Separation of cations

Indirect detection is commonly used for metal cations because most of these cations lack the high UV or visible absorptivity needed for direct photometric detection. Waters Associates introduced UV Cat 1 for indirect detection. This is an amine cation that absorbs strongly in the UV spectral region and has an electrophoretic mobility similar to 1+ and 2+ metal cations. Protonated phenylethylamine or 4-methylbenzyla-mine are suitable visualization reagents for indirect detection of metal cations at moderately acidic pH values. 

The great majority of chromatographic methods have used a reversed-phase silica column coated with a zwitterionic surfactant of the sulfobetaine type. Anions are generally retained more strongly than cations on this type of column. It has been stated that EIC is a method for the separation of anions. The strong retention of anions by the sulfobetaine phase is probably due largely to the close proximity of the hydrophobic chain (often called the head group ) to the N sites. 

A column coated with n-hexadecylphosphocholine has the hydrophobic head group adjacent to the negative phosphate group. The positive and negative sites on phosphocholine (PC) phases are separated by two -CHj groups. On the sulfo-betaine phases, the oppositely charged sites were separated by three -CH2 groups. 

sample cations and anions are taken up and eluted in pairs. The cation is attracted to the negative phosphate group and the anion pairs up with the positive nitrogen. Sample ions are retained more weakly by PC phases than with sul-fobetaine columns. This may be because of greater electrostatic repulsion on the PC columns. 

Since sample ions are both taken up and eluted in pairs with water as the eluent, the chromatographic peak for a sample cation (C ) wiU actually be a cation-anion pair (C A. The retention time for such a peak will be a function of how tightly A, as well as C, is retained by the phosphocholine. The retention time for C can be either increased or decreased by the choice of the accompanying anion, 

When a sample contains more than one anion, multiple cation-anion peaks can result when water alone is the eluent. For example, cations Ci and C2 can combine with anions Aj and A2 to form four peaks Cj Aj, Cj A2, C2 Aj and C2 A2. In order to obtain only one peak for each sample cation, the sample must be made to contain only a single artion, or an excess of the sodium salt of a strongly retained anion must be added to the sample. Since all sodium salts are eluted very rapidly by water, this extra peak will not interfere with the later sample peaks. 

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Wiktorowicz, J. E. and Colburn, J. C., Separation of cationic proteins via charge reversal in capillary electrophoresis, Electrophoresis, 11, 769, 1990. 

Fluorous Anions for the Separation of Cationic Hydrogenation Catalysts... 

Two types of conductivity detectors exist the contact conductivity detector, where the electrodes are in direct contact with the electrolyte, and the contactless coupled conductivity detector (C D also called oscillometric detector). With this detector, two stainless-steel tubes that act as electrodes are mounted on a capillary at a certain distance from each other. By applying an oscillation frequency, a capacitive transition occurs between the actuator electrode and the liquid inside the capillary. After having passed the detection gap between the electrodes, a second capacitive transition between the electrolyte and the pickup electrode occurs (see Figures 7 and 8 which is an example of separation of cations). In different reviews, Zemann and Kuban and Hauser discuss the advantages of this technique which include rather simple mechanical parts and electronics, and Kuban et al. compared several C D detectors. This technique has also been used as a detector for analysis by microchip CE. C" D detectors are available to be mounted on existing CE instruments. 

Stuff, J. R., Separation of cations in buffered l-methyl-3-ethylimidazolium chloride + aluminum chloride ionic liquids by ion chromatography, J. Chromatogr, 547,484-487,1991. 

Markuszewski, M. J., Stepnowski, R, and Marszall, R, Capillary electrophoretic separation of cationic constituents of imidazolium ionic liquids, Electrophoresis, 25, 3450-3454, 2004. 

Separation of cations Column IC-Pak (Soc. Waters) 150x3.9 mm Conductivity detection Elution 0.1 M EDTA/3 mM N03H Flow t mL/min... 

Figure 4.8—Membrane and electrochemically regenerated suppressors. Two types of membrane exist those that allow the permeation of cations (H+ and Na+) and those that allow the permeation of anions (OH and X ). a) The microporous cationic membrane model is adapted to the elution of an anion. Only cations can migrate through the membrane (corresponding to a polyanionic wall that repulses the anion in the solution) b) Anionic membrane suppressor placed after a cationic column and in which ions are regenerated by the electrolysis of water. Note in both cases the counter-current movement between the eluted phase and the solution of the suppressor c) Separation of cations illustrating situation b).

Since X-ray crystallography cannot observe the lone electron directly (Box 2.1), it is questionable whether it is really situated at such a distance from the Cs+ cation. If true, this would represent a very extreme example of the naked anion effect (Section 3.8.2). An alternative explanation localises the electron on the Cs+ cation, which would also account for the observed low conductivity. However, convincing evidence for the separation of cation and electron comes from the nearly isostructural sodide (Na ) and kalide (K ) analogues of [Cs ([18] crown-6) 2] + -e-. In these, species the alkali metal anions are situated in the same localised cavities as their electride analogues. 

