Cell potential conditions

Potentiometers Measuring the potential of an electrochemical cell under conditions of zero current is accomplished using a potentiometer. A schematic diagram of a manual potentiometer is shown in Figure 11.2. The current in the upper half of the circuit is... 

The photogenerated current is in the same direction as /, but is always less than because the battier potential under load conditions is always less than F, which results in a larger flow of majority carriers than that in a short-circuited cell. Thus, when a solar cell is under load, the current and voltage are always less than and lU, respectively this condition is the curve-factor loss. Depending on the characteristics of the particularp—n junction and on the cell operating conditions, there is an optimal load resistance that maximizes the power output of the cell, ie, the product of its current and voltage. 

The potential of the reaction is given as = (cathodic — anodic reaction) = 0.337 — (—0.440) = +0.777 V. The positive value of the standard cell potential indicates that the reaction is spontaneous as written (see Electrochemical processing). In other words, at thermodynamic equihbrium the concentration of copper ion in the solution is very small. The standard cell potentials are, of course, only guides to be used in practice, as rarely are conditions sufftciendy controlled to be called standard. Other factors may alter the driving force of the reaction, eg, cementation using aluminum metal is usually quite anomalous. Aluminum tends to form a relatively inert oxide coating that can reduce actual cell potential. 

The half-cell potential and the half-cell free energy change are related by the following relationship for reversible conditions ... 

The zinc-copper galvanic cell is under standard conditions when the concentration of each ion is 1.00 M, as shown in Figure 19-13. The cell potential under these conditions can be determined by connecting the electrodes to a voltmeter. The measured potential is 1.10 V, with the Zn electrode at the higher (more negative) potential, so Zn gives up electrons and E eii = 1.10 V ... 

Tabulated standard reduction potentials allow us to determine the potential of any cell under standard conditions. This net standard cell potential is obtained by subtracting the more negative standard reduction potential from the more positive standard reduction potential, giving a positive overall potential. 

Potentiometric methods are based on the measurement of the potential of an electrochemical cell consisting of two electrodes immersed in a solution. Since the cell potential is measured under the condition of zero cmrent, usually with a pH/mV meter, potentiometry is an equilibrium method. One electrode, the indicator electrode, is chosen to respond to a particular species in solution whose activity or concentration is to be measured. The other electrode is a reference electrode whose half-cell potential is invariant. 

The half-cell potentials of the two reference electrodes are constant sample solution conditions can often be controlled so that E,j is effectively constant and the composition of the internal solution can be maintained so that (ai)i , ai is fixed. Consequently Eq. (3) can be simplified to give... 

Over a long time period it may well not be possible to duplicate library cell culture conditions. What happens when the lot of media used in the final culture step prior to pyrolysis has been consumed Can culture media suppliers assure nutritional identity between batches Media types for growth of fastidious strains invariably include natural products such as brewer s yeast, tryptic soy, serum, egg, chocolate, and/or sheep blood. Trace components in natural products cannot be controlled to assure an infinite, invariable supply. The microtiter plate wells used here do not hold much media. Even so, the day will come when all media supplies are consumed and a change in batch is unavoidable. When that happens, if there were no effective way to compensate spectra for the resulting distortions, it would be necessary to re-culture and re-analyze replicates for every strain in the reference library. Until recently the potential for obsolescence was a major disincentive for developing PyMS spectral libraries of bacteria. Why this is no longer an insurmountable problem is discussed in the next section. 

Potentiometry is the most widely used electroanalytical technique. It involves the measurement of the potential of a galvanic cell, usually under conditions of zero current, for which purpose potentiometers are used. Measurements may be direct whereby the response of samples and standards are compared, or the change in cell potential during a titration can be monitored. 

The electrochemical experiments were conducted in an apparatus consisting of an electrochemical cell attached directly to a UHV system and has been described in detail elsewhere (16). The transfer between UHV and the EC was accomplished via a stainless steel air lock vented with ultra-pure Ar. Differentially pumped sliding teflon seals provided the isolation between UHV and atmospheric pressure. The sample was mounted on a polished stainless steel rod around which the teflon seals were compressed. All valves in the air lock were stainless steel gate valves with viton seals. Details of the electrochemical cell and conditions are contained in reference 16. Electrochemical potentials are referred to a saturated calomel electrode (SCE). 

