Diffusion metal cations

Dijfusion Dialy The propensity of and OH" to penetrate membranes is useful in diffusion dialysis. An anion-exchange membrane will block the passage of metal cations while passing hydrogen ions. This process uses special ion-exchange membranes, but does not employ an applied electric current. 

Fortunately the oxidation of many metals takes place by the diffusion of the metal cation . This flux is outwards through the oxide layer, and the work of adhesion" enables the loss of metal to be compensated for by a drift of the oxide towards the metal (Fig. 1.81). Thus the stresses set up in the maintenance of oxide/metal contact are compressive and, as such, can be more readily withstood by most oxides. Nevertheless, it is these general movements of the oxide scale which are ultimately responsible for discontinuities in the majority of cases and it is appropriate to discuss transport-induced flows before proceeding any further. 

In the context of Scheme 11-1 we are also interested to know whether the variation of K observed with 18-, 21-, and 24-membered crown ethers is due to changes in the complexation rate (k ), the decomplexation rate (k- ), or both. Krane and Skjetne (1980) carried out dynamic 13C NMR studies of complexes of the 4-toluenediazo-nium ion with 18-crown-6, 21-crown-7, and 24-crown-8 in dichlorofluoromethane. They determined the decomplexation rate (k- ) and the free energy of activation for decomplexation (AG i). From the values of k i obtained by Krane and Skjetne and the equilibrium constants K of Nakazumi et al. (1983), k can be calculated. The results show that the complexation rate (kx) does not change much with the size of the macrocycle, that it is most likely diffusion-controlled, and that the large equilibrium constant K of 21-crown-7 is due to the decomplexation rate constant k i being lower than those for the 18- and 24-membered crown ethers. Izatt et al. (1991) published a comprehensive review of K, k, and k data for crown ethers and related hosts with metal cations, ammonium ions, diazonium ions, and related guest compounds. 

These tetreihedra are tied together at the corners so that a silicate "backbone" forms the structure. The metal cations form "bridges" between backbone-layers and are much more free to move. However, it is well to note that a small amount of silicate does move, but the exact nature of the diffusing specie cannot be quantitatively defined (It may depend upon the nature of the compounds being formed. Most probably, the diffusing specie is actually SiOn but the charge of each actual specie may vary). In... 

Metal sorption on Fe/Al oxides is an inner sphere complexion. The formation of a surface-metal bond releases protons for every metal ion adsorbed. Heavy metal sorbed on Fe oxides can be exchanged only by other metal cations having a similar affinity or by H (McBride, 1989). Metal adsorption on Fe oxides is an initial rapid adsorption reaction, followed by slow diffusion (Barrow et al., 1989). Metal ions (Ni2+, Zn2+ and Cd2+) slowly... 

Electrodes based on solutions of cyclic polyethers in hydrocarbons show a selective response to alkali metal cations. The cyclic structure and physical dimensions of these compounds enable them to surround and replace the hydration shell of the cations and carry them into the membrane phase. Conduction occurs by diffusion of these charged complexes, which constitute a space charge within the membrane. Electrodes with a high selectivity for potassium over sodium (> 1000 1) have been produced. 

Figure 5.22 Formation of metal oxide films (a) parallel diffusion of cations and electrons (b) counterdiffusion of cations and holes (c) counterdiffusion of anions and electrons (d) parallel diffusion of anions and holes and (e) counterdiffusion of anions and cations.

When the kinetics of a sorption process do appear to separate according to very small and very large time scales, the almost universal inference made is that pure adsorption is reflected by the rapid kinetics (16,21,22,26). The slow kinetics are interpreted either in terms of surface precipitation (20) or diffusion of the adsorbate into the adsorbent (16,24). With respect to metal cation sorption, "rapid kinetics" refers to time scales of minutes (16,26), whereas for anion sorption it refers to time scales up to hours TT, 21). The interpretation of these time scales as characteristic of adsorption rests almost entirely on the premise that surface phenomena involve little in the way of molecular rearrangement and steric hindrance effects (16,21). 

Yeager and Steck derived diffusion coefficients for water in totally hydrated Nafion 120 membranes that were exchanged with alkali metal cations, using a radiotracer technique. At 25 °C, 77for the Na+ form was 2.65 X 10 cm /s and the values for the K+ and Cs+ forms were somewhat smaller, which would seem to reflect the lower maximal degree of hydration of these forms. 

Case 5 A fast reaction between the metal cation and the undissociated extracting reagent occurs at the interface or in proximity of the interface). The rate is controlled by the diffusion to and away from the interface of the species taking part in the reaction. 

The template synthesis method involves the diffusion of ligand precursors into the pores of a zeolite where they can assemble around an intrazeolite metal ion that acts as a template. This approach was first used in 1977 for the intrazeoHte synthesis of Cu, Co and Ni phthalocyanines by the condensation of four molecules of dicyanobenzene around the metal cation within the cages of NaY [132, 174]. 

Subsequently, it was found that F-centres can also be produced by heating a crystal in the vapour of an alkali metal this gives a clue to the nature of these defects. The excess alkali metal atoms diffuse into the crystal and settle on cation sites at the same time, an equivalent number of anion site vacancies are created, and ionisation gives an alkali metal cation with an electron trapped at the anion vacancy (Figure 5.24). In fact, it does not even matter which alkali-metal is used if NaCl is heated with potassium, the colour of the F-centre does not change because... 

Zeolites are somewhat like silica in their surface characteristics. Ketones and hydroxy-1,4-biradicals have very polar groups which can interact favorably with metal cations located along zeolite walls. The potential effect of the metal ions on the position of the reacting ketones is twofold. First, the cations may force a ketone molecule into a conformation or a site which it would normally not occupy based solely upon free-volume considerations. Second, the diffusion coefficient of a ketone or a hydroxy-1,4-biradical is probably much more than an order of magnitude smaller than that of benzene [289] so that the residence time of a ketone and its Norrish II intermediates in a zeolite site with at least one metal ion is expected to be closer to 100 ns than to 1 ns. 

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