Anion, increased concentration

As metal ion concentration increases in the crevice, a net positive charge accumulates in the crevice electrolyte. This attracts negatively charged ions dissolved in the water. Chloride, sulfate, and other anions spontaneously concentrate in the crevice (Figs. 2.4 and 2.5). Hydrolysis produces acids in the crevice, accelerating attack (Reactions 2.5 and 2.6). Studies have shown that the crevice pH can decrease to 2 or less in salt solutions having a neutral pH. 

The amount of chloride, sulfate, thiosulfate, or other aggressive anions dissolved in water necessary to produce noticeable attack depends on many interrelated factors. Extraordinarily, if the water is quite aggressive, general corrosion may occur so rapidly outside the crevice that concentration differences cannot easily develop between the crevice interior and exterior. However, it is usually safe to assume that as the concentration of aggressive anions increases in solution, crevice attack is stimulated. Seawater chloride concentrations produce severe attack in most stainless crevices in a few weeks. 

For both film-free and film-forming conditions a decrease in corrosion rate is observed as the concentration of the anion increases. For some anions the maximum in the corrosion rate may be attained at low concentrations depending on the species and concentration (Fig. 2.2). One form of inhibition... 

The effects of inhibitive and aggressive anions on the corrosion of zinc are broadly similar to the effects observed with iron. Thus with increasing concentration, anions tend to promote corrosion but may give inhibition above a critical concentration Inhibition of zinc corrosion is somewhat... 

The kinetics of desulphonation of sulphonic acid derivatives of m-cresol, mesitylene, phenol, p-cresol, and p-nitrodiphenylamine by hydrochloric or sulphuric acids in 90 % acetic acid were investigated by Baddeley et a/.701, who reported (without giving any details) that rates were independent of the concentration of sulphuric acid and nature of the catalysing anion, and only proportional to the hydrogen ion concentration. The former observation can only be accounted for if the increased concentration of sulphonic acid anion is compensated by removal of protons from the medium to form the undissociated acid this result implies, therefore, that reaction takes place on the anion and the mechanism was envisaged as rapid protonation of the anion (at ring carbon) followed by a rate-determining reaction with a base. 

Surfactants have a unique long-chain molecular structure composed of a hydrophilic head and hydrophobic tail. Based on the nature of the hydrophilic part surfactants are generally categorized as anionic, non-ionic, cationic, and zwitter-ionic. They all have a natural tendency to adsorb at surfaces and interfaces when added in low concentration in water. Surfactant absorption/desorption at the vapor-liquid interface alters the surface tension, which decreases continually with increasing concentrations until the critical micelle concentration (CMC), at which micelles (colloid-sized clusters or aggregates of monomers) start to form is reached (Manglik et al. 2001 Hetsroni et al. 2003c). 

Type II refers to the case in which the isotherms for both cations increase with increasing concentration of the respective cations. Such isotherms have been found for (Li, K)F,j2 (Li, K)(S04)i/2, (Na, K)OH, (Ag, Cs)Br, - (Ag, Na)I, - (Ag, K)I, - " and (Ag, CS)I. In charge asymmetric systems such as (K, Ca 2)CI, such isotherms also usually appear. A common feature of these type II systems is the particularly strong interaction of one cation with the common anion compared with that of the second cation with the anion. The strongly interacting cation will retard the internal mobility of the second cation. This is called the tranquilization effect, and will be explained in Section III.5( 70-... 

In such systems as (M, Mj (i/2))X (M, monovalent cation Mj, divalent cation X, common anion), the much stronger interaction of M2 with X leads to restricted internal mobility of Mi. This is called the tranquilization effect by M2 on the internal mobility of Mi. This effect is clear when Mj is a divalent or trivalent cation. However, it also occurs in binary alkali systems such as (Na, K)OH. The isotherms belong to type II (Fig. 2) % decreases with increasing concentration of Na. Since the ionic radius of OH-is as small as F", the Coulombic attraction of Na-OH is considerably stronger than that of K-OH. 

The adsorption of ions is determined by the potential of the inner Helmholtz plane 0n while the shift of Epzc to more negative values with increasing concentration of adsorbed anions is identical with the shift in 0(m). Thus, the electrocapillary maximum is shifted to more negative values on an increase in the anion concentration more rapidly than would follow from earlier theories based on concepts of a continuously distributed charge of adsorbed anions over the electrode surface (Stern, 1925). Under Stern s assumption, it would hold that 0(m) = 0X (where, of course, 0X no longer has the significance of the potential at the inner Helmholtz plane). 

The retarding effect of electrolytes in the application of basic dyes to acrylic fibres increases with increasing concentration of salt up to a certain level. Increasing the concentration beyond this point has no further effect on exhaustion with certain univalent anions, whilst with multivalent types there is an increase in dye sorption (Figure 12.2)... 

The situation is different for reactions of very hydrophilic ions, e.g. hydroxide and fluoride, because here overall rate constants increase with increasing concentration of the reactive anion even though the substrate is fully micellar bound (Bunton et al., 1979, 1980b, 1981a). The behavior is similar for equilibria involving OH" (Cipiciani et al., 1983a, 1985 Gan, 1985). In these systems the micellar surface does not appear to be saturated with counterions. The kinetic data can be treated on the assumption that the distribution between water and micelles of reactive anion, e.g. Y, follows a mass-action equation (9) (Bunton et al., 1981a). 

Anionic micelles strongly favor the two-proton mechanism, because of the increased concentration of hydronium ions at the micellar surface (Bunton and Rubin, 1976 Bunton et al., 1978b). 

The information obtained can be used to give interesting information upon the CO2 reduction mechanism. Because the radical anion increases in concentration in the negative direction, it cannot be in equilibrium with the electrode. The increase in anion concentration at cathodic potentials may, however, be explained if CO2 is formed as an intermediate radical. Thus from equations 5-7... 

The second-order rate constant for the methylation of sodium 9-fluorenone oximate in 33.5% acetonitrile/66.5% t-butyl alcohol solution was found to decrease with increasing concentration of the salt, suggesting an equilibrium (13) between the reactive free anion [109] and the less reactive ion pair [110]... 

Obtained in MeCN solution containing 0.1 mol dm 3 Bu°NBF4 as supporting electrolyte. Solutions were 1 x 10 3 mol dm 3 in compound and potentials were determined with reference to an Ag/Ag+ electrode at 21 1°C, 50 mV s scan rate. > p, and represent the anodic and cathodic peak potentials. Cathodic shifts in the metallocene redox couples produced by the presence of anion (5 equiv) added as their tetrabutylammonium salts. As the concentration of the anion increased, the cathodic current peak potential of the ferrocene/ferTOcenium redox couple began to exhibit the features of an EC mechanism. 

The increase in the anion concentration results in higher values of Cj at the maximum and in the shift of the maximum toward the zero charge density. The shift of the peak with increasing concentration is caused by the screening effect of the anions, accumulated in the diffuse layer and linked to the positive ends of the solvent dipoles. However, the changes in the shape of the C, vs. plots are greater on the negative than on the... 

It appears that the concentration of surface cations increases with increasing cathodic polarization (decreasing 4< >h ) whereas, the concentration of surface anions increases with increasing anodic polarization (increasing 4h). The dependence of the concentration of surface constituents on suggests that the dissolution rate of MX is determined by the transfer of cations at less anodic potentials and by the transfer of anions at more anodic potentials. 

---