The 2.2.2 column is especially advantageous in the determination of fluoride ion, which in traditional IC often elutes so early as to be masked by the injection peak. [32] In similar fashion, Tsai and Shih [33] derivatized polystyrene/divinylbenzene resin with cryptand 2.2.2 for the ion chromatographic separation of cations or anions. 

Ion-exchange chromatography is a liquid chromatography application that allows the separation of cationic and anionic species. Ion chromatography, a technique used for the separation of dissolved inorganic ions, is employed in EPA Methods 9056 and... 

Karadjova and coworkers [90] in a detailed and comprehensive investigation established a scheme for fractionation of wine components and Cu, Fe, and Zn determination in the different fractions. Like Fe, the other two metals may analogously exist in wines as free ions, as complexes with organic acids and as complexes with proteins, polyphenols and polysaccharides. The resin XAD-8 was used for the separation of wine polyphenols. Dowex ion exchange resins were used for the separation of cationic and anionic species of metals that were subsequently quantified off-line in Bulgarian and Macedonian wines by FAAS or ET-AAS (depending on their concentration levels). 

V.8 SEPARATION OF CATIONS INTO GROUPS Once the sample is dissolved, the separation of cations into groups can be attempted. This can be done according to the scheme outlined in Table V.12. 

Table V.12 Separation of cations into groups (Anions of organic acids, borates, fluorides, silicates, and phosphates being present). Add a few drops of dilute HC1 to the cold solution. If a ppt. forms, continue adding dilute HC1 until no further precipitation takes place. Filter. (1)...

Table V.12 Separation of cations into groups (contd.)...

V.18 PRELIMINARY TESTS FOR AND SEPARATION OF CERTAIN ANIONS After the systematic separation and detection of cations, the search for anions should be started. At this stage already considerable information is available about the presence or absence of certain anions. Not only the preliminary wet and dry tests supplied this information, but during the separation of cations a number of facts became available thus the presence of phosphate, borate, fluoride, citrate, tartrate, and oxalate has been detected, before the precipitation of Group III cations. Similarly, the presence of chromate (or dichromate), permanganate and arsenate has been established at various stages of these separations. When the systematic examination for anions is carried out, such findings should be kept in mind. 

When a suitable solvent has been found, prepare the solution for analysis using about 50 mg of the solid the volume of the final solution should be 1-1-5 ml. Use this solution for the separation of cations into groups according to Section VI.9. 

VI.9 SEPARATION OF CATIONS INTO GROUPS ON THE SEMIMICRO SCALE Once a solution is produced, the systematic search for cations can be started. As the first step, cations should be separated into groups later separations within the individual groups must be carried out. When making these separations, the results of preliminary tests must always be kept in mind. If we know for sure that certain cations are present or absent, we can make appropriate simplifications in our separation procedures, which will result in considerable gain of time. 

Separation of cations into groups can be carried out according to the instructions of Table VI. 11. Note, that this method is suitable only if anions of organic acids, borate, fluoride, silicate, and phosphate are absent. Modifications of the separation scheme in the presence of these anions are described in Section VI. 17. 

Table VI.ll Separation of cations into Groups on the semimicro scale (anions of organic adds, borate, fluoride, silicate, and phosphate being absent) Add 2 drops (1) of dilute HC1 to 1 ml of the clear solution in a 3 ml centrifuge tube (or a 4 ml test-tube). If a ppt. forms, stir and add a further 1-2 drops to ensure complete precipitation. Centrifuge (2) wash the ppt. with a few drops of cold water (3) and add washings to centrifugate.

VI.17 MODIFICATIONS OF SEPARATION PROCEDURES IN THE PRESENCE OF INTERFERING ANIONS The separation of cations, as described in Sections VI.9 to VI.16, is interfered with if anions of certain organic acids, borate, fluoride, silicate, and phosphate are present. During the course of... 

For the separation of cations into Groups IIA and IIB the prescriptions of Table VII.2 should be followed. 

It is assumed that the student is familiar with the laboratory operations described in Chapter II. First, most important reactions of a limited number of cations and anions should be studied, followed by preliminary tests, testing for anions in mixtures and separation of cations. 

Section VIII.4), followed by testing for anions (Sections VIII.5-VIII.7), and by separation and identification of the cations present (Section VIIL8 and VIII.9). Some teachers of qualitative inorganic analysis may prefer to start with the separation of cations - in this case a test for phosphate and fluoride has to be carried out before attempting the separation of Group III cations. 

Some teachers may prefer not to carry out a complete separation of cations, but to hand out separate unknown mixtures containing cations of one analytical group only. In this case precipitation is made of the particular group reagent (hydrochloric acid, hydrogen sulphide, ammonia, ammonium sulphide or ammonium carbonate) and the precipitate is examined by Group Separation Tables V. 12 to V. 19 as well as V.23 to V.28 for each group respectively. 

Table VII13 Separation of cations into groups (anions of organic acids, fluoride, and phosphate being absent) (contd.)...

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