In some cases, the cathodic reduction of dichlorosilane gives the corresponding disilene. For example, the electrolysis of dimesityldichlorosilane in a divided cell equipped with a mercury pool cathode and silver anode under controlled potential conditions (-3.2V vs Ag/Ag+) affords tetramesityldisilene in 20% yield (Scheme 42) [90]. 

Potentiometry deals with the electromotive force (EMF) generated in a galvanic cell where a spontaneous chemical reaction is taking place. In practice, potentiometry employs the EMF response of a galvanostatic cell that is based on the measurement of an electrochemical cell potential under zero-current conditions to determine the concentration of analytes in measuring samples. Because an electrode potential generated on the metal electrode surface,... 

We can use this table of standard reduction potentials to write the overall cell reaction and to calculate the standard cell potential, the potential (voltage) associated with the cell at standard conditions. There are a couple of things to remember when using these standard reduction potentials to generate the cell reaction and cell potential ... 

Thus far, we have based all of our calculations on the standard cell potential or standard half-cell potentials—that is, standard state conditions. However, many times the cell is not at standard conditions—commonly the concentrations are not 1 M. We may calculate the actual cell potential, Ecell, by using the Nemst equation ... 

Both half- and overall reaction tendencies change with temperature, pressure (if gases are involved), and concentrations of the ions involved. Thus far, we have only been concerned with standard conditions. Standard conditions, as stated previously, are 25°C, 1 atm pressure, and 1 M ion concentrations. An equation has been derived to calculate the cell potential when conditions other than standard conditions are present. This equation is called the Nernst equation and is used to calculate the true E (cell potential)... 

Standard reduction potentials are used to calculate the cell potential under standard conditions. All half-reactions are shown in the reduction form. 

Nernst equation The Nernst equation allows the calculation of the cell potential of a galvanic cell that is not at standard conditions, net ionic equation The net ionic equation is written by dropping out the spectator ions and showing only those chemical species that are involved in the chemical reaction. 

An electrical potential difference between the electrodes of an electrochemical cell (called the cell potential) causes a flow of electrons in the circuit that connects those electrodes and therefore produces electrical work. If the cell operates under reversible conditions and at constant composition, the work produced reaches a maximum value and, at constant temperature and pressure, can be identified with the Gibbs energy change of the net chemical process that occurs at the electrodes [180,316]. This is only achieved when the cell potential is balanced by the potential of an external source, so that the net current is zero. The value of this potential is known as the zero-current cell potential or the electromotive force (emf) of the cell, and it is represented by E. The relationship between E and the reaction Gibbs energy is given by... 

The numerical values of cell potentials and half-cell potentials depend on various conditions, so tables of standard reduction potentials are true when ions and molecules are in their standard states. These standard states are the same as for tables of standard enthalpy changes. Aqueous molecules and ions have a standard concentration of 1 mol/L. Gases have a standard pressure of 101.3 kPa or 1 atm. The standard temperature... 

You know that galvanic cells have positive standard cell potentials, and that these cells use spontaneous chemical reactions to produce electricity. You also know that electrolytic cells have negative standard cell potentials, and that these cells use electricity to perform non-spontaneous chemical reactions. Thus, you can use the sign of the standard cell potential to predict whether a reaction is spontaneous or not under standard conditions. 

The standard cell potential is positive, so the reaction is spontaneous under standard conditions. 

Refer to the table of half-cell potentials to determine if MnOz can oxidize Br to Br2 in acidic solution under standard conditions. 

Figure 2-1 shows that the reversible cell potential for a fuel cell consuming H2 and O2 decreases by 0.27 mV/°C under standard conditions where the reaction product is water vapor. However, as is the case in PAFC s, an increase in temperature improves cell performance because activation polarization, mass transfer polarization, and ohmic losses are reduced. 

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