Metal ions competing with

A red shift of the absorption band in the visible spectrum is also recorded after the anthocyanin-metal ion interaction. In such a configuration, metallic ions compete with protons and the flavonoids adopt the quinonoidal form (Figure 9.3). 

Operationally, there is a similarity between and metal ions (Lewis acids) and OH" and bases (Lewis bases). The OH group on a hydrous oxide surface has a complex-forming oxygen-donor group like OH" or OH groups attached to other elements (phosphate, silicate, polysilicate). Proton and metal ions compete with each other for the available coordinating sites on the surface ... 

Hessami and Tobias [70] extended the mechanisms of Bockris et al. and Matulis etal. of the deposition of single metals (Ni, Fe) to the mathematical modeling of codeposition of Ni—Fe alloys. This mathematical model for the anomalous alloy deposition describes the electrode processes using the calculated interfacial concentrations. The inhibition (reduction) of nickel partial current density during alloy deposition and the anomalous deposition are explained on the basis of the relative concentrations of metal-hydroxide ions, [MOH]+. Calculations show that the [FeOH]+ concentrations are higher than [NiOH]+ because it has a much smaller dissociation constant ([MOH]+ = (M2+)(OH )/A surface sites, and the result of this competition is inhibition (decrease) in Ni deposition in the presence of [FeOH]+ ions. Figure 30... 

Platinum ions reduce to metallic Pt by injecting holes into the Si valence band. Thus Pt ions act as an oxidizing agent for silicon, and result in the simultaneous formation of photoluminescent porous silicon under certain conditions. Nickel ions may exchange charge with both the conduction and the valence band. The reduction of Ni ions competes with hydrogen evolution, and the deposition of Ni can only be achieved at high pH where it is kinetically faster. The role of silicon surface states as reaction intermediates is discussed. 

The reaction of A-phosphorylmorpholine with pyridines is also affected by the presence of the metal ions. However, the complexes react more slowly than the uncomplexed phosphorylated morpholine. If the transition state of the rate-determining step involves considerable stabilization by electron density of the anionic oxygen centres then the metal ions compete for the electron density and destabilize the transition state. Alternatively, interaction of the solvent with the anion by hydrogen-bond formation may be significant, and this could be disrupted by the metals. 

Initially, iron(II) must travel into the protein shell and reach the ferroxidase centers. The X-ray crystallographic observation of metals such as Tb +, Cd +, Ca +, and Zn " " binding to the six conserved carboxylate residues supports the idea that the threefold channels are the likely entry route for iron into the protein shell in animal ferritins. Other experimental evidence to support this comes from Cd NMR studies indicating that Fe + ions compete with Cd binding to HoSF in the channels. Other studies have shown that substitution of the threefold... 

Competitive systems include suspensions with more than one solute (electrolyte ions, proton, and hydroxide not counted). These may be systems (1) in which two or more metal ions compete for surface sites (e.g., studied by Benjamin [117] or Yang and Davis [79]), (2) in which two or more ligands compete for surface sites (e.g., studied by Goldberg [81] or by Mesuere and Fish [118] and Hiemstra and van Riemsdijk [119]), and (3) systems in which metals and ligands are present, which at least under certain experimental conditions adsorb simultaneously without forming common surface complexes [14] (i.e., they compete). 

Chloride ions in the pore solution, having the same charge as OH ions, compete with these anions to combine with the Fe + cations. The resulting iron chloride complexes are thought to be soluble (unstable) therefore, further metal dissolution is not prevented, and ultimately the buildup of voluminous corrosion products takes place. Chloride ions also tend to be released from the unstable iron chloride complexes, making these harmful ions available for further reaction with the reinforcing steel. As the iron ultimately precipitates out in the form of iron oxide or hydroxide corrosion products, it can be argued that the consumption of hydroxide ions leads to localized pH reduction and therefore enhanced metal dissolution. 

The dashed lines ia Figure 4 are plots of equation 22 for Cu " and Mn and iadicate the concentration of the aquo metal ions ia equiUbrium with the sohd hydroxides as function of pH. At any pH where the soHd curve is above the dashed line for the same metal, the EDTA is holding the unchelated metal ion concentration at a value too low for the precipitation of the sohd hydroxide. Relatively large quantities of the metal can thus be maintained ia solution as the chelate at pH values where otherwise all but trace quantities of the metal would be precipitated. In Eigure 4, this corresponds to pH values where pM of the dashed curves is 4 or greater. At the pH of iatersection of the sohd and dashed lines for the same metal, the free metal ion is ia equihbrium with both the sohd hydroxide and the chelate. At higher pH the hydroxyl ion competes more effectively than the chelant for the metal, and only a trace of either the chelate or the aquo metal ion can exist ia solution. Any excess metal is present as sohd hydroxide. 

The presence of a Faradaic electrode reaction of any kind competing with the double layer charging presents a problem in determining the purely capacitive current needed to calculate the surface charge. From a plot of 1 vs. (/ = total electrode current) with a fixed concentration of the ions of the electrode metal dissolved in solution, the surface charge can be obtained [65Butl]. (Data obtained with this method are labelled TC). 

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