Other Chelating Reagents

Of the many chelating reagents studied, comparatively few offer appreciable advantages over EDTA as analytical reagents. Many of these other titrants have been reviewed by West.  

Historically, nitrilotriacetic acid (NTA) is next in importance to EDTA. It forms stable complexes with most metals, even to some extent with the alkali metals (the pJf for sodium is 2.1). Unfortunately, its tendency to form 2 1 complexes causes difficulty with the stoichiometry. 

beck Chemistry of Complex Equilibria, Van Nostrand Reinhold, London, 1970. 

PERRIN Masking and Demasking of Chemical Reactions, Wiley, New York, 1970. 

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The extraction of cadmium chelates with oxine, chloro-oxine, HTTA, dibenzoylmethane, anthranilic acid, and other chelating reagents has been investigated [14,15],... 

Crown Ethers, Cryptates, and Other Chelating Reagents... 

The pH value for which 50% of metal as its chelate is extracted, i.e., D = 1, and for [H2L] = 1, is equal to log Kex)/n- This is termed pH 1/2, and has a value characteristic for the particular reagent, solvent, and the metal ion being determined. For example, in the case of metal extraction in the form of dithizonates using carbon tetrachloride as a solvent, the pHj/2 values for Pb(II), Zn(II), In(III), and Bi(III) are 8.0, 5.0, 3.8, and 2.0, respectively. Similar considerations apply to other chelating reagents as, e.g., 8-hydro-xyquinoline, acetylacetone, and thenoyltrifluoroace-tone. Such relationships are often presented in graphical form (Figure 9). 

Several rare earths, including La, Gd, Lu, Y and Sc form intensely fluorescent chelates with reagents such as 8-hydroxyquinolines and various other chelating reagents. The analytical usefulness of methods based on the fluorescence of these chelates is limited, however, because other elements, notably the alkaline earths, also form fluorescent chelates and the presence of other rare earths, various anions, and other compounds tend to quench the fluorescence. 

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... 

Partition behaviour of americium(III) chelates with cupferron and other bidentate reagents was studied spectrophotometrically between a number of inert solvents and dilute HC104 solutions.98 Of special interest may be the data on their extractability and colours of chloroform extracts, collected in a tabular form for cupferronate derivatives of 58 metals. The pH ranges for the formation of cupferronates of 39 metal ions have been shown graphically in this publication.99 Solvent extraction and polarographic techniques were employed to study the possible adducts between technetium and cupferron.100 Evidence indicates a Tcm cupferronate and possibly a pertechnitate adduct, but no indication of a technetium(IV) complex was obtained. 

Analytical Reagents. Various chelators give specific color changes on combining with different metal salts, such as vanadium, iron, cobalt, nickel, copper, and palladium salts and thus identify the corresponding metal ions [74], Other chelators, such as C.I. Mordant Black 11 (Eriochrome Black T) (2 see Section 3.11.1), are employed as indicators for complexometric titrations. 

Solvent extraction is one of the methods widely used for concentration and separation. Most heavy metals are extracted with chelating reagents into organic solvents [35]. Some chelating agents commonly used in atomic absorption spectrometry are shown in Fig. 3. APDC and DDC are most commonly used in AAS. Solvents such as ketones, esters, ethers, alcohols, and other oxygen-containing hydrocarbons are suitable for the flame atomic absorption technique. Of these solvents, MIBK (methyl isobutyl ketone,... 

In other cases the adduct former may attach itself to the chelating reagent, and in still others self-adducts are formed that involve the extracting agent adding to itself. ... 

Hydroxylamine derivatives are used in determination of certain metals [112,113]. For example, the substitution of a benzoyl group (CeHsCO) for one of hydrogen atoms in hydroxylamine gives benzohydroxamic acid. Other important reagents in this group are N-benzoyl-N-phenylhydroxylamine (BPHA), also called N-phenylbenzohydroxamic acid [114,115] and N-furoylphenylhydroxylamine. All these reagents form extractable chelates and are used for determination of metals, e.g., V(V), Mn, Ti, Fe(III), and Nb. 

Triphenylmethane chelating reagents, other than CAS and ECR, have also been used to determine beryllium, e.g. Aluminon [34,35], Xylenol Orange [36], Brilliant Violet B [2], and Chromal Blue G (e = 3.1-10 ) [37]. A considerable increase of sensitivity is obtained by using Chromal Blue G in the presence of CTA (e = 9.4-10 at 626 nm) [38]. 

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