Extraction Schemes

Classification schemes are numerous and no one scheme covers all systems. The most common schemes are based on the form of the extracted element that transfers into the organic phase. This simplified classification scheme is adequate for the discussion here. However, even with simple schemes the categories are not exclusive, and some extraction agents could be classified into other categories. The experimental process of inorganic extraction of a neutral complex, regardless of the type of complex, is essentially the same. The neutral complex's interaction with the aqueous phase, including but not limited to the solubility, depends on the charge and polarity of the overall complex. The first step is to generate a neutral complex with the analyte of interest through one of the mechanisms listed above. A small volume of organic solvent is added to the sample mixture. For example, a 1 L aqueous sample may be extracted into 40 mL of organic solvent. Extraction can be performed in a separatory funnel or by using a mechanical shaker table. The pH of the mixture may need to be manipulated, depending on the exact extraction scheme used. In addition, masking agents may be used to obtain specificity (see below). After mixing, the two phases are separated and the procedure is generally repeated several times. The organic phase is combined from each extraction. The concentration of the elements in the sample is increased by 1-3 orders of magnitude in the organic phase. The extract can then be further pre-concen-trated if needed (back-extraction, evaporation, etc.) or analysed directly, for example by flame AAS.

Naturally occurring complexants Elements that can form neutral complexes can already exist in naturally occurring water systems. These complexes are formed essentially with covalent bonding between the element and naturally occurring ligand(s). Ligands are molecules or ions bonded to a central metal ion and tend to be Lewis bases; also included in this category would be undissociated covalent species. Examples of this category would include I2 and B2, the halides of some metals (GeCl4 HgCl2, AsCl3) and oxides of some metals (OsO4). The extraction of these types of compounds would proceed in the same manner as for chelates and ion-associated complexes.

Chelates A chelate is a type of ligand. A multiden-tate (dentate is Latin for tooth) ligand that uses more than one atom to bind to a metal in a coordination complex, see Figure 2. The metal is the electron-pair acceptor and the chelating agent the electron-pair donor. When binding to the metal ion, the chelate (ligand) forms a ring of atoms, of which the metal is one member. The chelate complex charge exactly neutralizes the charge on the metal ion. Most rings contain > 4 and < 8 atom members; the most stable

Et2N

Dithiocarbamate:

A/,/V'-Diethylthiocarbamate ion

Oxime:

8-Hydroquinoline ion

Oxime:

8-Hydroquinoline ion

Figure 2 Chemical structures of typical chelate-metal complexes.

typically is a 5-membered ring. Bidendate describes a chelate where two atoms from the chelate complex bond to the metal and tridentate would indicate three coordinating atoms. Many chelating extractants are weak acids, therefore, control of pH is important in many extracting schemes.

An exhaustive treatment of every chelate system is beyond the scope of this chapter. Table 2 lists a selection of chelate types with one or two specific chelate agents listed below these. To describe the selectivity of each is not possible in a brief chapter, a sense of the ability of each chelate reagent is given by listing the wide range of complex-forming metals that are possible. Detailed information about the selectivity, solvent and other experimental conditions can be found in the references listed in Further Reading. The list in Table 2 include inorganic extraction procedures for a wide array of instrumental methods including: flame AAS, electrothermal (ET)-AAS, inductively coupled plasma-atomic emission spectroscopy, (ICP-AES), neutron activation analysis (NAA), spectrophotometry chromatographic, flame photometry, and polarography. In addition, most of the chelate groups listed in Table 2 are compatible with more than one organic solvent. Solvent flexibility in an analytical scheme allows an extended range of instrumental methods which can be used for the determination.

Inorganic extraction, utilizing chelates, for analytical separation and/or preconcentration has been exploited for many instrumental systems. For flame AAS analysis, often the inorganic solvent extraction is designed to increase the concentration of elements of interest and, most importantly, reduce the concentration of alkali and alkaline earth elements (i.e. leave most of them in the aqueous phase). This separation

P-Diketone:

Acetyiacetone ion

P-Diketone:

Acetyiacetone ion

Table 2 A select list of inorganic extraction systems

Metals extracted

Chelating agents

Oxines -8-Hydroxyquinoline -(and derivatives)

a-Dioximes -Dimethylglyoxime

Dithizones -Diphenyldithiocarbazone

Dithiocarbamates -sodium diethyldithiocarbamate -Sodium WW-phenylacetyldithiocarbamate

P-Diketones -Acetylacetone -Thenoyltrifluoroacetone

Witrosoarylhydroxylamines -Ammonium N/-Nitroso-N/-phenylthydroxylamine (cupferron)

Organophosphorus acids -di-n-butylphosphoric acid -Di(2-ethylhexyl)phosphoric acid

1-Nitroso-2-naphthol

1-(2-Pyridylzao)-2-naphthol (PAN)

Ion-pair agents

Chelated ion-pairs -ethylenediaminetetraacetic acid (EDTA)/halide -1,10-phenoanthroline/perchlorate

Non-chelated ion-pairs -tetraalkylammonium salts -tetraphenylarsonium salts

Halide ion pairs -HCI -HF -HI

> 50 metals

> 30 metals

> 50 metals

> 50 metals

> 50 metals

> 50 metals

> 30 metals

> 30 metals

> 30 metals

> 50 metals

> 50 metals

> 50 metals

> 50 metals

> 50 metals

> 50 metals is especially necessary for many natural water samples such as seawater, brines, etc. Trace element analyses of clinical samples such as blood, urine, etc., also benefit from inorganic extraction for flame AAS analysis as well as other determination techniques (ET-AAS, ICP-AES, etc.). Radiochemistry separations for NAA also often use inorganic extraction techniques that utilize chelating schemes.

Extraction schemes have also been developed which leave the analyte of interest in the aqueous phase and remove interferences through the organic phase. This technique has limited applicability owing to the limited solubility of the (starting reagent) chelate in organic solvents.

Ion association (ion pair) Neutral complexes can be formed through ion association (ion-pair) and extracted from an aqueous solution into an organic solvent. Ion association inorganic extracts encompass a wide range of extraction schemes. General sub-groupings include chelated ion pairs, nonchelated ion pairs and halide-cation ion-pairs. The halide-cation pairs are typically extracted into oxygen-containing solvents, such as methyl isobutyl ketone, diethyl ether and alcohols. A select list of ion pair extracting agents is given in Table 2.

Maximizing the coulombic forces of attraction between the ion pairs facilitates extraction of the ion pairs. The dielectric constant of the solvent is a large contributor to the overall extractability of a scheme. Enhancement of the extraction of ion-associated complexes is increased by the addition of electrolytes, called 'salting-out'. The salting-out effect may be attributed to the increase in anion concentration, as well as the decrease of the dielectric constant of the aqueous phase. Complexes can be formed by ligands coordinated to the metal and an appropriate counter anion that neutralizes the total charge. One of the ions (either the complexed ligand or the anion) typically contains a large hydrophobic group(s) which further enhances extraction of the ion pair into the organic phase.

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