Extraction with slow chemical reaction

Countercurrent Extraction Cascade with Slow Chemical Reaction... 

Figure 3.41. Concentrations in the solvent phases of stages 1, 2 and 3 in a countercurrent extraction column with slow chemical reaction.

In 1985, mono-segmented flow analysis was proposed [64] as a means of achieving extended sample incubation times without excessive sample dispersion. The sample was inserted between two air bubbles into an unsegmented carrier stream therefore the innovation combined the favourable characteristics of both segmented and unsegmented flow systems. Further development revealed other potential applications, especially with regard to relatively slow chemical reactions, flow titrations, sample introduction to atomic absorption spectrometers, liquid-liquid extraction and multi-site detection (Chapters 7 and 8). This innovation was also referred to as segmental flow injection analysis [65]. 

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. 

When one or more of the chemical reactions is sufficiently slow in comparison with the rate of diffusion to and away from the interface of the various species taking part in an extraction reaction, such that diffusion can be considered instantaneous, the solvent extraction kinetics occur in a kinetic regime. In this case, the extraction rate can be entirely described in terms of chemical reactions. This situation may occur either when the system is very efficiently stirred and when one or more of the chemical reactions proceeds slowly, or when the chemical reactions are moderately fast, but the diffusion coefficients of the transported species are very high and the thickness of the two diffusion films is close to zero. In practice the latter situation never occurs, as diffusion coefficients in liquids generally do not exceed 10 cm s, and the depth of the diffusion films apparently is never less than 10 cm. 

For chemical and physical processes across microdroplet/solution interfaces, obs having dimensions of s or dm mor s is often proportional to r " ( = 0, 1 or 2). A linear relationship between obs and has been reported for the extraction of a neutral compound such as ferrocene derivatives from water into a micro-oil-droplet without adsorption at the microdroplet/water interface [18,19] and for a photographic dye formation reaction in an oil-in-water emulsion [23]. The proportionality of a kobs versus r plot has been reported for a relatively slow process such as a photographic dye formation reaction [23,29,42], electron transfer [43-45] and adsorption at the micro-oil-droplet/water interface [19,20]. For the chemical reaction with the rate-determining step in a solution phase or a microdroplet in a microdroplet/solution system, fcobs is independent of r[23]. Based on the droplet size dependence of the reaction rate, the rate-determining step of the overall reaction processes across a microdroplet/solution interface is analysed and the reaction mechanism can be discussed in detail. 

Because the metal-extraction separation process involves chemical reactions, rates may be slow compared with ordianry liquid extraction. Although slow kiostics have important process design implications, more attention has been focused on mechanistic interpretations than on quantitative phenomenonological characterization. Cox and Fieri1 hava reviewed some chemical aspects of extraction kinetics. 

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. 

However, in some cases (e.g. [38]) the model was not successful. This happened in the case of the extraction of zinc and nickel with HDEHP using the RDC technique, and this was interpreted as follows in the case of zinc the transfer was found to be controlled by mass transfer alone because the chemical reaction was too fast to limit the kinetics in the case of nickel the reaction was too slow and much extractant was partitioning to the aqueous phase without complexing the metal cation, thus making it impossible to use the MTWCR model of Rod [57] presented above. 

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