transfer factors of... 

In order to achieve high phase transfer factors, the extraction coil lengths are usually substantially longer than normal FI reaction coils. This feature would have been a principle source of analyte dispersion in the extraction system however, the phase... 

Like most other FI analytical processes, the separations are also almost always performed under non-equiiibrated conditions, and the phase transfer factors P are rarely higher than 0.3. usuall> being in the range 0.05-0.2. While this sometimes may have some unfavourable effects on sensitivity, they may be compensated for whenever necessary, by preconcentration measures during the gas-liquid separation. On the other hand, the non-equilibrium conditions may be exploited favourably to improve selectivity through kinetic discrimination (cf. Sec. 5.5.1 Tolerance of interferences in FI hydride generation systems). 

The highest phase transfer factor is obtained when the flow-rate ratio of donor and acceptor streams is one, and decreases with a deviation from unity to either direction. 

The phase transfer factor P (cf. Sec. 1.4.6) in on-line dialysis may be conveniently expressed by the dialysis factor, defined as the output concentration of the detector channel. Q, normalized by division with the initial sample channel concentration, Cs [5]. The phase transfer efficiency may also be described using dialysis percentage by multiplying the dialysis factor with 100. 

Factors Influencing the Usefulness of Phase-Transfer Catalysis... 

Phase transfer catalysis has been used with success to prepare N- substituted pyrazoles (78MI40403, 79MI40408, 70JHC1237, 80JOC3172) and this procedure can be considered the simplest and most efficient way to obtain these compounds. Experimental design methodology has been used to study the influence of the factors on the reaction between pyrazole and -butyl bromide under phase transfer conditions (79MI40408). 

Pressure is similar to temperature as a rate limiting factor since the diffusibility of a gas is inversely related to its pressure. For instance, loweringthe pressure 760 Torr(l atm)to 1 Torr increases the gas-phase transfer of reactants to the deposition surface and the... 

The presence of two phases in the reaction mixture may seem to be a mass-transfer engineering problem, but even moderate stirring of the mixture produces an emulsion, which greatly facilitates the phase transfer steps of the reaction mechanism. In our fixed-bed reactor, the turbulence resulting from the flow rates used seemed to suffice to eliminate external mass transfer hmitations. At MeOH SA of 20 and identical LHSV values, similar acid conversions were observed for two linear flow velocities differing by a factor of two. 

Phase transfer catalysis. As well as their use in homogeneous reactions of the type just described, polyethers (crowns and cryptands) may be used to catalyse reactions between reagents contained in two different phases (either liquid/liquid or solid/liquid). For these, the polyether is present in only catalytic amounts and the process is termed phase transfer catalysis . The efficiency of such a process depends upon a number of factors. Two important ones are the stability constant of the polyether complex being transported and the lipophilicity of the polyether catalyst used. 

Onium salts, crown ethers, alkali metal salts or similar chelated salts, quaternary ammonium and phosphonium are some of the salts which have been widely used as phase transfer catalysts (PTC). The choice of phase transfer catalysts depends on a number of process factors, such as reaction system, solvent, temperature, removal and recovery of catalyst, base strength etc. 

The unique ability of crown ethers to form stable complexes with various cations has been used to advantage in such diverse processes as isotope separations (Jepson and De Witt, 1976), the transport of ions through artificial and natural membranes (Tosteson, 1968) and the construction of ion-selective electrodes (Ryba and Petranek, 1973). On account of their lipophilic exterior, crown ether complexes are often soluble even in apolar solvents. This property has been successfully exploited in liquid-liquid and solid-liquid phase-transfer reactions. Extensive reviews deal with the synthetic aspects of the use of crown ethers as phase-transfer catalysts (Gokel and Dupont Durst, 1976 Liotta, 1978 Weber and Gokel, 1977 Starks and Liotta, 1978). Several studies have been devoted to the identification of the factors affecting the formation and stability of crown-ether complexes, and many aspects of this subject have been discussed in reviews (Christensen et al., 1971, 1974 Pedersen and Frensdorf, 1972 Izatt et al., 1973 Kappenstein, 1974). 

The choice of the catalyst is an important factor in PTC. Very hydrophilic onium salts such as tetramethylammonium chloride are not particularly active phase transfer agents for nonpolar solvents, as they do not effectively partition themselves into the organic phase. Table 5.2 shows relative reaction rates for anion displacement reactions for a number of common phase transfer agents. From the table it is clear that the activities of phase transfer catalysts are reaction dependent. It is important to pick the best catalyst for the job in hand. The use of onium salts containing both long and very short alkyl chains, such as hexade-cyltrimethylammonium bromide, will promote stable emulsions in some reaction systems, and thus these are poor catalysts. 

Conventional Systems. In the conventional antifouling compositions, the organotin compound (TBTO, TBTF, TBTC1, TBTOAc) is mechanically mixed into the paint vehicle. When the TBT species is completely soluble in the polymer matrix, factors (a) and (b) become unimportant in most cases. The mobile species is already present its diffusion in the matrix, phase transfer and migration across the boundary layer into the ocean environment may be represented by Figure 2a. When the organotin compound forms a dispersed second phase, rate of its dissolution in the polymer matrix becomes another factor to consider. 

The mode of action of the antifouling polymers thus conforms to the bulk abiotic bond cleavage model. All the controlling factors, viz., diffusion of water into the polymer matrix, hydrolysis of the tributyltin carboxylate, diffusion of tributyltin species from the matrix to the surface, phase transfer of the organotin species, and its migration across the boundary layer, are analyzed. It is found that the transport of the mobile tributyltin species in the matrix is the rate limiting factor. 

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