Anodic cations

It is always true that in an electrochemical cell, anions move to the anode cations move to the cathode. 

Strategy Split the reaction into two half-reactions. Remember that oxidation occurs at the anode, reduction at the cathode. Anions move to the anode, cations to the cathode. Electrons are produced at the anode and transferred through the external circuit to the cathode, where they are consumed. 

Ionic transport occurs in both directions across the skin. For example, to preserve electroneutrality at the anode, cations migrate into the body and anions migrate from the body into the electrode chamber. Hence, iontophoresis can be used for both drug delivery and noninvasive sampling [6,7,17],... 

Scheme 60. The precedent for anodic cation radical cyclizations.

To simplify the following discussion, we restrict it to the case of a cathode. Obviously, however, the results and phenomena directly apply to anodes, cations being replaced by anions, and vice versa. 

The anodic cation transfer rate, vj, and the cathodic anion transfer rate, v(), both depend on the interfacial potential, Acj>H, between the ionic solid and the aqueous solution as described in the following equations ... 

Because of electrical neutrality, the anodic cation transfer current is always equal to the cathodic anion transfer current = v. Equations 22.41 and 22.42, then, yield the interfacial potential,... 

Figure 13.34 Use of cationic surfactant to reverse electroosmotic flow (a) normal EOF toward cathode (no surfactant) and (b) reversed EOF toward anode (cationic surfactant bilayer). (Katz et al., used with permission.)...

Anions migrate towards the anode, cations migrate towards the cathode. 

If the electrode above the center of the gel is anode, cationic surfactant molecules adsorb on the reverse side of the anionic gel facing the electrodes, which cause the gel to deform into concave shape. This is because the anode electrode above the center of the gel repels the surfactant molecules. 

Anions with more positive E xidation values will be discharged first at the anode. Cations with more negative values will be discharged first at the cathode. 

Electrodialysis consists of applying a direct current across a lx)dy of water separated into vertical layers by membranes alternately permeable to cations and anions. Cations migrate toward the cathode and anions toward the anode. Cations and anions both enter one layer of water, and both leave the adjacent layer. Thus, layers of water enriched in salts alternate with those from which salts have been removed. The water in the brine-enriched layers is recirculated to a certain extent to prevent excessive accumulation of brine. 

When we replace the voltmeter with a lightbulb, electrons flow from the zinc electrode (the anode) to the copper electrode (the cathode). The flow of electrons lights the bulb. Anions in the salt bridge migrate toward the anode cations migrate toward the cathode. 

Note that chemists tend to refer to positive ions as cations (attracted to the cathode m electrolysis) and negative ions as anions (attracted to an anode). In this section of the encyclopedia, the temis positive ion and negative ion will be used for the sake of clarity. 

Anodic-stripping voltaimnetry (ASV) is used for the analysis of cations in solution, particularly to detemiine trace heavy metals. It involves pre-concentrating the metals at the electrode surface by reducmg the dissolved metal species in the sample to the zero oxidation state, where they tend to fomi amalgams with Hg. Subsequently, the potential is swept anodically resulting in the dissolution of tire metal species back into solution at their respective fomial potential values. The detemiination step often utilizes a square-wave scan (SWASV), since it increases the rapidity of tlie analysis, avoiding interference from oxygen in solution, and improves the sensitivity. This teclmique has been shown to enable the simultaneous detemiination of four to six trace metals at concentrations down to fractional parts per billion and has found widespread use in seawater analysis. 

As a furtlier example for tire meaning of ex situ investigations of emersed electrodes witli surface analytical teclmiques, results obtained for tire double layer on poly crystalline silver in alkaline solutions are presented in figure C2.10.3. This system is of scientific interest, since tliin silver oxide overlayers (tliickness up to about 5 nm) are fonned for sufficiently anodic potentials, which implies tliat tire adsorjDtion of anions, cations and water can be studied on tire clean metal as well as on an oxide covered surface [55, 56]. For tire latter situation, a changed... 

