Potential of a cell

The net electrochemical driving force is determined by two factors, the electrical potential difference across the cell membrane and the concentration gradient of the permeant ion across the membrane. Changing either one can change the net driving force. The membrane potential of a cell is defined as the inside potential minus the outside, i.e. the potential difference across the cell membrane. It results from the separation of charge across the cell membrane. 

The EMF of a galvanic cell is a thermodynamic equilibrium quatity. Thus, the potential of a cell must be measured under equilibrium conditions, i.e. without current flow. The measured EMF must be compensated by a known external potential difference. The measurement of the EMF of a cell is thus based on determination of a potential difference that exactly compensates the measured potential difference so that no current passes. This is easily achieved by the Poggendorf compensation method (see Fig. 3.13). 

Nernst equation an equation to calculate the actual potential of a cell in which the concentrations or pressures differ from 1.00 M or 1.00 atm. 

Calculate the potential of a cell using the following half-cells ... 

Beyond the Hayflick limit, cells that have escaped senescence and/or crisis must progress beyond mortality stage 2 (Figure 26.1). Recent studies have shown that human telomerase reverse transcriptase cDNA (hTERT) in combination with viral oncogenes also has the potential to immortalize human airway epithelial cells [15, 78-82], However, even though hTERT alone will cause enhanced growth potential of a cell line, with the exception of one study [81],... 

This procedure of using a finite difference between two quantities, both of which become infinite, is of general usefulness. We also will use it in Chapter 19 to obtain the standard potential of a cell. 

First of all, the important role of platinum as the metal part of the standard hydrogen electrode (SHE), which is the primary standard in electrochemistry should be mentioned. The standard potential of an electrode reaction (standard electrode potential) is defined as the value of the standard potential of a cell reaction when that involves the oxidation of molecular hydrogen to solvated (hydrated) protons (hydrogen ions) ... 

One more example demonstrates how to use standard reduction potentials to determine the standard potential of a cell. Let s say you wanted to construct a cell using silver and zinc. This cell resembles the Daniell cell of the previous example except that a silver electrode is substituted for the copper electrode and a silver nitrate solution is used in place of copper sulfate. From Table 14.2, it is determined that when silver and copper interact silver is reduced and copper oxidized. The two relevant reactions are... 

The membrane potential of a cell in the resting state is predominantly determined by two processes ... 

Calculate the standard cell potential of a cell constructed with nickel (Ni) and copper (Cu) electrodes. 

Thus, suppose that when one wishes to measure an interfacial potential difference PDm /s, one connects the electrode with another electrode and measures the potential of this cell. Suppose one always keeps this second electrode constant in nature (i.e, the algebraic sum of the potentials associated with it is kept constant). Then, measurements of the potential of a cell in which the one (same) electrode and its associated solution were always present and the other [i.e., the first electrode mentioned here (Mt)] and its solution were changed would clearly reflect the changing interfacial potential difference FDM /s. This is in fact what is done to measure the relative values of PDM /sas Mjor S (or both) are varied. 

The membrane potential of a cell at a given time is the result of the types and numbers of ion channels open at that instant. In most cells at rest, more K+ channels... 

The standard potential of an electrode is the standard potential of a cell in which the other electrode is a hydrogen electrode. The latter is assigned zero potential. If the test electrode is found to be the anode, then it is assigned a negative potential if it is the cathode, then its standard potential is positive. 

What is the standard potential of a cell that uses the Zn2+/Zn and Ag+/Ag couples When the cell is allowed to produce an electric current, which metal is deposited ... 

Calculate the potential of a cell consisting of two Ag/AgCl electrodes, one in contact with 0.100 M KC1, the other in contact with 1.0 x 1 O 5 M NaCl. 

In (19-6), n has been made dimensionless. For a half-reaction, n is the number of electrons in the half-equation for the whole-cell reaction, n is the number of electrons in one of the multiplied half-equations before canceling the electrons. The Nernst equation is closely related to the laws of chemical equilibrium. Le Chatelier s principle applies to the potential of a cell in the same sense as it applies to the yield of an equilibrium process. Since Q is a fraction that has product concentrations in the numerator (top) and reactant concentrations in the denominator (bottom), an increased concentration of the product reduces the potential and an increased concentration of reactant raises the potential. 

What is the potential of a cell containing two hydrogen electrodes, the negative one in contact with 10-8 M H+ and the positive one in contact with 0.025 M H+ ... 

Fig. 16. Top The illustration shows the variation of the induced transmembrane potential of a cell with frequency and with the conductivity of the external medium. The induced potential increases with external conductivity. However, it also decreases with frequency and above about 5 or 10 MHz the induced potential has fallen below 10 mV in most practical cases. Cells can be cultivated for days in fields of high MHz frequencies.

The half-cell potentials listed in Table lenable us to attach numbers to our predictions. As was implied in the preceding problem example, if the potential of a cell made up of the two half-cell reactions is positive, then the reaction will proceed spontaneously to the right if it is negative, the reverse reaction will be spontaneous. 

Note that E° for a concentration cell is always zero, since this would be the potential of a cell in which the electroactive species are at unit activity in both compartments. 

Table 2.1 lists half-reactions for electrodes of the second type and their potential for unit activities. These electrodes have, in the majority of cases, their own electrolyte associated with them. So, to calculate the potential of a cell in relation to the standard hydrogen electrode it is necessary to take the liquid junction potential between the two electrolytes into account (Section 2.10). 

The standard potential of an -> electrode reaction (standard electrode potential) is defined as the value of the standard potential of a cell reaction (E ) when that... 

Nernst equation (17.3) An equation relating the potential of a cell or half-cell to the concentrations or pressures of its reagents. 

This chapter is concerned primarily with the computation of potentials of a cell using the hydrogen electrode as a probe for studying ionic equilibrium processes in mixed-organic-aqueous solvent systems. Computation of a number of other thermodynamic functions of the ionic process under investigation or of the solvent used is rather straightforward once the standard potential of the measuring cell has been calculated. 

Using this relationship we can calculate the potential of a cell in which some or all of the components are not in their standard states. 

The potential of a cell depends.on the concentrations or partial pressures of the reacting substances. The standard concentrations of the dissolved substances in Table 32-2 are taken to be approximately 1 M (more accurately, unit activity, correction being made for deviation from the perfect-solution law), and the standard pressure for gases is 1 atm (corrected in very accurate work for deviation from the perfect-gas law). 

One useful aspect of the ISE approach is the ease with which selectivity testing can be performed. Once the polymer is employed as the active ingredient in a polymer membrane electrode, the binding can be examined by measuring the potential of a cell as outlined below. We have seen that the selectivity obtained by batch extraction procedures gives the same affinity series as that measured by using the polymer in an electrode [11]. 

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