Electrochemical potential reference solutions

Finding the End Point Potentiometrically Another method for locating the end point of a redox titration is to use an appropriate electrode to monitor the change in electrochemical potential as titrant is added to a solution of analyte. The end point can then be found from a visual inspection of the titration curve. The simplest experimental design (Figure 9.38) consists of a Pt indicator electrode whose potential is governed by the analyte s or titrant s redox half-reaction, and a reference electrode that has a fixed potential. A further discussion of potentiometry is found in Chapter 11. 

Basic properties of semiconductors and phenomena occurring at the semiconductor/electrolyte interface in the dark have already been discussed in Sections 2.4.1 and 4.5.1. The crucial effect after immersing the semiconductor electrode into an electrolyte solution is the equilibration of electrochemical potentials of electrons in both phases. In order to quantify the dark- and photoeffects at the semiconductor/electrolyte interface, a common reference level of electron energies in both phases has to be defined. 

For a metal, the negative of the work function gives the position of the Fermi level with respect to the vacuum outside the metal. Similarly, the negative of the work function of an electrochemical reaction is referred to as the Fermi level Ep (redox) of this reaction, measured with respect to the vacuum in this context Fermi level is used as a synonym for electrochemical potential. If the same reference point is used for the metal s,nd the redox couple, the equilibrium condition for the redox reaction is simply Ep (metal)= Ep(redox). So the notion of a Fermi level for a redox couple is a convenient concept however, this terminology does not imply that there are free electrons in the solution which obey Fermi-Dirac statistics, a misconception sometimes found in the literature. 

Potentiometric measurements are based on the determination of a voltage difference between two electrodes plunged into a sample solution under null current conditions. Each of these electrodes constitutes a half-cell. The external reference electrode (ERE) is the electrochemical reference half-cell, which has a constant potential relative to that of the solution. The other electrode is the ion selective electrode (ISE) which is used for measurement (Fig. 18.1). The ISE is composed of an internal reference electrode (IRE) bathed in a reference solution that is physically separated from the sample by a membrane. The ion selective electrode can be represented in the following way ... 

The first term refers to the electrolyte. Accordingly, the sum runs over all ion types present in the electrolyte. The second term contains the contribution of the electrons in the metal. T and Te are the interfacial excess concentrations of the ions in solution and of the electrons in the metal, respectively, /x is the chemical potential of the particle type i, Fa is Faradays constant, and /x is the electrochemical potential of the electrons. Substitution leads to... 

Any surface (typically a piece of metal) on which an electrochemical reaction takes place will produce an electrochemical potential when in contact with an electrolyte (typically water containing dissolved ions). The unit of the electrochemical potential is volt (TV = 1JC1 s 1 in SI units).The metal, or strictly speaking the metal-electrolyte interface, is called an electrode and the electrochemical reaction taking place is called the electrode reaction. The electrochemical potential of a metal in a solution, or the electrode potential, cannot be determined absolutely. It is referred to as a potential relative to a fixed and known electrode potential set up by a reference electrode in the same electrolyte. In other words, an electrode potential is the potential of an electrode measured against a reference electrode. The standard hydrogen electrode (SHE) is universally adopted as the primary standard reference electrode with which all other electrodes are compared. By definition, the SHE potential is OV, i.e. the zero-point on the electrochemical potential scale. Electrode potentials may be more positive or more negative than the SHE. 

As the name suggests the SCE contains a saturated solution of KC1. Therefore, all SCEs have the same electrochemical potential value at the same temperature. Ag/AgCI/KCI electrodes, on the other hand, exist with a variety of KC1 concentrations, ranging from 0.1 M solution to saturated solution. Consequently, Ag/AgCl/KCl electrode potentials vary according to type, as shown in Table 2.1. Due to the variety of Ag/AgCl/KCl electrodes, care must be taken when dealing with potentials obtained with this kind of reference electrode. 

It is good practice to check carefully the electrochemical potential of the embeddable reference electrode against an accurate reference (SCE or Ag/AgCl), preferably in a laboratory, before the electrode is embedded in concrete. Normally, a saturated Ca(OH)2 solution is used as a test solution. By prolonging the exposure time in the solution, the magnitude of shortterm potential drift can be detected (be aware of temperature dependence). Potential values should always be compared with data provided by the supplier of the reference electrode. It is recommended that the functional and/or calibration check procedures given by the supplier are followed. 

Activity effects. The exchange of trace ions in solution with others in the polymer film might, simplistically, be expected to lead to a linear uptake/solution concentration relationship. Unfortunately, this is seldom the case. The thermodynamic restraint is that of electrochemical potential. Thus electroneutrality is not the sole constraint on the ion exchange process. A second thermodynamic requirement is that the activity of mobile species in the polymer and solution phases be equal. (Temporal satisfaction of these two constraints is discussed below, with reference to Figure 4.) The rather unusual, high concentration environment in the polymer film can lead to significant - and unanticipated - activity effects (8). 

The - electrode potential, with respect to a given reference is determined by the - electrochemical potentials of the redox couple in solution. In case of doped semiconductor electrodes, the introduced impurities determine the semiconductor electrical properties, but have no influence on the equilibrium electrode potential, despite influencing the semiconductor work function. 

Unfortunately, a problem arises when attempting to compare the electrochemical potential of the solntion and the electrochemical potential of the semiconductor. Like most electronic energy levels for molecules, the Fermi level of the semiconductor is usually determined relative to the vacuum level. Experimental measurements to determine fp.sc for semiconductors (generally through determination of the semiconductor work function and dopant density) yield values that can be related to the energy of an electron in vacuum. However, electrochemical potentials of liquid phases can only be measured as potential differences between the test solution and a solution that is nsed as a reference. Since it is not possible to measure directly the energy of an individual redox couple relative to the vacuum level, it is not possible to determine directly the desired relationship between the energy level on the solid side of the junction and that on the liquid side. 

Typically, the reference level for the solution redox potential is chosen to be the normal hydrogen electrode (NHE). Some tabnlations nse the saturated calomel electrode (SCE) as the reference level with the difference between these two scales well-known to be NHE = —0.2412 V versus SCE. The fundamental problem lies in the determination of the absolnte energy of the NHE relative to vacuum. Although a method to determine directly the absolute electrochemical potential of an NHE has not yet been described, a recent indirect measnrement has indicated that it is approximately 4.4 eV below the vacnum level. This value is often used to relate the solution electrochemical potential scale to the solid electrochemical potential scale. It provides the best approximation that is presently available to calculate the... 

The reaction free energy is hence directly related to the electrochemical-potential difference measured between the two metal electrodes with a voltmeter (see Eqs. 32-34). Furthermore, the electrochemical potential of a redox system in solution is probed by measurement of the potential of a metal phase, in equilibrium with the redox system, with respect to a reference electrode. 

Potentiometry is the measurement of an electrical potential difference between two electrodes (half-ceUs) in an electrochemical cell (Figure 4-1) when the cell current is zero (galvanic cell). Such a cell consists of two electrodes (electron or metallic conductors) that are connected by an electrolyte solution (ion conductor). An electrode, or half-cell, consists of a single metallic conductor that is in contact with an electrolyte solution. The ion conductors can be composed of one or more phases that are either in direct contact with each other or separated by membranes permeable only to specific cations or anions (see Figure 4-1). One of the electrolyte solutions is the unknown or test solution this solution may be replaced by an appropriate reference solution for calibration purposes. By convention, the cell notation is shown so that the left electrode (Mi,) is the reference electrode the right electrode (Mr) is the indicator (measuring) electrode (see later equation 3). ... 

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