Absolute half-cell potential, determination

A problem that has fascinated surface chemists is whether, through suitable measurements, one can determine absolute half-cell potentials. If some one standard half-cell potential can be determined on an absolute basis, then all others are known through the table of standard potentials. Thus, if we know E for... 

The intimate relationship between double layer emersion and parameters fundamental to electrochemical interfaces is shown. The surface dipole layer (xs) of 80SS sat. KC1 electrolyte is measured as the difference in outer potentials of an emersed oxide-coated Au electrode and the electrolyte. The value of +0.050 V compares favorably with previous determinations of g. Emersion of Au is discussed in terms of UHV work function measurements and the relationship between emersed electrodes and absolute half-cell potentials. Results show that either the accepted work function value of Hg in N2 is off by 0.4 eV, or the dipole contribution to the double layer (perhaps the "jellium" surface dipole layer of noble metal electrodes) changes by 0.4 V between solution and UHV. 

Double layer emersion continues to allow new ways of studying the electrochemical interphase. In some cases at least, the outer potential of the emersed electrode is nearly equal to the inner potential of the electrolyte. There is an intimate relation between the work function of emersed electrodes and absolute half-cell potentials. Emersion into UHV offers special insight into the emersion process and into double layer structure, partly because absolute work functions can be determined and are found to track the emersion potential with at most a constant shift. The data clearly call for answers to questions involving the most basic aspects of double layer theory, such as the role water plays in the structure and the change in of the electrode surface as the electrode goes frcm vacuum or air to solution. 

Although it is not difficult to measure relative half-cell potentials, it is impossible to determine absolute half-cell potentials because all voltage-measuring devices measure only differences in potential. To measure the potential of an electrode, one contact of a voltmeter is connected to the electrode in question. The other contact from the meter must then be brought into electrical contact with the solution in the electrode compartment via another conductor. This second contact, however, inevitably involves a solid/solution interface that acts as a second half-cell when the potential is measured. Thus, an absolute half-cell potential is not obtained. What we do obtain is the difference between the halfcell potential of interest and a half-cell made up of the second contact and the solution. 

Reference half-cells The fact that individual half-cell potentials are not directly measurable does not prevent us from defining and working with them. Although we cannot determine the absolute value of a half-cell potential, we can still measure its value in relation to the potentials of other half cells. In particular, if we adopt a reference half-cell whose potential is arbitrarily defined as zero, and measure the potentials of various other electrode systems against this reference cell, we are in effect measuring the half-cell potentials on a scale that is relative to the potential of the reference cell. 

In Feature 18-3, we showed that absolute values for individual half-cell potentials cannot be determined in the laboratory. That is, only relative cell potentials can be measured experimentally. Figure 21-1 shows a typical cell for potentiometric analysis. This cell can be represented as... 

Determining Ehaif-ceii The Standard Hydrogen Electrode What portion of ceii for the zinc-copper reaction is contributed by the anode half-cell (oxidation of Zn) and what portion by the cathode half-cell (reduction of Cu ) That is, how can we know half-cell potentials if we can only measure the potential of the complete cell Half-cell potentials, such as Ezine and °opper. are not absolute quantities, but rather are values relative to that of a standard. This standard reference halfcell has its standard electrode potential defined as zero (E fereiice — 0.00 V). The standard reference half-cell is a standard hydrogen electrode, which consists of a specially prepared platinum electrode immersed in a 1 M aqueous solution of a strong acid, H (fl ) [or H30 (a )], through which H2 gas at 1 atm is bubbled. Thus, the reference half-reaction is... 

We must emphasize that no melhod can determine the absolute value of the potential of a single electrode. because all voltage-measuring devices determine only differences in potential. One conductor from such a device is connected to the electrode under study To measure a potential difference, however, the second conductor must make contact with the electrolyte solution of the half-cell under study. This second contact inevitably creates a solid-solution interface and hence acts as a second half-cell in which a chemical reaction must also lake place if charge is to flow. A potential is associated with this second reaction. Thus, we cannot measure the absolute value for the desired half-cell potential. Instead, we can measure only the difference between the potential of interest and the half-cell potential for the contact between the voltage-measuring device and the solution. 

There have been a number of theoretical approaches to the determination of absolute electrode potentials (relative to the electric potential at a location infinitely distant from all charges). All of them require the use of nonthermodynamic theories. One work cites a value of —4.43 V (absolute) for the standard hydrogen electrode. Other workers have come up with values ranging from this value to —4.73 V. We will use only half-cell potentials relative to the standard hydrogen electrode. 

In galvanic cells it is only possible to determine the potential difference as a voltage between two half-cells, but not the absolute potential of the single electrode. To measure the potential difference it has to be ensured that an electrochemical equilibrium exists at the phase boundaries, e.g., at the electrode/electrolyte interface. At the least it is required that there is no flux of current in the external and internal circuits. 

Since in experiments such as the one we have just discussed, it is only possible to determine potential differences between two electrodes (and not the absolute potential of each half cell), it is now useful to choose a reference system to which all measured potential differences may be related. In accord with the IUPAC 1953 Stockholm convention, the standard hydrogen electrode (SHE) is commonly selected as the reference electrode to which we arbitrarily assign a zero value of electrical potential. This is equivalent to assigning (arbitrarily) a standard free energy change, ArG°, of zero at all temperatures to the half reaction ... 

In Galvanic cells it is only possible to determine the potential difference as a voltage between two half cells, but not the absolute potential of the single electrode. 

The electrochemical potential difference AV between two electrodes in a galvanic cell can be determined by experiments however, it is not possible to measure the absolute potential y of a single electrode ( half-cell ). In many calculation problems, however, it is expedient to be able to assign an absolute electrochemical potential to a single electrode. Conventionally, therefore, an arbitrary zero in the electrochemical series has been established by the following definition... 

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