Extraction reagents

The mode of extraction in these oxonium systems may be illustrated by considering the ether extraction of iron(III) from strong hydrochloric acid solution. In the aqueous phase chloride ions replace the water molecules coordinated to the Fe3+ ion, yielding the tetrahedral FeCl ion. It is recognised that the hydrated hydronium ion, H30 + (H20)3 or HgO,, normally pairs with the complex halo-anions, but in the presence of the organic solvent, solvent molecules enter the aqueous phase and compete with water for positions in the solvation shell of the proton. On this basis the primary species extracted into the ether (R20) phase is considered to be [H30(R20)3, FeCl ] although aggregation of this species may occur in solvents of low dielectric constant. 

This section provides a brief review of a number of chelating and other extraction reagents, as well as some organic solvents, with special interest as to their selective extraction properties. The handbook of Cheng et al. should be consulted for a more detailed account of organic analytical reagents.11 

An interesting application is the separation of cobalt and nickel neither Co(II) nor Ni(II) forms extractable chelates, but Co(III) chelate is extractable extraction is therefore possible following oxidation. 

Thenoyltrifluoroacetone(TTA), C4H3S,CO,CH2,COCF3. This is a crystalline solid, m.p. 43 °C it is, of course, a /1-diketone, and the trifluoromethyl group increases the acidity of the enol form so that extractions at low pH values are feasible. The reactivity of TTA is similar to that of acetylacetone it is generally used as a 0.1-0.5 M solution in benzene or toluene. The difference in extraction behaviour of hafnium and zirconium, and also among lanthanides and actinides, is especially noteworthy. 

Other fluorinated derivatives of acetylacetone are trifluoroacetylacetone (CF3COCH2COCH3) and hexafluoroacetylacetone (CF3COCH2COCF3), which form stable volatile chelates with aluminium, beryllium, chromium(III) and a number of other metal ions. These reagents have consequently been used for the solvent extraction of such metal ions, with subsequent separation and analysis by gas chromatography [see Section 9.2(2)]. 

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In connexion with the above chemical methods of separation, it is important to note that sufficient of the extracting reagent must be used to remove completely the component which it dissolves or with which it reacts. 

Although phosphine [7803-51-2] was discovered over 200 years ago ia 1783 by the French chemist Gingembre, derivatives of this toxic and pyrophoric gas were not manufactured on an industrial scale until the mid- to late 1970s. Commercial production was only possible after the development of practical, economic processes for phosphine manufacture which were patented in 1961 (1) and 1962 (2). This article describes both of these processes briefly but more focus is given to the preparation of a number of novel phosphine derivatives used in a wide variety of important commercial appHcations, for example, as flame retardants (qv), flotation collectors, biocides, solvent extraction reagents, phase-transfer catalysts, and uv photoinitiators. 

Solvent Extraction Reagents. Solvent extraction is a solution purification process that is used extensively in the metallurgical and chemical industries. Both inorganic (34,35) and organic (36) solutes are recovered. The large commercial uses of phosphine derivatives in this area involve the separation of cobalt [7440-48-4] from nickel [7440-02-0] and the recovery of acetic acid [61-19-7] and uranium [7440-61-1]. 

Recently it has been shown that rotating coiled columns (RCC) can be successfully applied to the dynamic (flow-through) fractionation of HM in soils and sediments [1]. Since the flow rate of the extracting reagents in the RCC equipment is very similar to the sampling rate that is used in the pneumatic nebulization in inductively coupled plasma atomic emission spectrometer (ICP-AES), on-line coupling of these devices without any additional system seems to be possible. 

Sodium diethyldithiocarbamate, (C2H5)2N CS S Na+. This reagent is generally used as a 2 per cent aqueous solution it decomposes rapidly in solutions of low pH. It is an effective extraction reagent for over 20 metals into various organic solvents, such as chloroform, carbon tetrachloride, and ethanol. The selectivity is enhanced by the control of pH and the addition of masking agents. 

The stability of the reagent in acid solution, together with its ability to complex a wide range of metals, make it a very useful general extracting reagent, especially for heavy metals. The chief applications of APDC in quantitative analysis are as follows ... 

Discussion. Because of the specific nature of atomic absorption spectroscopy (AAS) as a measuring technique, non-selective reagents such as ammonium pyrollidine dithiocarbamate (APDC) may be used for the liquid-liquid extraction of metal ions. Complexes formed with APDC are soluble in a number of ketones such as methyl isobutyl ketone which is a recommended solvent for use in atomic absorption and allows a concentration factor of ten times. The experiment described illustrates the use of APDC as a general extracting reagent for heavy metal ions. 

Almela, A. Elizalde, M. P. Interactions of metal extractant reagents. Part VIII. Comparative aggregation equilibria of Cyanex 302 and Cyanex 301 in heptane. Anal. Proc. 1995, 32, 145-147. 

