Ions, softness

Metal ions Softness parameter Metal ions Softness parameter... 

Electrospray ionization ESI Spray Nonvolatile molecular ions Soft method, LC-MS Large molecules... 

Identify the molecular ion This is an important inital step, because it is needed to derive the molecular composition (Chap. 6.6.4). If the El spectrum does not allow for the identification of the molecular ion, soft ionization methods should be employed in addition. 

Similarly, relevant electrophiles (Lewis acids) including A-type metal cations (hard), bivalent transition metal ions (borderline), and B-type metal ions (soft) can be categorized (see Stumm and Morgan 1996). Note that in organic molecules, the atom where a nucleophile attacks (i.e., the electrophilic site) may possess harder (e.g., C=0, P=0) or softer (e.g., CH3-X) character. 

Pearson designated the class (a) ions hard acids and class (b) ions soft acids. Bases are also classified as hard or soft. The halide ions range from F, a very hard base, through less hard Cl and Br to 1 , a soft base. Reactions are more favorable for hard-hard and soft-soft interactions than for a mix of hard and soft in the reactants. For example, in aqueous solution ... 

Copper occurs in soil solids and solutions almost exclusively as the divalent cation Cu ". However, reduction of Cu " (cupric) to Cu (cuprous) and Cu (metallic copper) is possible under reducing conditions, especially if halide or sulfide ions ( soft bases) are present to stabilize Cu" (a soft acid). Copper is classified as a chalcophile, owing to its tendency to associate with sulfide in the very insoluble minerals, CU2S and CuS. In reduced soils, then, copper has very low mobility. Most of the colloidal material of soils (oxides of Mn, Al, and Fe, silicate clays, and humus) adsorb strongly, and increasingly so as the pH is raised. For soils with high Cu accumula-... 

For the same reason, x for Fe is the smallest in Table 3. Hard cations rely on ionic bonding, where an oxide ion has the advantage over a sulfide ion. Soft cations rely more on covalent bonding, where sulfur has the advantage because it is a better donor (softer and less electronegative). But Fe is not as good an acceptor as Hg, for example, because of its low value of %. Hence FeO and FeS are more ionic than HgO and HgS, and A for Fe " is larger than for Hg ". ... 

In order to detect intact molecular ions soft ionisation techniques had to be developed. Field desorption (FD) was the first ionisation technique established to produce mass spectra with little or no fragment-ion content FD is based on electron tunnelhng from an emitter biased at a high electrical potential [24]. The filament of the emitter is heated and the sample is evaporated into the gas-phase. Typically, intact molecular ions are detected. This method is limited to relatively low molecular weight compounds, which additionally have to be thermally stable to some extent. 

Ion exchange is a unit operation in which ions held on a solid resin are exchanged for ions in the feed solution. For most people the most familiar ion exchange system is water softening, which replaces calcium and magnesium ions ( hard water ions) in the feed water with sodium ions ( soft water ions). 

Effect of donor atom type on selectivity. Changing the type of donor atom at the binding site alters the nature of the ligand-cation interactions and can lead to quite subtle changes in complexation selectivity (15,26). For example, the replacement of oxygen (hard base) as the donor atom by sulphur (soft base) in 18-crown-6 enhances the complexation of transition metal ions (soft acids), for example Ag, and reduces that of alkali metals (hard acids), for example (Table 2.2) (17, 27). With nitrogen as a donor atom, however, the complexation of transition metal ions (Ag" ) is also promoted without substantially diminishing that of alkali metals (K ) (17). 

FIGURE 1.4 The percentage of injected metal bound to gastropod hemolymph protein as related to the metal ion softness. 

FIGURE 1.5 Elimination of metal ions from crayfish hemolymph is related to metal ion softness. Strong covalent bonding slows elimination. In this illustration, the iron datum was omitted because it was derived from the citrate salt, whereas the other metals were prepared from chloride salts. Based on trends shown here and in Figure 1.3, one could incorrectly assume that metal softness might always be the best descriptor for predicting trends. However, as Ahrland explains, soft and polarizable are not synonymous a soft acceptor is certainly always polarizable, but a highly polarizable acceptor need not necessarily be soft, i.e., have (b) properties. For metal ion acceptors, the outer d-electrons are as essential as the polarizability (Ahrland, S. 1968. Thermodynamics of complex formation between hard and soft acceptors and donors. Struct. Bond. 5 118-149). 

Kinraide, T.B. 2009. Improved scales for metal ion softness and toxicity. Environ. Toxicol. Chem. 28 525-533. 

The relationship between metal ionic characteristics and the maximum biosorption capacity was estabUshed using QSAR models based on the classification of metal ions (soft, hard, and borderhne ions). Ten kinds of metal were selected and the waste biomass of Saccharomyces cerevisiae obtained from a local brewery was used as biosorbent. Eighteen parameters of physiochemical characteristics of metal ions were selected and correlated with Ths suggestion was made that classification of metal ions could improve the QSAR models and different characteristics were significant in correlating with ax, such as polarizing power Z /r or the first hydrolysis constant logRo or ionization potential IP. 

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