Capillary zone electrophoresis also can be accomplished without an electroosmotic flow by coating the capillary s walls with a nonionic reagent. In the absence of electroosmotic flow only cations migrate from the anode to the cathode. Anions elute into the source reservoir while neutral species remain stationary. 

Deposition of MnO from a solution containing Mn cations on the anode is not considered the primary electrode process. Initially the Mn (ITT) ion is formed on the anode (73). MnO formation arises from Mn(TTT) disproportionation ... 

The anode and cathode chambers are separated by a cation-permeable fluoropolymer-based membrane (see Membrane technology). Platinum-electroplated high surface area electrodes sold under the trade name of TySAR (Olin) (85,86) were used as the anode the cathode was formed from a two-layer HasteUoy (Cabot Corp.) C-22-mesh stmcture having a fine outer 60-mesh stmcture supported on a coarse inner mesh layer welded to a backplate. The cell voltage was 3.3 V at 8 kA/m, resulting ia a 40% current efficiency. The steady-state perchloric acid concentration was about 21% by weight. 

Sodium nitrite has been synthesized by a number of chemical reactions involving the reduction of sodium nitrate [7631-99-4] NaNO. These include exposure to heat, light, and ionizing radiation (2), addition of lead metal to fused sodium nitrate at 400—450°C (2), reaction of the nitrate in the presence of sodium ferrate and nitric oxide at - 400° C (2), contacting molten sodium nitrate with hydrogen (7), and electrolytic reduction of sodium nitrate in a cell having a cation-exchange membrane, rhodium-plated titanium anode, and lead cathode (8). 

If the cations in solution are condensable as a soHd, such as copper, they can plate out on the cathode of the cell. As the same time, perhaps some hydrogen is also produced at the cathode. The SO can react with a copper anode material by taking it into solution to replace the lost copper ions. Thus the anode is a consumable electrode in the process. 

Electrodialysis. In electro dialysis (ED), the saline solution is placed between two membranes, one permeable to cations only and the other to anions only. A direct electrical current is passed across this system by means of two electrodes, causiag the cations ia the saline solution to move toward the cathode, and the anions to the anode. As shown ia Figure 15, the anions can only leave one compartment ia their travel to the anode, because a membrane separating them from the anode is permeable to them. Cations are both excluded from one compartment and concentrated ia the compartment toward the cathode. This reduces the salt concentration ia some compartments, and iacreases it ia others. Tens to hundreds of such compartments are stacked together ia practical ED plants, lea ding to the creation of alternating compartments of fresh and salt-concentrated water. ED is a continuous-flow process, where saline feed is continuously fed iato all compartments and the product water and concentrated brine flow out of alternate compartments. 

Electrodialysis. Electro dialysis processes transfer ions of dissolved salts across membranes, leaving purified water behind. Ion movement is induced by direct current electrical fields. A negative electrode (cathode) attracts cations, and a positive electrode (anode) attracts anions. Systems are compartmentalized in stacks by alternating cation and anion transfer membranes. Alternating compartments carry concentrated brine and purified permeate. Typically, 40—60% of dissolved ions are removed or rejected. Further improvement in water quaUty is obtained by staging (operation of stacks in series). ED processes do not remove particulate contaminants or weakly ionized contaminants, such as siUca. 

Electrically assisted transdermal dmg deflvery, ie, electrotransport or iontophoresis, involves the three key transport processes of passive diffusion, electromigration, and electro osmosis. In passive diffusion, which plays a relatively small role in the transport of ionic compounds, the permeation rate of a compound is deterrnined by its diffusion coefficient and the concentration gradient. Electromigration is the transport of electrically charged ions in an electrical field, that is, the movement of anions and cations toward the anode and cathode, respectively. Electro osmosis is the volume flow of solvent through an electrically charged membrane or tissue in the presence of an appHed electrical field. As the solvent moves, it carries dissolved solutes. 

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