Nd-Fe-B permanent magnets are easily available, powerful, and inexpensive and generate strong quantifiable convective effects during electrolysis, similar to those obtained with rotating electrodes or large electromagnets. The magnetic field of Nd-Fe-B magnets has been simulated numerically and mapped. Its most characteristic difference from the field of most commercial electromagnets is the presence of magnetic field gradients, which introduce additional body forces in the electrolytic solution and create new modes of mass transfer due to the attraction of electrogenerated radicals into areas of stronger field. The effect of those new forces on the radial distribution of the flow profile in the vicinity of the electrode has been monitored with generation-collection experiments and optical photography. The emerging utility of Nd-Fe-B magnets in systems of chemical interest is demonstrated with flow control and delivery devices, based on galvanic cells configured as self-powered magnetohydrodynamic pumps.

f i (E)) [(E-A i) 2 /E] ∑ k)0 ∞ a i, k + 1 (E-A i) k /(k + 1) 9362

Generally, faradaic current passing through an electrolytic cell placed in a magnetic field causes stirring of the electrolytic solution and current−voltage characteristics similar to those obtained with rotating disk electrodes. It is reported herein that the intensity of the hydrodynamic convection generated by conventional disk millielectrodes in magnetic fields is intimately related to the nature of the faradaic process, and that the mass-transfer limited current, i l, is proportional to n 3/2 where n is the number of electrons involved in the heterogeneous electron transfer. That finding has been justified on the basis of a feedback mechanism that relies on the dependence of the faradaic current on the hydrodynamic velocity profile within the electrolytic conductor, and of the hydrodynamic velocity profile on the current. The implications of the nonlinear dependence of i l on n have been discussed in terms of a moving-boundary diffusion-layer model which is introduced into digital simulations and reproduces the main features of magnetic field voltammograms. Combination of the new findings with our previous results leads to the following expression for disk millielectrodes in transverse magnetic fields at room temperature:  i l = 4.31 × 102 n 3/2 F A 3/4 |B|1/3 D ν-1/4 C bulk 4/3, where A is the electrode area, F the Faraday constant, |B| the magnetic field strength, D the diffusion coefficient, C bulk the bulk concentration of the redox-active species, ν the kinematic viscosity of the electrolyte, and where the numerical constant has units of cm T-1/3 s-1/4 mol-1/3.

In anhydrous CH 3 CN, 4-benzoyl-N-methylpyridinium cations undergo two reversible, well-separated (∆E 1/2 ∼ 0.6 V) one-electron reductions in analogy to quinones and viologens. If the solvent contains weak protic acids, such as water or alcohols, the first cyclic voltammetric wave remains unaffected while the second wave is shifted closer to the first. Both voltammetric and spectroelectrochemical evidence suggest that the positive shift of the second wave is due to hydrogen bonding between the two-electron reduced form of the ketone and the proton donors. While the one-electron reduction product is stable both in the presence and in the absence of the weak-acid proton donors, the two-electron reduction wave is reversible only in the time scale of cyclic voltammetry. Interestingly, at longer times, the hydrogen bonded adduct reacts further giving nonquaternized 4-benzoylpyridine and 4-(R-hydroxybenzyl)pyridine as the two main terminal products. In the presence of stronger acids, such as acetic acid, the second wave merges quickly with the first, producing an irreversible two-electron reduction wave. The only terminal product in this case is the quaternized 4-(Rhydroxybenzyl)-N-methylpyridinium cation. Experimental evidence points toward a common mechanism for the formation of the nonquaternized products in the presence of weaker acids and the quaternized product in the presence of CH 3 CO 2 H.

Cyclic voltammetry with Nd-Fe-B disk magnet electrodes (3.2 mm diameter) at slow sweep rates (< or = 0.01 V s(-1)) in relatively concentrated solutions (e.g., 80 mM) of diamagnetic redox-active species (e.g., TMPD) is controlled by diffusion. Under similar conditions, cyclic voltammetry with conventional noble metal disk millielectrodes is characterized by the absence of diffusion waves and the presence of density gradient driven natural convection. Although the magnetic field in the vicinity of Nd-Fe-B electrodes is relatively strong (approximately 0.5 T at the surface of the magnet electrode), the absence of magnetohydrodynamic stirring effects is attributed to the fact that the i and B vectors are almost parallel, and therefore the magnetohydrodynamic force F(B) (=i x B) is very small. On the other hand, the absence of natural convection is attributed to the two possible paramagnetic body forces, F(inverted Delta B) and F(inverted Delta C), exerted by the magnet electrode on the diffusion layer. Of those two forces, the former depends on field gradients (F(inverted Delta B) approximately B x inverted Delta B), while the latter depends on concentration gradients (F(inverted Delta C) approximately inverted Delta C(j)) and is directed toward areas with higher concentration of paramagnetic j. Through thorough analysis of the magnetic field and its gradients, it is found that the average F(inverted Delta C) force acting upon the entire diffusion layer is approximately 1.75 times stronger than F(inverted Delta B). Nevertheless, it is calculated that either force independently is strong enough and would have been able to hold the diffusion layer by itself. Further evidence suggests that, integrated over the entire solution, F(inverted Delta B) is the dominant paramagnetic force when the redox-active species is paramagnetic, e.g., [Co(bipy)(3)](ClO(4))(2) (bipy = 2,2'-bipyridine). Finally, convective behavior with diamagnetic redox-active species and magnet millielectrodes can be observed by holding closely (2-3 mm away) a repelling second magnet that bends the induction B to the point that the i x B product is not equal to 0. with Nd-Fe-B disk ma