Cell voltage concentration dependence

Electrochemical cells may consist of two electrodes of the same type, but with different concentrations of the electroactive species in the electrolyte. Such cells are known as concentration cells. For example, two platinum electrodes operate in two H+/H2 solutions of different activity, separated by a membrane. The equilibrium cell voltage is defined by Equation (21a). As the standard potential is the same for both electrode reactions, the measurable cell voltage will depend only on the activity ratios, Equation (21b). If in this system both electrolytes were in equilibrium with the same EE pressure, the measured E would respond linearly to the pH difference between the two electrolytes, Equation (21c) (i.e. a pH electrode). 

The cell voltage required depends on the product of current and voltage drop in the dilute streams, particularly toward the end of the process when the total ion concentration is very low. Hence, the cell must be designed to minimize this term, i.e., the gap between the membranes must be as small as possible. 

In fact, some care is needed with regard to this type of concentration cell, since the assumption implicit in the derivation of A2.4.126 that the potential in the solution is constant between the two electrodes, caimot be entirely correct. At the phase boundary between the two solutions, which is here a semi-pemieable membrane pemiitting the passage of water molecules but not ions between the two solutions, there will be a potential jump. This so-called liquid-junction potential will increase or decrease the measured EMF of the cell depending on its sign. Potential jumps at liquid-liquid junctions are in general rather small compared to nomial cell voltages, and can be minimized fiirther by suitable experimental modifications to the cell. 

One of the most important characteristics of a cell is its voltage, which is a measure of reaction spontaneity. Cell voltages depend on the nature of the half-reactions occurring at the electrodes (Section 18.2) and on the concentrations of species involved (Section 18.4). From the voltage measured at standard concentrations, it is possible to calculate the standard free energy change and the equilibrium constant (Section 18.3) of the reaction involved. 

The driving force behind the spontaneous reaction in a voltaic cell is measured by the cell voltage, which is an intensive property, independent of the number of electrons passing through the cell. Cell voltage depends on the nature of the redox reaction and the concentrations of the species involved for the moment, we ll concentrate on the first of these factors. 

A voltmeter joined between the two electrodes of a galvanic cell shows a characteristic voltage, which depends on the concentration and nature of participating reactants. For example, in the Cu-Zn cell, if Cu2+ and Zn2+ are at 1 mol dm-3 (1 M) concentrations and the temperature is 298 K, the voltage measured would be 1.10 V. This voltage is characteristic of the reaction as shown below ... 

In the discussion of the Daniell cell, we indicated that this cell produces a voltage of 1.10 V. This voltage is really the difference in potential between the two half-cells. The cell potential (really the half-cell potentials) is dependent upon concentration and temperature, but initially we ll simply look at the half-cell potentials at the standard state of 298 K (25°C) and all components in their standard states (1M concentration of all solutions, 1 atm pressure for any gases and pure solid electrodes). Half-cell potentials appear in tables as the reduction potentials, that is, the potentials associated with the reduction reaction. We define the hydrogen half-reaction (2H+(aq) + 2e - H2(g)) as the standard and has been given a value of exactly 0.00 V. We measure all the other half-reactions relative to it some are positive and some are negative. Find the table of standard reduction potentials in your textbook. 

The value of /jim is determined by the discontinuity in the dependence of cell current on applied cell voltage which occurs when the interfacial concentration approaches zero. The polarisation parameter is convenient in the design and scale-up of electrodialysis equipment. It can be easily measured in small-scale stacks at a given value of bulk concentration and then used to predict limiting current densities in larger stacks at other concentrations. Most stacks use operating values of the polarisation parameter that are 50-70 per cent of the limiting values. 

CONCENTRATION DEPENDENCE OF EQUILIBRIUM CELL VOLTAGE THE GENERAL NERNST EQUATION... 

Equation (5.9) is the general Nemst equation giving the concentration dependence of the equilibrium cell voltage. It will be used in Section 5.4 to derive the equilibrium electrode potential for metal/metal-ion and redox electrodes. 

In order to confirm the possible interaction of ethanol and crocin on NMDA receptors, we also performed whole-cell patch recording with primary cultured hippocampal neurons and measured membrane currents induced by the application of NMDA in a voltage-clamped condition. Application of 100 pM NMDA induced an inward current of 100.2 9.8 pA (n=10) at a holding potential of -60 mV. The NMDA-induced inward current was not affected by 10 pM CNQX (data not shown), but was completely abolished by 30 pM APV, supporting the fact that the response was mediated by NMDA receptors. Ethanol inhibited NMDA-induced currents in a concentration-dependent manner. Crocin (10 pM) had no effect on NMDA-induced currents by itself (data not shown), but attenuated the inhibitory effect of ethanol on NMDA-induced currents. The concentration-effect curve for ethanol was shifted to the right by the presence of crocin [22]. 

The cell potential E (also called the cell voltage or electromotive force) is an electrical measure of the driving force of the cell reaction. Cell potentials depend on temperature, ion concentrations, and gas pressures. The standard cell potential E° is the cell potential when reactants and products are in their standard states. Cell potentials are related to free-energy changes by the equations AG = —nFE and AG° = —mFE°, where F = 96,500 C/mol e is the faraday, the charge on 1 mol of electrons. 

Electrochemical cells are made of two conducting electrodes, called the anode and the cathode. The oxidation reaction takes place at the anode, where electrons are released to flow through a wire to the cathode. At the cathode, reduction takes place. For the oxidation and reduction reactions to occur, the electrodes must be in a conducting solution called an electrolyte. The electrochemical cell voltage depends on the types of materials, usually conducting metals, used as electrodes, and the concentration of the electrolyte solution. (See Figure 6.5.)... 

Unlike genistein (42), quercetin (29) [308,309] and myricetin (31) [310] enhanced the calcium current carried by L-type calcium channels of the myocytes from rat tail artery. The character of voltage-, concentration-, and frequency-dependent calcium current changes induced by myricetin (31) [310] allowed the conclusion that this flavonoid exerted its effect on L-type calcium channels by binding preferentially to the channels in the inactivated state. Quercetin stimulated L-type calcium channels in rat pituitary GH3 cells in a dose-dependent manner [309]. The EC50 value was about 7 xM, and many facts suggested direct interaction between ion channels and flavonoid. In NG 108-15 neuronal cells quercetin (29) caused inhibition of TTX-sensitive sodium current. 

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