Membrane voltage drop

The power consumption of a membrane chlor-alkali cell which uses this material can be estimated from these voltage and transport number results. Cell voltage is the sum of membrane voltage drop, galvanic voltage, electrode overpotentials, and solution and structural IR drops. When these are summed, cell power consumption is calculated by ... 

In addition, these results suggest that the pH gradient across such membranes in operating chlor-alkali cells will be an important factor in determining the membrane voltage drop. 

Figure 4.10. Current density contours computed using (a) model of Springer et al. and (b) BFM2. Computations performed for fixed membrane voltage drop of 0.1 F.

The percutaneous absorption picture can be qualitatively clarified by considering Fig. 3, where the schematic skin cross section is placed side by side with a simple model for percutaneous absorption patterned after an electrical circuit. In the case of absorption across a membrane, the current or flux is in terms of matter or molecules rather than electrons, and the driving force is a concentration gradient (technically, a chemical potential gradient) rather than a voltage drop [38]. Each layer of a membrane acts as a diffusional resistor. The resistance of a layer is proportional to its thickness (h), inversely proportional to the diffusive mobility of a substance within it as reflected in a... 

Srinivasan et al. [17] have described a four-electrode potentiostat system which is suited to maintaining a constant voltage drop across a membrane in a two-chamber diffusion cell. This system was evaluated in connection with trans-dermal iontophoretic drug delivery of polypeptides. 

In order to determine the thermal time constant of the microhotplate in dynamic measurements, a square-shape voltage pulse was applied to the heater. The pulse frequency was 5 Hz for uncoated and 2.5 Hz for coated membranes. The amplitude of the pulse was adjusted to produce a temperature rise of 50 °C. The temperature sensor was fed from a constant-current source, and the voltage drop across the temperature sensor was amplified with an operational amplifier. The dynamic response of the temperature sensor was recorded by an oscilloscope. The thermal time constant was calculated from these data with a curve fit using Eq. (3.29). As already mentioned in the context of Eq. (3.37), self-heating occurs with a resistive heater, so that the thermal time constant has to be determined during the cooHng cycle. 

The current signal, on the other hand, can be reduced drastically. A first, obvious, method is reducing the working electrode surface. A second possibility involves reducing the amount of reacting electroactive component which is transported to the electrode surface, e.g. by means of a membrane or a porous wall. When the working electrode, the reference electrode and the counter electrode are positioned at the same side of the membrane, the resistance of the solution between the electrodes is not influenced by the membrane, which is not the case when the electrodes are positioned at different sides of the membrane, and the influence on the IR voltage drop can... 

Power Supply and Process Control Unit. Electrodialysis systems use large amounts of direct current power the rectifier required to convert AC to DC and to control the operation of the system represents a significant portion of a plant s capital cost. A typical voltage drop across a single cell pair is in the range 1 -2 V and the normal current flow is 40 mA/cm2. For a 200-cell-pair stack containing 1 m2 of membrane, the total voltage is about 200-400 V and the current about... 

At low frequencies, the external field induces an alternating voltage which is superposed on the resting (dc) transmembrane potential. As frequencies rise and the membrane capacitance dominates its electrical behaviour, this induced voltage drops to a very low level. 

In the polarization curve, three parts can be observed kinetic, ohmic, and mass transfer. In the kinetic part, the cell voltage drop is due to the charge-transfer kinetics, i.e., the 02 reduction and H2 oxidation rate at the electrode surface, which is dominated by the kinetic I-rj equation (Equation 1.37). In the ohmic part, the cell voltage drop is mainly due to the internal resistance of the fuel cell, including electrolyte membrane resistance, catalyst layer resistance, and contact resistance. In the mass transfer part, the voltage drop is due to the transfer speed of H2 and 02 to the electrode surface. 

Figure 3.14. Percentage of cell individual voltage drop caused by charger transfer, membrane, and mass transfer resistances at different current densities and 80°C [21]. (Reproduced by permission of ECS—The Electrochemical Society, from Tang Y, Zhang J, Song C, Liu H, Zhang J, Wang H, Mackinnon S, Peckham T, Li J, McDermid S, Kozak P. Temperature dependent performance and in situ AC impedance of high-temperature PEM fuel cells using the Nafion-112 membrane.)...

Because of the high resistance of the glass membrane (10 to 100 MO), it is not practical to measure the emf directly. Instead, pH meters either use a direct-reading electronic voltmeter or electronically amplify the small current that flows through the cell and detect the voltage drop across a standard resistor potentiometrically. Both battery-operated and ac line-operated pH meters are available connnercially from such firms as Beckman Coulter, Thermo Orion, and Coming. Such pH meters are calibrated to read directly in pH units, have internal compensation for the temperature coefficient of emf, and have provision for scale adjustments. 

It may be noted that we define AG such that it equals the chemical potential of the substrate minus the chemical potential of the product. We noted above that the possibility of free-energy dissipation drives a reaction. Free-energy differences like ACr and A/tg in the above equation embody such a possibility they act as forces that drive the reaction. Other examples are the contractile force on a muscle the voltage drop across an electrical resistance the osmotic pressure on a semipermeable membrane. The dissipation function consists of the sum of the products of fluxes (currents) and the (thermodynamic) forces that drive them [4]. 

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