Metal-organic complexes extraction

TABLE 1. METAL-ORGANIC COMPLEXES EXTRACTABLE WITH CHLOROFORM FROM JULIA CREEK SHALE... 

Armannsson [659] has described a procedure involving dithizone extraction and flame atomic absorption spectrometry for the determination of cadmium, zinc, lead, copper, nickel, cobalt, and silver in seawater. In this procedure 500 ml of seawater taken in a plastic container is exposed to a 1000 W mercury arc lamp for 5-15 h to break down metal organic complexes. The solution is adjusted to pH 8, and 10 ml of 0.2% dithizone in chloroform added. The 10 ml of chloroform is run off and after adjustment to pH 9.5 the aqueous phase is extracted with a further 10 ml of dithizone. The combined extracts are washed with 50 ml of dilute ammonia. To the organic phases is added 50 ml of 0.2 M-hydrochloric acid. The phases are separated and the aqueous portion washed with 5 ml of chloroform. The aqueous portion is evaporated to dryness and the residue dissolved in 5 ml of 2 M hydrochloric acid (solution A). Perchloric acid (3 ml) is added to the organic portion, evaporated to dryness, and a further 2 ml of 60% perchloric acid added to ensure that all organic matter has been... 

This example illustrates a case of considerable analytical importance, especially for the determination of complex formation constants for hydrophilic complexes, as discussed in section 4.12, when the equilibrium constants for the stepwise metal-organic complexes are of secondary interest. values are tabulated in several reference works. is a conditional constant and only valid provided no other species are formed besides the extracted one. 

Solvent extraction has become a common technique for the determination of formation constants, P , of aqneons hydrophilic metal complexes of type MX , particularly in the case when the metal is only available in trace concentrations, as the distribntion can easily be measnred with radioactive techniques (see also section 4.15). The method reqnires the formation of an extractable complex of the metal ion, which, in the simplest and most commonly used case, is an nn-charged lipophilic complex of type MA. The metal-organic complex MA serves as a probe for the concentration of metal ions in the aqueous phase through its equilibrium with the free section 4.8.2. This same principle is used in the design of metal selective electrodes (see Chapter 15). Extractants typically used for this purpose are P-diketones like acetylacetone (HAA) or thenoyltrifluoroacteone (TTA), and weak large organic acids like dinonyl naph-talene sulphonic acid (DNNA). 

Few solubility parameters are available for the metal-organic complexes discussed in this chapter. Another approach is then necessary. The distribution constant for the reagent (extractant), R, can be expressed as ... 

In Chapters 2 and 3 several physicochemical factors of importance to solvent extraction have been described, and many of their effects have been illustrated in this chapter. Here, we summarize some observed regularities. The effect of various stractures on the bond strengths in metal organic complexes has been extensively treated in other publications [47-49]. 

The extractabilities of metal-organic complexes depend on whether inner or outer sphere complexes are formed. Case 1, section 4.2.1, the extraction of ura-nyl nitrate by TBP, is a good example. The free uranyl ion is surrounded by water of hydration, forming U02(H20)f, which from nitric acid solutions can be crystallized out as the salt U02(H20)6 (N03), though it commonly is written U02(N03)2(H20)6. Thus, in solution as well as in the solid salt, the UOf is surrounded by 6 HjO in an inner coordination sphere. In the solid nitrate salt, the distance du.o(nitrate) between the closest oxygen atoms of the nitrate anions, (0)2N0, and the U-atom is longer than the corresponding distance, du-o(water), to the water molecules, OH2, i.e., du.o(nitrate) > 4u.o(water) thus the nitrate anions are in an outer coordination sphere. 

Donat, J. R., Statham, P. J., and Bruland, K. W. (1986) An Evaluation of a C-18 Solid Phase Extraction Technique for Isolating Metal-Organic Complexes from Central North Pacific Ocean Waters, Mar. Chem. 18, 85-99. 

As described in 9.2.6 the solv it extraction technique requires (i) a liquid two-phase system consisting of an organic solvent in contact with an aqueous solution, and (ii) the presence of an extractant (commonly a weak organic acid, abbreviated HA), which reacts with the metal ion to form an imcharged metal-organic complex MA, that preferentially dissolves in the organic phase. The distribution of the metal between the organic and aqueous phases can be shown to be fit the relation... 

Donat JR, Statham PJ, and Bruland KW (1986) An evaluation of a C-18 solid phase extraction technique for isolating metal-organic complexes from central North Pacific Ocean waters. Marine Chemistry 18 ... 

The ionic liquids show excellent extraction capabilities and allow catalysts to be used in a biphasic system for convenient recycling. For example, the hydrovinylation of styrene with ethene can be carried out successfully using an ionic liquid and supercritical CO2 as solvent (Eq. 10-15). The ionic liquid dissolves the metal organic complex catalyst and SC-CO2 facilitates mass transfer and continuous processing. 

Neutral metal chelate complexes, extractable into organic solvents. Intense color of many facilitates colorimetric determinations. 

The relevant equilibria for extracting a neutral metal-ligand complex from an aqueous solution into an organic phase are shown in the following diagram. 

Quantitative. Classically, silver concentration ia solution has been determined by titration with a standard solution of thiocyanate. Ferric ion is the iadicator. The deep red ferric thiocyanate color appears only when the silver is completely titrated. GravimetricaHy, silver is determined by precipitation with chloride, sulfide, or 1,2,3-benzotriazole. Silver can be precipitated as the metal by electro deposition or chemical reduciag agents. A colored silver diethjldithiocarbamate complex, extractable by organic solvents, is used for the spectrophotometric determination of silver complexes. 

Thiocyanates are rather stable to air, oxidation, and dilute nitric acid. Of considerable practical importance are the reactions of thiocyanate with metal cations. Silver, mercury, lead, and cuprous thiocyanates precipitate. Many metals form complexes. The deep red complex of ferric iron with thiocyanate, [Fe(SCN)g] , is an effective iadicator for either ion. Various metal thiocyanate complexes with transition metals can be extracted iato organic solvents. 

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