Ion-association systems

Dagnall and West8 have described the formation and extraction of a blue ternary complex, Ag(I)-l,10-phenanthroline-bromopyrogallol red (BPR), as the basis of a highly sensitive spectrophotometric procedure for the determination of traces of silver (Section 6.16). The reaction mechanism for the formation of the blue complex in aqueous solution was investigated by photometric and potentiometric methods and these studies led to the conclusion that the complex is an ion association system, (Ag(phen)2)2BPR2, i.e. involving a cationic chelate complex of a metal ion (Ag + ) associated with an anionic counter ion derived from the dyestuff (BPR). Ternary complexes have been reviewed by Babko.9... 

These include the simplest ion-association systems in which bulky cations and anions are extracted as pairs or aggregates without further coordination by solvent molecules. An example of this type of system is the extraction of manganese or rhenium as permanganate or perrhenate into chloroform by association with the tetraphenylarsonium cation derived from a halide salt... 

Non-chelated ion-association systems tetraphenylarsonium and tetraalkyl-ammonium salts... 

Quinolones (including ciprofloxacin) have been shown to be separable by TLC using thin layers of Diol-silica adsorbent, and solutions of di-(2-ethylhexyl)-orthophosphoric acid (HDEHP, an ion-pairing reagent) in polar solvents as the mobile phase [23]. Retention and selectivity in the adsorption-ion-association system can be controlled by adjusting the concentration of HDEHP (typically 5 10%), or by changing the polar diluents. 

Another group of ion-association systems is represented by solvated salts (usually nitrates, but also halides and sulphates). Solutions (1-50%) of tri-n-butyl phosphate (TBP, formula 1.11) in hexane, CCI4, and solutions (1-20 %) of tri-n-octylphosphine oxide (TOPO, formula 1.12) in cyclohexane are most often used as the extractants. Solvation with TBP or TOPO (through the strongly basic oxygen atom of the phosphoryl group) enables metal salts. 

By the incorporation of the various equilibrium expressions as indicated by the above scheme, the distribution ratio D may be expressed in terms of the chloride ion concentration. With ion association systems, the extracted ion pair-complex can dimerize, particularly in solvents of low dielectric constant. This would result in the dependence of D on the total iron concentration. 

Other types of organic reagent include those used to form colored ion-association systems, precipitants forming insoluble normal salts, and the insoluble salts of chloroanilic acid. 

Some dyestuffs form ion-association systems with large inorganic anions. Brilliant green co-extracts stoichiometrically with hexachloroantimonate ion, providing a method for the determination of antimony. 

Further, the nature of the reagents used in spec-trophotometric appHcations, used in catalytic procedures or to form complexes, allows chemical reactions involving mixed Hgand and ion-association systems, organized media, etc. to be carried out to determine different analytes, thus increasing the applicability of spectrophotometry. 

Silicate has also been determined based on the ion-association system formed with molybdate and rhodamine B. The proposed method has been applied to iron alloys, and the analytical range was up to 3.5 pg/50ml with a wavelength of 585 nm. 

Bieganowska, M.L. Petmczynik, A. Thin-layer reversed phase chromatography of some alkaloids in ion-association systems. Part II. Chem. Anal. (Warsaw) 1994,39,445-454. 

As an example of separations in ion-association systems, a series of alkaloids was separated by ion-pair reversed-phase TLC using silanized silica gel either alone or containing a counterion as the sorbent, with phosphate buffer-organic modifier mixtures containing low concentrations of anionic ion-pairing reagent as the mobile phase (Bieganowska and Petruczynik, 1994). 

Bieganowska and Petruczynik (1994) studied the separation of 15 alkaloids on unmodified silica gel in reversed-phase ion-association systems. Tombesi et al. (1994) purified 15 alkaloids (including atropine, cocaine, codeine, and quinine) by TLC decomposition of their picrates using silica gel and alumina sorbents identification was based on melting points and UV-visible spectra. 

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