Separation and preconcentration of elements

Zygmunt Marczenko; Maria Balcerzak    Department of Analytical Chemistry, Warsaw University of Technology, Naokowskiego 3, 00-664 Warsaw, Poland

The spectrophotometric determination of elements is usually preceded by their separation from major components (matrix) and from interfering elements, the effects of which cannot be eliminated by other methods such as masking or change of pH of the medium. In the trace analysis of high-purity materials, separation from the matrix involves simultaneous concentration of the trace components. General methods of preconcentrating and separating elements have been outlined in several monographs and reviews [1–4].

The present Section provides a discussion of the following separation and preconcentration methods: solvent extraction, precipitation and co-precipitation with collectors, volatilization, and methods based on the use of ion-exchangers and other sorbents. These methods are used not only with spectrophotometry, but also in conjunction with other methods of determination.

1.1 Solvent extraction

1.1.1 Introduction

The extraction process and extractive methods for separation and preconcentration of elements are described in several monographs and reviews [5,6].

Solvent extraction separation is based on differences in the solubilities of elements and their compounds between two immiscible liquid phases. Usually, the initial phase is an aqueous solution and the second phase is an organic solvent, immiscible with water. Some properties of the more common organic solvents are listed in Table 1.1. The ion to be extracted into the non-aqueous phase should first be transformed into an uncharged species.

Stripping (“re-extraction”, “back-extraction”, or “scrubbing”) involves bringing the element from the organic extract back into the aqueous phase.

The extraction efficiency, i.e., the degree of transfer of the species from the aqueous to the organic phase, is defined in terms of the distribution- (or extraction-) coefficient, (D). The quantity D is the ratio of total concentration (i.e., the concentration of all the existing forms) of the element in the organic phase (Σc0) to the total concentration in the aqueous phase (Σcw) in the aqueous phase, at equilibrium

=∑co∑cw

The extraction efficiency (%E) is also expressed as the extraction percent

E=100DD+vw/vo

where D is the distribution coefficient, and vw and vo are the volumes of the aqueous and the organic phases, respectively.

When the distribution coefficient of a given element in a specified system is large (e.g., 1,000, i.e., log D = 3), a single extraction will suffice. In most extraction systems the partition coefficients change as the concentration of the substance extracted changes; in most cases they decrease with decreasing concentration [7].

Shaking the phases in a separating funnel during the extraction or re-extraction must be continued until equilibrium is attained. The time required for the system to reach equilibrium varies from seconds to several minutes, depending on the kinetics of the process [8–10], When the shaking time recommended is more than two minutes, it is advisable to use a mechanical shaker.

Extraction is equally useful in the preconcentration and separation of small amounts of elements, and in the separation of macrocomponents from traces. Extraction methods generally require less time than precipitation methods. The former give also “purer” separation of elements owing to the small area of phase contact. Co-extraction occurring in some cases [11] has not been widely used in extraction separations.

1.1.2 Extraction systems

Extraction systems may be divided into two classes: (1) uncharged covalent species (simple molecules and chelates), and (2) ion associates (ion pairs).

Simple molecules (e.g., I2, HgCl2, AsCl2, BiI3, GeCl4, OSO4) are extracted with nonpolar solvents such as benzene, CHCl3, CCl4. The extraction of this type of compound is comparatively selective and is widely applied in separation of some elements [5,12,13].

Inner chelates (uncharged chelates) are formed when metal ions react with bifunctional ligands, such as dithizone (formula 1.1), 8-hydroxyquinoline [14] (formula 1.2), dithiocarbamates (formulae 1.3 and 1.4), ethyl xanthates [15,16], cupferron (formula 1.5), BPHA (N-benzoyl-n-phenylhydroxylamine) [17,18], acetylacetone (formula 1.6) and thenoyltrifluoroacetone (HTTA) (formula 1.7) [19,20].

  (1.1)

  (1.2)

  (1.3)

  (1.4)

  (1.5)

  (1.6)

  (1.7)

Inner chelates are extracted with non-polar solvents (mostly with CHCl3 and CCl4). Synergism [21,22] is important in the extraction of some chelates.

Some inner chelates were extracted into chloroform solutions of diantipyrylmethane (DAM) (formula 1.8) [23]. The effects of salting-out agent, solvent, and temperature on the

  (1.8)

extraction have been discussed [20]. Selectivity can be increased by using the exchange technique, in which a less-stable metal chelate is the source of the chelating agent [24].

The extraction of chelates is usually applied to preconcentration and separation of small amounts of metals. Owing to their low solubility in organic solvents, most chelates can not be used for the extraction of macrocomponents. Cupferronates and acetylacetonates are exceptions.

Chelates of metal ions with alkyl- and arylphosphoric and thiophosphoric acids can be extracted into chloroform and other solvents [25,26], Such systems enable one to separate, by means of extraction procedures, many metals from strongly acid solutions. Examples of such reagents are di-(2-ethylhexyl)phosphoric acid (HDEHP) and di-n-butyldithiophosphoric acid (formulae 1.9 and 1.10). HDEHP is a viscous liquid (density...