Electrotransport

The C23 term is associated with electrotransport in which the electrons in the current flow interact with impurities and tend to drag them along. From the matrix Equation 17.38, 

The complement of electrotransport predicts a current flow resulting from a concentration gradient as the diffusion current of atoms or ions drag electrons along 

Again because solid state diffusion is so slow, the effect is negligible. 

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Refining of Vanadium. In addition to the purification methods described above, vanadium can be purified by any of three methods iodide refining (van Arkel-deBoer process), electrolytic refining in a fused salt, and electrotransport. 

Electron-beam melting of zirconium has been used to remove the more volatile impurities such as iron, but the relatively high volatiUty of zirconium precludes effective purification. Electrorefining is fused-salt baths (77,78) and purification by d-c electrotransport (79) have been demonstrated but are not in commercial use. 

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. 

Eig. 6. Electrically assisted transdermal therapeutic system. Electrotransport faciUtates passage of dmgs (D ) through the skin and into adjacent tissue and... 

Electrotransport technology offers a number of benefits for therapeutic appHcations, including systemic or local adininistration of a wide variety of therapeutic agents with the potential adininistration of peptides and proteins long-term noninvasive administration, improving convenience and compliance controlled release, providing a desired deflvery profile over an extended period with rapid onset of efficacious plasma dmg levels and in some cases reduced side effects and a transport rate relatively independent of skin type or site. Additional benefits include easy inception and discontinuation of treatment, patterned and feedback-controlled deflvery, and avoidance of first-pass hepatic metaboHsm. 

Commercially available electrotransport systems are bulky and limited to acute appHcations (96). One example, the Drionic system used for the treatment of hyperhidrosis (excessive perspiration), is presoaked in water for 30 min before each 20- to 30-min treatment. Another system, the Phoresor, approved for the deHvery of Hdocaine [137-58-6] for local anesthesia, and of dexamethasone [50-02-2] for treatment of local inflammation such as bursitis or tendinitis, is powered by a 9 V replaceable battery and features a disposable, fiHable dmg electrode. 

Euture electrotransport therapeutic systems will differ substantially from those just described. They will draw on advances in microelectronics and transdermal system technology to provide transdermal therapy for compounds with low passive permeation rates, patterned or pulsed dmg deHvery,... 

Advances in experimental techniques, including pulsed-field gradient NMR, and theoretical methods, including volume averaging, macrotransport, and variational methods, that may lead to the resolution of a number of the fundamental issues in gel electrophoresis and to improvements in the practical application of electrotransport in polymeric media... 

Electrothermic process, for zinc, 26 577 Electrothermic zinc smelting, 26 612 Electrotransport technique, for purifying vanadium, 25 522... 

Electrotransport In this process too a super-clean atmosphere is necessary. In electrotransport (a kind of solid-state electrolysis) a large dc current (typically 200 A cm-2) is passed through a rod of the metal at a temperature 100-200°C below its melting point. In the rare earth metals the interstitial impurities slowly move towards the anode, while several metallic impurities move towards the cathode. In this case too, as in zone melting, the purest portion of the bar is its central part. 

D. Electrorefining, Zone Melting, and Solid-State Electrotransport... 

If an actinide metal is available in sufficient quantity to form a rod or an electrode, very efficient methods of purification are applicable electrorefining, zone melting, and electrotransport. Thorium, uranium, neptunium, and plutonium metals have been refined by electrolysis in molten salts (84). An electrode of impure metal is dissolved anodically in a molten salt bath (e.g., in LiCl/KCl eutectic) the metal is deposited electrochemically on the cathode as a solid or a liquid (19, 24). To date, the purest Np and Pu metals have been produced by this technique. 

Ultrapure Th metal has been processed at the Ames Laboratory by solid-state electrotransport under very low pressures (on the order of 0.3 nPa), which has produced the purest Th metal known, that with a resistivity ratio of 4200 for doubly refined metal (99-101). This resistivity ratio of 4200 translates into probably <50 ppm total impurities in the metal (see footnote 1) (87-90, 104). Single crystals measuring 0.25 cm in diameter by 1.1 cm in length with resistivity ratios of 1700-1800 have also been grown (55). 

The method of choice for the preparation of Th metal is reduction of the tetrachloride (Section II,B) by Mg (55), followed by refinement using electrotransport purification (Section III,D) (87, 88, 90). 

Secondary refining processes such as zone melting and solid-state electrotransport (Section III,D) should yield ultrahigh-purity Pa metal. 

Dnomc system -electrotransport systems [CONTROLLED RELEASE TECHNOLOGY - PHARMACEUTICAL] (Vol7)... 

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