Anion separation

B. A. Moyer, P. V. Bonnesen, in Physical Factors in Anion Separations (A. Bianchi, K. Bowman-James,... 

As pointed out in Chapter 2, when an ionic solid dissolves in water, the cations and anions separate from each other. This process can be represented by a chemical equation in which the reactant is the solid and the products are the positive and negative ions in water (aqueous) solution. For the dissolving of MgCl2, the equation is... 

A salt is an ionic solid containing a cation other than H+ and an anion other than OH or O2-. When a salt such as NaCl, K2CO3, or A1(N03)3 dissolves in water, the cation and anion separate from one another. 

Common eluents in suppressor ion chromatography are dilute solutions of mineral acids or phenylenediamine salts for cation separations and sodium bicarbonate/sodium carbonate buffers for anion separations. These eluents are too highly conducting to be used without a suppressor column or conductivity detection. Fritz et al. [54-56] have... 

The unusual rate enhancement of nucleophiles in micelles is a function of two interdependent effects, the enhanced nucleophilicity of the bound anion and the concentration of the reactants. In bimolecular reactions, it is not always easy to estimate the true reactivity of the bound anion separately. Unimolecular reactions would be better probes of the environmental effect on the anionic reactivity than bimolecular reactions, since one need not take the proximity term into account. The decarboxylation of carboxylic acids would meet this requirement, for it is unimolecular, almost free from acid and base catalysis, and the rate constants are extremely solvent dependent (Straub and Bender, 1972). 

For a pure salt dissolved in water it is not feasible to determine the activity coefficient, V , for the cation and anion separately so that the mean activity coefficient concept has been defined for a salt s cation-anion pair as... 

Quaternary onium salts were the first phase-transfer catalysts used subsequently, a number of compounds (linear polyethers, polypodands, crown-ethers, cryptands, cage-compounds, etc.) were found effective for the anion activation in two-phase systems. These structurally different systems must satisfy at least two fundamental conditions in order to behave as phase-transfer catalysts i) solubility in the organic phase ii) steric hindrance around the cationic center leading to a good cation-anion separation within the ion-pair. 

The direct quaternization of chloromethylated polystyrenes by tertiary amines or phosphines represents the easiest way to obtain polymer-supported quaternary onium salt (12,13). A lipophilic character of quaternary cation and a topology allowing sufficient cation-anion separation also play an important role (35,36). A linear spacer chain (of about 10 carbon atoms) between the catalytic site and the polymer backbone substantially increases the reaction rates. The loading of quaternary onium groups also affects catalytic efficiency, the influence being different for directly bonded and spaced groups, e.g. 10 and 11, respectively (37). 

Nairn, A., and Pretsch, E. (1994). Potentiometric detection of anions separated by capillary electrophoresis using an ion-selective microelectrode. /. Chromatogr. A 676, 437-442. 

Organic ion-radicals exist as salts with counterions. As seen in the preceding chapters, neutral molecules with strong acceptor/donor properties form rather stable ion-radical salts. Under certain conditions (see later), components of an ion-radical salt pack up in a crystal lattice in a special manner. Different ion-radical parts (cations and anions separately) line up one over the other in the form of endlessly long, practically linear one-dimensional (1-D), piles-chains. These piles-chains form a crystal. 

Flow Rate 92 ml/hr Columns 3 x 500 mm Anion Separator 6 X 250 mm Anion Suppressor Injection Volume 100 1 Meter Full Scale Setting 10 /iMHO... 

Here, Tj and 2 correspond to the reactant and product state, respectively, and T3 is a zwitterionic state having two chloride anions separated by a carbocation. 

Anion Separation by Solid Support Bound Sapphyrins.131... 

Samples and standards are injected into the IC in 100-yL aliquots. The ions are separated by their varying affinites for the ion exchange resin in the anion separator column, the oppositely charged ions are stripped away by the suppressor column leaving only the separated anions and water to be detected by the conductivity cell. Samples are quantitated by comparison with a calibration curve. 

The conditions of IC analysis (Table I) chosen were based on the need for separation of formate ion from common atmospheric contaminants, such as chloride ion, and other organic anions, such as acetate. The conditions were also optimized to obtain a suitable detection limit. Thus, a weak borate eluent (0.005 combined with a 500 mm anion separator column (and a 150 mm precolumn) was chosen. Conversion of the pre-column and separator column to their borate forms (from the normal carbonate form) was necessary. The process of continually passing the borate eluent through the columns until a stable baseline was obtained required several hours. 

Two ion chromatographic techniques were utilized to quantify formic acid in both diesel engine exhaust and mine air subjected to diesel emissions. A commonly used anion separation system utilizing a weak borate eluent adequately separated the acids of interest in diesel exhaust. It was, however, affected by the presence of strong acids during subsequent consecutive analyses. 

Samples were collected in midget bubblers containing dilute sodium carbonate solutions. Analysis of formate ion in these solutions was performed using a 500 mm anion separator column and sodium borate eluent. Analysis of formate in these solutions was also performed using the ion chromatography exclusion mode (ICE) using the Dionex IE-C-1 column with dilute hydrochloric acid eluent. 

The oxidation of 8 resulted in the mixed-valence compound 9 [38] which contain the same cation as the complexes 5 and 7 described above and an AumCI4 anion. The main interesting feature is the one-dimensional chain arrangement in the crystal, with alternating cations and anion separated by intermetallic distances of 3.404(2) A (Au -Au111) and 4.658 A (Hgn-Auln) (Figure 4.5). 

Figure 26-5 illustrates a student experiment to measure ions in pond water. Eluent for the anion separation was NaHC03/Na2C03 buffer. The product from eluent after passing through the suppressor is H2C03, which has low conductivity. 

Figure 26-7 Anion separation by ion chromatography with a gradient of electrolytically generated KOH and conductivity detection after suppression. Column Dionex lonPac AS11 diameter = 4 mm flow = 2.0 mL/min. Eluent 0.5 mM KOH for 2.5 min, 0.5 to 5.0 mM KOH from 2.5 to 6 min 5.0 to 38.2 mM KOH from 6 to 18 min. Peaks (1) quinate, (2) F, (3) acetate, (4) propanoate, (5) formate,...

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