Water, coextraction

When coextraction of water is an important consideration, the diluent can heve a large effect on the amonm of water coextracted and also can improve the separation of water in a subrequeni distillation step. It can be worthwhile to choose a diluanl with a lower equilibrium distribution coefficient for the solute, if the ratio of solute to water extracted is increased. 

Extraction properties, including efficiency, selectivity, water coextraction, and temperature dependence, are strongly affected by the properties of the extracting medium. These depend on the amine concentration and on the diluent used. In a nonpolar solvent, charged species such as ion pairs tend to stabilize by aggregation. Polar and particularly protic solvents solubilize ion pairs, reduce their aggregation, and substantially enhance their extraction efficiency. The effects of diluents are in many cases so strong that they are frequently refered to as extractant modifiers or enhancers. 

A complete analysis of a system with reactions (1)—(19) is very complex. For the purpose of analyzing the effects of aqueous phase concentration and of HA acidity on the selectivity, one can use the simplifying assumptions that (1) aggregation and water coextraction do not affect the selectivity, and (2) the presence of HA in the organic phase does not affect the HCl/HjPO i preference in binding to the amine (ion-pair formation) or to the ion pair formed. With these assumptions the effect of HA at the low aqueous phase concentration range will be discussed first. 

Metal Extraction. As with other carboxyhc acids, neodecanoic acid can be used in the solvent extraction of metal ions from aqueous solutions. Recent appHcations include the extraction of zinc from river water for deterrnination by atomic absorption spectrophotometry (105), the coextraction of metals such as nickel, cobalt, and copper with iron (106), and the recovery of copper from ammoniacal leaching solutions (107). 

In almost all theoretical studies of AGf , it is postulated or tacitly understood that when an ion is transferred across the 0/W interface, it strips off solvated molecules completely, and hence the crystal ionic radius is usually employed for the calculation of AGfr°. Although Abraham and Liszi [17], in considering the transfer between mutually saturated solvents, were aware of the effects of hydration of ions in organic solvents in which water is quite soluble (e.g., 1-octanol, 1-pentanol, and methylisobutyl ketone), they concluded that in solvents such as NB andl,2-DCE, the solubility of water is rather small and most ions in the water-saturated solvent exist as unhydrated entities. However, even a water-immiscible organic solvent such as NB dissolves a considerable amount of water (e.g., ca. 170mM H2O in NB). In such a medium, hydrophilic ions such as Li, Na, Ca, Ba, CH, and Br are selectively solvated by water. This phenomenon has become apparent since at least 1968 by solvent extraction studies with the Karl-Fischer method [35 5]. Rais et al. [35] and Iwachido and coworkers [36-39] determined hydration numbers, i.e., the number of coextracted water molecules, for alkali and alkaline earth metal... 

TABLE 2 Numbers (w) of Coextracted Water Molecules in NB and Radii (ri,) of Hydrated Ions at 25°C... 

FIG. 7 Plots of the increase of water concentration in NB (A[H20]) with extraction of cations with DPA against the equilibrium cation concentration in NB. Each value in the parentheses shows the number (n) of coextracted water molecules per ion. (From Ref. 46. Copyright 1997 American Chemical Society.)... 

The numbers ( ) of coextracted water molecules shown in Table 3 are from Table 2 [46]. In the case of -Pr4N, metal complex cations, larger anions of r > 0.23 nm, and polyanions, it is assumed that n = 0. Although the n value of [Fe(phen)3] or CIO4 was reported to be as small as 0.3 or 0.2 [46], these ions have been classified as nonhydrated (i.e., = 0) so that comparatively better results may be obtained. 

The sediment sample is allowed to dry in open air and then sieved. To 20g of the sample 20% distilled water is added for deactivation purposes and the excess water is then bound to active silica (Siloxid), so that a powdery consistency is obtained. The insecticides studied are extracted with petroleum ether (b.p. 30-60°C) in a Soxhlet apparatus. The extract is concentrated using the vacuum rotary evaporator and the coextractants are separated on a Celite oleum column. The petroleum ether eluate is then concentrated to a volume of 1ml and used for gas chromatography under the following conditions [10, 31-33],... 

