Cations in water

The complexes on surface of chelate-functionalized silica often include ligands available in solution in the coordination sphere. Use of a chromophore reagent as a ligand leads to the formation of colored mixed ligand complexes (MLC). The phenomena can be used as a basis for developing test-systems for visual determination of microquantities of inorganic cations in water. 

Fig, 3. Ultraviolet spectra of quinazoline cations in water. A, Typical hydrated cation B B, typical mixture of hydrated and anhydrous cation, and C, typical anhydrous cation. 

The ultraviolet spectra of the neutral molecule and the cation in water show very similar behavior to quinazoline and its cation (see Fig. 4). By analogy, the quinazoline 3-oxide cation must be co-... 

You may be surprised to learn that many metal cations act as weak acids in water solution. A 0.10 M solution of A12(S04)3 has a pH close to 3 you can change the color of hydrangeas from red to blue by adding aluminum salts to soil At first glance it is not at all obvious how a cation such as Al3+ can make a water solution addic. However, the aluminum cation in water solution is really a hydrated species, A1(H20)63+, in which six water molecules are bonded to the central Al3+ ion. This spedes can transfer a proton to a solvent water molecule to form an H30+ ion ... 

A very similar equation can be written to explain why solutions of zinc salts are acidic. Here it appears that the hydrated cation in water solution contains four H20 molecules bonded to a central Zn2+ ion. The Bransted-Lowry equation is... 

One way to separate two cations in water solution is to add an anion that precipitates only one of the cations. This approach is known as selective precipitation. To see how it works, consider a simple case, a solution containing Mg2+ and Na+ ions. Referring back to Table 16.1 (p. 433), you can see that Mg2+ forms a couple of insoluble compounds MgC03 (K = 6.8 X 10-6) and Mg(OH)2 (K = 6 X 10-12). In contrast, all of the common compounds of sodium are soluble, including the carbonate and hydroxide. It follows that you could readily separate Mg2+ from Na+ by adding either C032- or OH- ions to the solution. In either case, Mg2+ will precipitate while Na+ remains in solution. 

Using the data in this table and in Table 18.1 (page 487), it is possible to decide on the relative stabilities of different transition metal cations in water solution. 

TABLE 10.7 Acidic Character and ICa Values of Common Cations in Water ... 

Figure 4. The solute-oxygen distribution function for a linear chain of cations in water (1 g/cm ) at 298 K, (a) single ion (b) chain of ions. Circles simulation solid lines polymer RISM. (Reproduced with permission from Ref. 8. Copyright 1987 North Holland Press.)...

Piquemal J-P, Perera L, Cisneros GA, Ren P, Pedersen LG, Darden TA (2006) Towards accurate solvation dynamics of divalent cations in water using the polarizable Amoeba force field from energetics to structure. J Chem Phys 125 054511... 

Environmental Fate. It is not known if 3,3 -dichlorobenzidine, like benzidine, is oxidized by clay minerals or if cations in water ean have the same oxidizing effect. 3,3 -Dichlorobenzidine does not appear to biodegrade easily, but the few studies in this area did not state the type(s) or concentrations of mieroorganisms used in eaeh study. More systematic studies with other organisms may prove useful. A reeent study (Nyman et al. 1997) provides evidence that in the span of a year up to 80% of 3,3 -dichloro-benzidine can degrade to benzidine in anaerobic mixtures of sediment/water. Further research to identify the pathways and produets of deeomposition of 3,3 -dichlorobenzidine in various soils is needed. The toxieologieal profile for benzidine eontains information on the environmental fate of that compound (ATSDR 1995). 

Figure 8.14 Effect of polarizing power on degree of hydrolysis of cations in water. Reprinted from D. R. Turner, M. Whitfield, and A. G. Dickson, Geochimica et Cosmochimica Acta, 45, 855-881, copyright 1981, with kind permission from Elsevier Science Ltd., The Boulevard, Langford Lane, Kidlington 0X5 1GB, UK.

This is what analytical chemistry is all about, the selection from the methods that are available, the one which is most suitable for the task i.e. a method that is fit for purpose. For example you would probably never choose NMR for the determination of cations in water (ICP-OES or AAS are certainly more suitable). 

Table 6.2. Long wavelength uv absorption bands of benzene radical-cations in water, determined by pulse-radiolysis techniques.

