Factors favouring solvent extraction

The concentration in the aqueous layer after three extractions with 8.33 mL of carbon tetrachloride is given by  

If we confine our attention to the distribution of a solute A between water and an organic solvent, we may write the percentage extraction E% as  

If the solution contains two solutes A and B it often happens that under the conditions favouring the complete extraction of A, some B is extracted as well. The effectiveness of separation increases with the magnitude of the separation coefficient or factor ft, which is related to the individual distribution ratios as follows  

1 per cent of B (ratio 10 1) a second extraction of the same aqueous phase will bring the total amount of A extracted up to 99.2 per cent, but increases that of B to 17.4 per cent (ratio 5.7 1). More complete extraction of A thus involves an increased contamination by B. Clearly, when one of the distribution ratios is relatively large and the other very small, almost complete separation can be quickly and easily achieved. If the separation factor is large but the smaller distribution ratio is of sufficient magnitude that extraction of both components occurs, it is necessary to resort to special techniques to suppress the extraction of the unwanted component. 

Ionic compounds would not be expected to extract into organic solvents 

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Clearly the most popular separation and preconcentration technique for atomic absorption analysis is solvent extraction. In this case it is easy to identify extraction systems which will remove a broad range of impurities from matrices such as acids, bases and alkali metal salt solutions. In addition to the advantages to be gained from separation, especially valuable in furnace work, and the concentration factors available, solvent extraction confers an additional advantage (typically a factor of 3—5) in flame analysis arising from the favourable nebulisation characteristics of several organic solvents. 

Essentially, extraction of an analyte from one phase into a second phase is dependent upon two main factors solubility and equilibrium. The principle by which solvent extraction is successful is that like dissolves like . To identify which solvent performs best in which system, a number of chemical properties must be considered to determine the efficiency and success of an extraction [77]. Separation of a solute from solid, liquid or gaseous sample by using a suitable solvent is reliant upon the relationship described by Nemst s distribution or partition law. The traditional distribution or partition coefficient is defined as Kn = Cs/C, where Cs is the concentration of the solute in the solid and Ci is the species concentration in the liquid. A small Kd value stands for a more powerful solvent which is more likely to accumulate the target analyte. The shape of the partition isotherm can be used to deduce the behaviour of the solute in the extracting solvent. In theory, partitioning of the analyte between polymer and solvent prevents complete extraction. However, as the quantity of extracting solvent is much larger than that of the polymeric material, and the partition coefficients usually favour the solvent, in practice at equilibrium very low levels in the polymer will result. 

At pH 9.05, the phenol-phenolate equilibrium favours the phenol by a factor of 7 1, and the amine-ammonium ion equilibrium favours the amine by a factor of 7 1. In other words, the non-ionized morphine predominates, and this can thus be extracted into the organic phase. What about the amounts in ionized form are these not extractable By solvent extraction of the non-ionized morphine, we shall set up a new equilibrium in the aqueous phase, so that more non-ionized morphine is produced at the expense of the two ionized forms. A second solvent extraction will remove this, and we shall effectively recover almost all the morphine content. A third extraction would make certain that only traces of morphine were left as ionized forms. 

The most widely applied reagents have been chelating agents which will complex with many metals, e.g. dithizone and the various thiocarbamate derivatives such as diethyldithiocarbamate and pyrrolidine dithiocarbamate. The latter agent as the ammonium salt (APDC) has been shown to complex some thirty elements [19] most of which can be readily extracted into various solvents. 4-Methylpentane-2-one (methyl isobutyl ketone or MIBK) is usually the favoured solvent because of its excellent compatibility with flames. The solubility of MIBK in water is not negligible and this limits the available concentration factor to ten higher molecular weight ketones (e.g. decan-2-one) offer better concentration factors and chloroform up to fifty times, but this latter solvent is only really suitable for electrothermal atomisation. 

Polarity is no longer the main factor in the choice of solvent when the ion pair that must be extracted is extremely lipophilic, as in the case of Bu N Br", due to a favourable structural combination of both cation and anion. This aspect is of particular interest if we consider that the polarity of the organic medium can both determine the extractability of the ion pair and influence its chemical reactivity. For example, in the substitution reaction of the methanesulfonate group of... 

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