Inoue, K. Koba, M. Yoshizuka, K. Tazaki, M. Solvent-extraction of platinum(IV) with a novel sulfur-containing extracting reagent. Solvent Extr. Ion Exch. 1994, 12, 55-67. 

Other methods reported for the determination of beryllium include UV-visible spectrophotometry [80,81,83], gas chromatography (GC) [82], flame atomic absorption spectrometry (AAS) [84-88] and graphite furnace (GF) AAS [89-96]. The ligand acetylacetone (acac) reacts with beryllium to form a beryllium-acac complex, and has been extensively used as an extracting reagent of beryllium. Indeed, the solvent extraction of beryllium as the acety-lacetonate complex in the presence of EDTA has been used as a pretreatment method prior to atomic absorption spectrometry [85-87]. Less than 1 p,g of beryllium can be separated from milligram levels of iron, aluminium, chromium, zinc, copper, manganese, silver, selenium, and uranium by this method. See also Sect. 5.74.9. 

The extraction of a solute A may be improved by its reaction with another solute ( extraction reagent , or extractant), B, forming an adduct compound, AB. This occurs through chemical interaction between A and B. 

Interfacial tension studies are particularly important because they can provide useful information on the interfacial concentration of the extractant. The simultaneous hydrophobic-hydrophilic nature of extracting reagents has the resulting effect of maximizing the reagent affinity for the interfacial zone, at which both the hydrophobic and hydrophilic parts of the molecules can minimize their free energy of solution. Moreover, as previously mentioned, a preferential orientation of the extractant groups takes place at the interface. Conse-... 

In this section, we describe three simple cases of rates and mechanisms that have been found suitable for the interpretation of extraction kinetic processes in kinetic regimes. These simple cases deal with the exuaction reaction of a monovalent metal cation (solvation water molecules are omitted in the notation) with a weakly acidic solvent extraction reagent, BH. The overall extraction reaction is... 

Case 1 The rate-determining step of the extraction reaction is the aqueous phase complex formation between the metal ion and the anion of the extracting reagent. Even if at very low concentration, BH will always be present in the aqueous phase because of its solubility in water. The rate-determining step of the extraction reaction is as follows ... 

It must be observed that when this mechanism holds, the rate of the extraction reaction is independent of the interfacial area, Q, and the volume of the phases, V. The expected logarithmic dependency of the forward rate of extraction on the specific interfacial area S = Q/V), the organic concentration of the extracting reagent and the aqneons acidity, is shown in Fig. 5.7, case 1. 

Equation (5.46) shows a further difference relative to Eq. (5.34). Equation (5.46) indicates a zero reaction order relative to [BH] in the forward extraction rate, reflecting the complete saturation of the interface with the extracting reagent. 

Case 3 There are two interfacial rate-determining steps, consisting of 1) formation of an interfacial complex between the interfacially adsorbed molecules of the extractant and the metal ion and (2) transfer of the interfacial complex from the interface to the bulk organic phase and simultaneous replacement of the interfacial vacancy with bulk organic molecules of the extractant. For this mechanism, we distinguish two possibilities. The first (case 3.1) describes the reaction with the dissociated anion of the extracting reagent, B"(ad). The second (case 3.2) describes the reation with the undissociated extractant, BH(ad). 

Case 5 A fast reaction between the metal cation and the undissociated extracting reagent occurs at the interface or in proximity of the interface). The rate is controlled by the diffusion to and away from the interface of the species taking part in the reaction. 

A mathematically simple case, that occurs frequently in solvent extraction systems, in which the extracting reagent exhibits very low water solubility and is strongly adsorbed at the liquid interface, is illustrated. Even here, the interpretation of experimental extraction kinetic data occurring in a mixed extraction regime usually requires detailed information on the boundary conditions of the diffusion equations (i.e., on the rate at which the chemical species appear and disappear at the interface). 

Perhaps the area of environmental pollution most pertinent to solvent extraction processing is that of water pollution and, consequently, the toxicity of solvent extraction reagents to aquatic life then become important. Thus, the primary consideration in the development of water quality criteria for these reagents is to determine their toxicity toward fish and their biodegradability [70]. 

Ashbrook, A. W. Commercial chelating solvent extraction reagents. 1. o-Hy-droxyoximes Purification and isomer separation/extraction. Metallurgy Division, Mines Branch, Dept. Energy, Mines and Resources, Canada, Report EMA 73-10, 1973. 

Ddx distribution constant for x (specified) for example, v = R, or = HA, for undissociated extractant (reagent), = C for neutral (e.g., metal-containing) complex Dsp solubility product Kyj ionic product of water... 

McDowell, W. J., Shoun, R. R., An evaluation of crown compounds as solvent extraction reagents. Proc. ISEC 77,1,95-97,1977. 

McDowell, W. J., Crown ethers as solvent extraction reagents Where do we stand, Sep. Sci. Technol, 23,1251-1268,1988. 

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