Extraction of metals, solvent

The value of solvent extraction arises from the ease with which it lends itself to multistage operation without increased consumption of heat or chemicals. This makes solvent extraction particularly usefiil when either extreme purification is necessary or when the metals are so similar in their properties that a single precipitation or crystallization would not give the requisite degree of separation. 

Ion exchange is another method that lends itself to multistage operation. Generally speaking, solvent extraction is preferable when large amounts of metals are to be separated, and ion exchange is preferable for small quantities or low concentrations or for separating the alkalies or alkaline earths, to which sdvent extraction is not readily applicable. 

The separation can be accomplished in several ways. You have noted above that the uncharged organic molecules tend to dissolve in the organic layer while 

In one method, the metal ion is incorporated into a bulky molecule and then associates with another ion of the opposite charge to form an ion pair, or the metal ion associates with another ion of great size (organiclike). For example, it is well known that iron(III) can be quantitatively extracted from hydrochloric acid medium into diethyl ether. The mechanism is not completely understood, but evidence exists that the chloro complex of the iron is coordinated with the oxygen atom of the solvent (the solvent displaces the coordinated water), and this ion associates with a solvent molecule that is coordinated with a proton  

Similarly, the uranyl ion is extracted from aqueous nitrate solution into isobutanol by associating with two nitrate ions (U02 , 2N03 ), with the uranium probably being solvated by the solvent to make it solventlike. Permanganate forms an ion pair with tetraphenylarsonium ion [(C6H5)4As , Mn04 ], which makes it organiclike, and it is extracted into methylene chloride. There are numerous other examples of ion-association extractions. 

The most widely used method of extracting metal ions is formation of a chelate molecule with an organic chelating agent. 

Most chelating agents are weak acids that ionize in water the ionizable proton is displaced by the metal ion when the chelate is formed, and the charge on the organic compound neutralizes the charge on the metal ion. An example is diphenyl-thiocarbazone (dithizone), which forms a chelate with lead ion  

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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). 

The nature of the donor atoms in the chelating agent. Ligands which contain donor atoms of the soft-base type form their most stable complexes with the relatively small group of Class B metal ions (i.e. soft acids) and are thus more selective reagents. This is illustrated by the reagent diphenylthiocarbazone (dithizone) used for the solvent extraction of metal ions such as Pd2+, Ag+, Hg2+, Cu2+, Bi3+, Pb2+, and Zn2 +. ... 

The choice of a satisfactory chelating agent for a particular separation should, of course, take all the above factors into account. The critical influence of pH on the solvent extraction of metal chelates is discussed in the following section. 

A K De, S M Khopkar and R A Chalmers, Solvent Extraction of Metals, Van Nostrand Reinhold, London, 1970... 

J Stary, The Solvent Extraction of Metal Chelates, Pergamon Press, Oxford, 1964... 

The solvent extraction of metal ions by crown compounds. Y. Takeda, Top. Curr. Chem., 1984,... 

Analytical applications of substituted cinnamohydroxamic acids in spectrophotometry and solvent extraction of metal ions. Y. K. Agrawal and R. K. Jain, Rev. Anal. Chem., 1982, 6,49-64 (71). 

Solvent extraction of metal carboxylates. H. Yamada and M. Tanaka, Adv. Inorg. Chem. Radiochem., 1985, 29,143 (159). 

Takeda, Y. The Solvent Extraction of Metal Ions by Grown Compounds. 121, 1-38 (1984). 

The catalytic role of the liquid-liquid interface in the solvent extraction of metal ions described in this chapter and some important remarks on the interfacial phenomena reported in other studies are summarized ... 

The pH-metric technique used to determine partition coefficients was first used in the 1950s in solvent extraction of metal complexes [280-282], but it is in pharmaceutical research that it is most widely used thanks to the recent development of a fully automated and computer-controlled apparatus [125,283]. The potentiometric approach has been validated in various solvent systems [284-287], and it has become a relevant and expanding experimental technique to obtain lipophilicity descriptors [257,287-289]. 

V. Marcus and A. S. Kertes, Ion Exchange and Solvent Extraction of Metal Complexes, Wiley-Intersdence, New York, 1969. 

Also of great interest is the so-called FIA scanning as a method for investigating, for instance, the influence of pH on the solvent extraction of metal dithizonates95 by controlled-potential continuous alteration of the pH of the carrier stream. Many other investigations can thus be made, such as the catalytic activity of enzymes and the influence of pH on ion-selective electrodes. 

Preston, J. S. Solvent extraction of metals by carboxylic acids. Hydrometallurgy 1985, 14, 171-188. 

A variety of preconcentration procedures has been used, including solvent extraction of metal chelates, coprecipitation, chelating ion exchange, adsorption onto other solids such as silica-bonded organic complexing agents, and liquid-liquid extraction. 

The major uses of non-ionizing solvents in chemical analysis are twofold. They may be used simply to provide media for the dissolution and reaction of covalent materials, or they may play a more active part in a chemical process. For example, oxygen-containing organic solvents can be used to effect the solvent extraction of metal ions from acid aqueous solutions the lone pair of electrons possessed by the oxygen atom forming a dative bond with the proton followed by the extraction of the metal ion as an association complex. 

Uses Plasticizer for lacquers, plastics, cellulose esters, and vinyl resins heat-exchange liquid carbonless copy paper systems in aircraft hydraulic fluids solvent extraction of metal ions from solution of reactor products uranium extraction and nuclear fuel reprocessing pigment grinding assistant antifoaming agent solvent for nitrocellulose and cellulose acetate. 

Computational Chemistry in Modeling Solvent Extraction of Metal Ions... 

This example indicates that in solvent extraction of metal complexes with acidic ligands, it can be more advantageous to plot log D vs. log[A ], rather than against pH, which is the more common (and easy) technique. 

Marcus, Y. Kertes, S. Ion Exchange and Solvent Extraction of Metals, WUey-Interscience, Chichester, 1969. 

Various instruments of theoretical chemistry have been widely to describe separate steps of solvent extraction of metal ions. Because of the complexity of solvent extraction systems, there is still no unified theory and no successful approach aimed at merging the extraction steps. It has already been pointed out that the challenging problem for theoreticians dealing with solvent extraction of metals, in particular with thermodynamic calculations, is to evaluate correctly solvent effects by the use of the most accurate explicit solvation models and QM calculations. However, such calculations on extremely large sets consisting of hundreds or even thousands of molecules, necessary to model all aspects of the extraction systems, are still impossible due to both hardware and software limitations. 

Stary, J. The solvent extraction of metal chelates. New York The Macmillan Co.1964. 

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