Competition between anions

The practical significance of such competition evolves from the experience that silicate and (anionic) humics can increase the efficiency of phosphate fertiliser because these compounds occupy sites suitable for phosphate adsorption (Kingston et ak, 1968 Schwertmann, 1995). Hydroxyl is another anion that competes effectively with adsorbing anions, owing to its location in the inner Helmholz layer. The release of adsorbed phosphate after liming a soil or after inflow of acidic surface soil into weakly alkaline surface waters due to erosion, can be considered as the result of competition between OH and phosphate ions. 

Ghromate adsorption by iron oxides is suppressed by a large excess of carbonate or silicate species. Gompetition between silicate and borate adsorption on ferrihydrite was reported by McPhail et ak (1972). Low levels of sulphate suppress uptake of phthalic and chelidimic acids by goethite (Ali Dzombak, 1996 a). 

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Barrow NJ (1992) The effect of time on the competition between anions for sorption. J Soil Sci 43 424-428... 

From gas phase measurements CO is known to prefer top sites on all three low index faces, with the CO molecule perpendicular to the surface and bonded through the carbon end of the molecule except at high coverages (27). It is likely that HCOOH and COOH are adsorbed in a similar way. It is not likely that they could "enter the "troughs , which seems to be possible for anions. For Pt(100) on the other hand, upon sweep reversal and gradual oxide reduction, anions are immediately adsorbed on that "flat" surface. They block adsorption of HCOOH. Adsorption of anions decreases as potential becomes more negative. The oxidation of HCOOH commences and the rate increases as at more negative potentials, i.e. at lower overpotential. A competition between anions and HCOOH adsorption explains this apparently anomalous behaviour. The explanation of the "anomalous behaviour of the Pt(110) surface can be also found in the data for stepped surface vicinal to the (100) and (110) orientations. 

There have been a number of studies of the reaction of diazoacetic ester in aprotic solvents, mainly with carboxylic acids (Bronsted and Bell, 1931 Hartman et al., 1946 and references cited). However, the information available hardly justifies conclusions about the mechanism. Addition of relatively basic phenols causes an acceleration in rate which can be interpreted in terms of nucleophilic catalysis of a rate-determining displacement of nitrogen, but the kinetic order in acid varies between one and two. Formally, a mixed order would result if proton loss from the diazonium ion was effected by carboxylate ions alone, while the less discriminating displacement of nitrogen involved competition between anions and unionized molecules. However, there are examples of high or mixed orders in other acid-catalysed reactions (Bronsted and Bell, 1931 Bell, 1941 1959) and in all probability large medium effects play a role. 

This section reports studies of the effect of strongly interacting adsorbates on the sorption of other adsorbates representing the same class (small cations, small anions, surfactants, polymers), e.g. sorption of copper is studied in absence and in the presence of other heavy metal cations at otherwise identical conditions (solid to liquid ratio, pH, equilibration time, etc.). A few examples of adsorption competition between anions or cations of inert electrolytes are also presented. This limitation does not imply that actual adsorption competition occurs only between adsorbates representing the same class. 

Barrow, N. J. (1989). Testing a mechanistic model. IX. Competition between anions for sorption by soils. J. Soil Sci. 40 415-25. 

These results, which revealed the competition between cathodic anion release and cathodic cation, e.g. Cs", binding into PP" FCN, led us to a further examination of this phenomenon using PP" CIO4 and PP OTs. The results demonstrated that the nature of the anion caused major differences in the competition between anion loss and cation gain. Perchlorate, for example, was more easily lost than tosylate into aqueous NaCl solution. It is hypothesized that the more hydrophobic tosylate is more strongly bound and preferentially retained. 

There is a competition between anion and t-butyl alcohol. The sugar and nitrate ion are separated by the plane of the macrocycle. 

Section 8 13 When nucleophilic substitution is used for synthesis the competition between substitution and elimination must be favorable However the normal reaction of a secondary alkyl halide with a base as strong or stronger than hydroxide is elimination (E2) Substitution by the Sn2 mechanism predominates only when the base is weaker than hydroxide or the alkyl halide is primary Elimination predominates when tertiary alkyl halides react with any anion... 