B) The AG -values derived from formation constants of complexes in methanol (cf. Table 2) are not identical with the free energies of transfer AG, as defined by Eq. (30), because the cations are already solvated by the nonaqueous solvent prior to the complexation. These values are representative, none the less, since the solvation of cations in water and methanol is very similar (95). 

Tab. 1 Standard potentials (in V versus standard hydrogen electrode (SHE)) for alkali metals and their monovalent cations in water at 298.15 K...

Yanes and coworkers [43] demonstrated an application of IL for aqueous CE for fhe separation of phenolic compounds (flavonoids) found in grape seed exfracfs. By using [C Qlm] (n = 2,4) ILs as additives for the running electrolyte, a simple and reproducible electrophoretic method for the separation of polyphenols was developed. If was speculated that the separation mechanism was based on an association between the imidazolium cations and the polyphenols. The role of fhe alkyl substituents on the imidazolium cations was investigated and discussed [43]. The anion has little effect on the separation while a related study demonstrated that interaction between phenolic compounds and the IL cations in water occurred through n-n interactions. 

The value of Av and its sign can be used to discriminate between possible mechanisms for a reaction. An example taken from the literature [14,15] is the decomposition of diazonium cations in water. The mechanism of this reaction could be bimolecular ... 

The mechanism of base-catalysed deprotonation of the a-CH of 4-methoxybenzyl alcohol radical cations in water has been examined. There is no direct attack of HO- at the a-CH as was believed, but reaction occurs via deprotonation of the OH to produce the benzyloxy radical, which then forms the carbon-centred radical by a 1,2-hydrogen... 

For phenol one can compare the effects of hydroxy and methoxy substituents. Scheme 19 shows effects of O-methyl substitution on pATas for protonation of a benzene ring containing one, two, and three hydroxy substituents. The pK s for di- and trihydroxy-substituted and methoxy-substituted benzenes were measured directly by Kresge et al.68 Again the stabilities of the hydroxy-substituted cations in water are consistently greater than methoxy. The importance of solvation in controlling these effects is demonstrated by the inversion of relative pK s of trihydroxy and trimethoxy benzene in concentrated solutions of perchloric acid.68 Thus the difference in p/fas is matched by a... 

Baciocchi E, Bietti M, Steenken S (1997) Base-catalyzed C-H deprotonation of 4-methoxbenzyl alcohol radical cations in water evidence for a carbon-to-oxygen 1,2-H-shift mechanism. J Am Chem Soc 119 4078-4079... 

Mohr M, Marx D, Parrinello M, Zipse H (2000) Solvation of radical cations in water-reactive or unre-active solvation Chem Eur J 6 4009-4015... 

The Cadarache group39 focused its modeling contribution on di(isopropoxy)-calix[4]arene-crown-6. It was deduced from these simulations that the 1,3-alternate conformation is much more preorganized than the cone conformation to bind a large cation in water and in vacuo. It can be explained by the fact that the isopropoxy groups, which are near to the crown basis, can prevent the complexation of small cations by shielding the complexation site constituted by the four phenoxy oxygen atoms. 

Diffuse reflectance spectra (DRS) of CV+ (1.4 X 10-6 mol/L of guest per gram of a-ZrP) adsorbed on the external surface of a-ZrP differed from that of the cation in water in three ways (1) The monomer and dimer bands, both, disappeared, (2) two new bands with maxima at 430 nm and 640 nm appeared, and (3) the DRS spectrum in the blue region (< 400 nm) also drastically changed. The appearance of new bands was likely due to the protonation of the dye by the acidic P—OH groups of a-ZrP layers (Fig. 27) and the strong band around 430 nm was assigned to the doubly protonated form (CV3+). 

Thus a large binding constant corresponds to a high equilibrium concentration of bound metal, and hence a more stable metal-macrocycle complex. Typical binding constants for crown ethers and alkali metal cations in water are in the range 101 102. In methanol, this increases up to 106 for [K([18] crown-6)]+. The binding constant for K+ and [2.2.2] cryptand is about 1010. Some other examples are given in Table 1.3. 

Hardness Concentration of cations in water that will react with a sodium soap to precipitate an insoluble residue. Total hardness is a measure of the concentration of calcium and magnesium ions in water, usually expressed as mg/L CaC03. Volume 1(10). 

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