In this flow sheet, the aqueous raffinate from extraction is acidified to 5-6 mol dm with hydrochloric acid to optimize platinum extraction by the solvating extractant TBP. The coextraction of iridium is prevented by reduction with sulfur dioxide, which converts the iridium(IV) to the (III) species, which is not extractable. Once again, kinetics are a factor in this reduction step because, although the redox potentials are quite similar, [Ir(IV)/(III) —0.87 V Pt(IV)/(II) —0.77 V], iridium(IV) has a relatively labile configuration, whereas platinum(IV) has the inert arrangement. The species H2PtCl6 is extracted by TBP, from which platinum can be stripped by water and recovered by precipitation as (NH3)2PtCl2. 

To avoid coextraction of iron, the iron is initially reduced to its Il-valent state. Then zinc is extracted as a zinc chloride complex into an organic solution containing tributylphosphate (TBP). Zinc is stripped from the organic solution with water or dilute hydrochloric acid (Fig. 14.5). The resulting strip solution is evaporated, either (1) after addition of sulfuric acid, giving a dilute hydrochloric acid condensate and a zinc sulfate precipitate, or (2) directly without any addition, giving a dilute hydrochloric acid condensate and a concentrate zinc chloride product solution. 

For difficult separations, multiple extractions are frequently carried out, although in many cases the background is also coextracted. Using multiple extractions, polar interferences may sometimes be transferred from the aqueous into organic solvents that can dissolve minute amounts of water. This problem cannot be eliminated by simple presaturation of the extraction solvent but only by washing the extract with small amounts of water (58). Another relevant issue to be considered in trace residue analysis concerns the purity of the organic solvents, since they can introduce solvent impurities into the sample extract. Therefore, the need for high solvent purification should not be overlooked in some applications. 

A mixture of petroleum ether/diethyl ether (1 + 1) is suitable for extracting vitamin D from the unsaponifiable material and allows vitamins A and D to be coextracted. For the determination of vitamin D alone in fortified milks, margarine, and infant formulas, Thompson et al. (70) extracted the unsaponifiable matter three times with hexane in the presence of a 6 4 ratio of water to ethanol. The combined hexane layers were then washed with 55% aqueous ethanol, after the initial 5% aqueous KOH and water washes, to remove material, including 25-hydroxyvitamin D, that was more polar than vitamin D. This extraction process was based on partition studies that showed that insignificant amounts of vitamin D were extracted from hexane by aqueous ethanol when the ratio of ethanol to water was less than 6 4. 

The most suitable solvents for extracting SPA from fats are acetonitrile (113,125,126, 139,142) and water-alcohol mixtures. The fat is usually dissolved in hexane or petroleum ether, and SPA are extracted into acetonitrile (105,110-113,125,126,128). The disadvantages of acetonitrile extraction are that (a) BHT recovery is low and (b) moderately high levels of interfering compounds are coextracted. The advantage of aqueous methanolic extraction of SPA from nonpolar solvent is that the fat is mostly excluded (99,114). Hammond (99) described a methanolic extraction of a melted fat sample, heated to 40-50°C, followed by transfer of the mixture to a deep freeze for a few hours to aid the solidification of any excess fat from methanol. The methanol layer was then decanted and filtered prior to the addition of an internal standard and direct injection. 

A study of the metabolism of SPI in pig liver was conducted. The polar character of the cysteyl derivatives makes them difficult to extract in chlorinated solvents (dichloromethane, chloroform). As a consequence, extraction with pure MeOH was considered, because it extracted both SPI and its cysteyl metabolites. However, it also extracted other biomolecules interfering with the metabolites. Extractions with pure MeCN were unsuccessful, since liver tissues tended to agglomerate in this medium. Only 40% of the cysteyl conjugates were then extracted. The property of water to disperse the liver tissue was used to develop extraction conditions with MeCN-water (90 10). A good dispersion of the liver was then obtained, and pollution by polar interfering compounds coextracted from the liver was limited. Acetonitrile was evaporated and MeOH was added... 

Driss et al. proposed an analytical procedure that, when applied to tap water, gives coextracted compounds with a large number of unresolved peaks at the start of the chromatogram, the intensity of which depends on the untreated water volume (48). 

The Sumitomo Metal Mining Co. in Japan uses a 40% solution of tri- -octylamine in xylene to separate cobalt and nickel in chloride media.44-163 194 The traces of zinc that are present are coextracted with the cobalt and, owing to the strong extraction of zinc, remain in the organic phase when the cobalt is stripped with water (cf. Figure 11). The zinc is then removed from a bleed stream of the organic phase by deprotonation stripping with sodium hydroxide solution ... 

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