If the cation has been unchanged, its ability to act as a hydrogen-bond donor has been unchanged, so why is an effect seen at all I propose that there is competition between the anion and the Reichardt s dye solute for the proton. Thus, the values of the ionic liquids are controlled by the ability of the liquid to act as a hydrogen bond donor (cation effect) moderated by its hydrogen bond acceptor ability (anion effect). This may be described in terms of two competing equilibria. The cation can hydrogen bond to the anion [Equation (3.5-2)] ... 

The solvents themselves are adsorbed on the electrode surface, as is shown by the capacitance-potential graphs illustrated in Fig. 9 (Payne, 1967, 1970) potassium hexafluorophosphate, the electrolyte in each of the solvents, is thought to be adsorbed only very weakly. The solvents show somewhat differing curves and the peaks have been interpreted both in terms of competition between the solvent and anions for sites at the surface and also in terms of solvent reorientation. Ethers are adsorbed from the amide solvents most strongly at the potentials around the peaks and this has been postulated to be due to an increase in freedom for the solvent to rotate at these potentials (Dutkiewicz and Parsons, 1966). 

The results of Table 1 show clearly that electronation of nitrocumene (39) (Scheme 15) does compete with reaction with chemisorbed hydrogen, M(H), at some stage in the electrohydrogenation process. The simplest interpretation is a direct competition between electronation of the nitro compound (eq. [7]) and reaction of the adsorbed nitro compound with chemisorbed hydrogen, M(H) (eq. [13]). However, it is quite possible that the electronation of the adsorbed nitro compound (eq. [20]) could be faster than its reaction with M(H) (eq. [13]) and the competition would then be between the cleavage of the adsorbed radical anion (eq. [21]) and its reaction with M(H) (eq. [22]). 

The effects of organic molecules and phosphate on the adsorption of acid phosphatase on various minerals, and kaolinite in particular, have been investigated by Huang et al. [97]. The Langmuir affinity constant for AcP adsorption by kaolinite follows the series tartrate (K — 97.8) > phosphate (K= 48.6) > oxalate (K — 35.6) > acetate (K= 13.4). At low concentration, acetate even promoted the adsorption of acid phosphatase. It was considered that competitive interactions between anionic adsorbates can occur directly through competition for surface sites and indirectly through effects of anion adsorption on the surface charge and protonation. 

The results of a recent investigation of the dependence of the cleavage rate constant upon the solvent of two similar anion radicals, those of 3-nitrobenzyl chloride and 3-chloroacetophenone,85 may likewise be interpreted as the outcome of a competition between the Lewis acid solvation of the developing halide ion and of the negatively charged oxygen atoms in the initial state. 

The use of an anionic reagent for addition at carbonyl carbon rather than a fully esterified form of a trivalent phosphorus acid obviates a troublesome aspect of the Abramov reaction. Specifically no dealkylation step is required. Mechanistic investigations257 258 indicate that the reaction proceeds much as a simple "aldol"-type reaction in which the anionic phosphorus site adds directly to the carbonyl center. While the initial efforts concerned with the "Pudovik reaction"259 were directed toward the use of sodium salts of the simple dialkyl phosphites, as shown in Equation 3.17,260 266 with a, 5-unsaturated carbonyl systems (vide infra) competition between sites for addition can occur. Addition at the carbonyl carbon site is the kinetically favored route.267-270... 

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). 

Reactions of hydrophilic anions in microemulsions can be treated in terms of the ion-exchange formalism. The extent of ion binding to microemulsion droplets has been estimated conductimetrically, and the rates of reactions of OH- and F with p-nitrophenyl diphenyl phosphate are consistent with the extent of ion binding and competition between reactive and inert anions. Alternatively the data could be accommodated to variations in the estimated surface potential of the droplet (Mackay and Hermansky, 1981 Mackay, 1982). 

Addition of NaN02 (50 pM) had no effect on the reaction profile with NO present, and no reaction was observed (on the time scale of the stopped-flow experiment) when NO was absent. However, at higher concentrations, anions, including the conjugate bases of various buffers (B ), slowed down the reaction. This was attributed to the competition between water and the anions for the labile 5th coordination site of Cu(dmp)2(H20)2+. 

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