Counterion sensitivity

For analysis of a major compound that is a counterion, sensitivity is not an issue. As a general rule, one should not inject more than 2% of the capillary volume however, one may explore to inject up to 8% to decrease the LOD. Alternatively, a capillary with a larger diameter and/or greater length may he used. This allows injection of more samples. An extended pathlength may also he considered. 

Metalloprotein Approx. coordination Ligands pH sensitive Counterion sensitive chemical shift Footnote reference... 

In general, (Z)-[2-(4-methylphenylsulfmyl)ethenyl]benzenes undergo addition with higher diastereoselectivity than their. -counterparts. The stereochemical outcome for ( )-[2-(4-methyl-phenylsulfinyl)ethcnyl]benzene is more sensitive to the metal counterion and reaction solvent1,2. For ( )-[2-(4-methylphenylsulfmyl)ethenyl]benzene a reverse in product diastereoselection is observed when the metal counterion/solvent is changed from Na/ethanol to Li/THF1. 

The ionic or polar substances can be seperated without any reaction on specially treated chromatographic columns and detected refractometrically. This is necessary because alkyl sulfosuccinates show only small absorption in the UV-visible region no sensitive photometric detection can be obtained. Separation problems can arise when common steel columns filled with reverse phase material (or sometimes silica gel) are used. This problem can be solved by adding a suitable counterion (e.g., tetrabutylammonium) to the mobile phase ( ion pair chromatography ). This way it is possible to get good separation performance. For an explanation of separation mechanism see Ref. 65-67. A broad review of the whole method and its possibilities in use is given in an excellent monograph [68]. 

The critical value for Q is 1/z. There is a proportional increase in the number of free counterions, f/z, as Q increases from zero, reaching a plateau when Q = z. Also, below this value the degree of dissociation,) , increases as the concentration decreases, and tends to unity as v tends to zero. When Q> /z, p decreases with 0 and tends to /zQ as 0 tends to zero. The number of free ions caimot exceed njz Q. Note that this number is inversely proportional to the square of the valence. The condensation of ions is thus very sensitive to valence for multivalent counterions it takes place at a lower value of Q and the number of free ions is much smaller... 

The selective binding of cations is not as sensitive to size as to valence. The value of Q for the condensation of counterions of the same valence is unaffected. In the case of monovalent cations, the dissociation of all counterions is complete at infinite dilution, when 2 1 When Q the... 

The most important factor determining the sensitivity of the conformation to the concentration of polyions is the change in ion activity or osmotic pressure with conformation. If the activity coeflScient of the counterions is sensitive to conformation then conformational change resulting from concentration changes of polyions becomes large. 

Specific-ion electrodes are expensive, temperamental and seem to have a depressingly short life when exposed to aqueous surfactants. They are also not sensitive to some mechanistically interesting ions. Other methods do not have these shortcomings, but they too are not applicable to all ions. Most workers have followed the approach developed by Romsted who noted that counterions bind specifically to ionic micelles, and that qualitatively the binding parallels that to ion exchange resins (Romsted 1977, 1984). In considering the development of Romsted s ideas it will be useful to note that many micellar reactions involving hydrophilic ions are carried out in solutions which contain a mixture of anions for example, there will be the chemically inert counterion of the surfactant plus the added reactive ion. Competition between these ions for the micelle is of key importance and merits detailed consideration. In some cases the solution also contains buffers and the effect of buffer ions has to be considered (Quina et al., 1980). 

Co/pH and V o/pH results are sensitive to different aspects of the surface chemistry of oxides. Surface charge data allow the determination of the parameters which describe counterion complexation. Surface potential data allow the determination of the ratio /3 —< slaDL- Given assumptions about the magnitude of the site density Ns and the Stern capacitance C t, this quantity can be combined with the pHp2C to yield values of Ka and Ka2. Surface charge/pH data contain direct information about the counterion adsorption capacitances in their slope. To find the equilibrium constants for adsorption, a plot such as those in Figures 7 and 8 can be used, provided that Ka and Kai are independently known from V o/pH curves. 

From Figure 1.3, it becomes also evident that, while the slopes of the distinct solutes are different, those of the corresponding enantiomers are nearly the same. This means that for both enantiomers an ion-exchange process is at work and both isomers respond almost equally sensitive to the variation of the counterion concentration. In other words, the separation factors are usually almost unaffected by the counterion concentration, which opens up the possibility for a flexible adjustment... 

After extensive experimentation, a simple solution for avoiding catalyst deactivation was discovered, when testing an Ir-PHOX catalyst with tetrakis[3,5-bis (trifluoromethyl)phenyl]borate (BArp ) as counterion [5]. Iridium complexes with this bulky, apolar, and extremely weakly coordinating anion [18] did not suffer from deactivation, and full conversion could be routinely obtained with catalyst loadings as low as 0.02 mol% [19]. In addition, the BArp salts proved to be much less sensitive to moisture than the corresponding hexafluorophosphates. Tetrakis (pentafluorophenyl)borate and tetrakis(perfluoro-tert-butoxy)aluminate were equally effective with very high turnover frequency, whereas catalysts with hexafluorophosphate and tetrafluoroborate gave only low conversion while reactions with triflate were completely ineffective (Fig. 1). 

It is evident from the preceding discussion that some, but not all, reaction mechanisms are sensitive to the level of solvation present. What of the counterion, the. oppositely charged ion in solution often regarded merely as a spectator In ICR spectrometry, only one type of ion, positive or negative, is normally trapped in the cell at a time. The chemistry of ions independent of any counterion can thus be examined. 

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