Reference electrodes potential measurement

Concentration polarization and overpotential can both occur at the working and auxiliary electrodes. There is an ohmic potential drop between working and auxiliary electrodes. To obtain the best measurement of the working electrode potential, the reference electrode should be placed as close as possible to the working electrode (Figure 17-4). 

Monitoring of the electrochemical potential of steel reinforcement in concrete is a well established technique for assessing the severity of corrosion and for controlling cathodic protection systems. A reference electrode is the electrochemical device used for measuring these potentials. The reference electrode is either placed on the concrete surface during the measurements or permanently embedded in the concrete in close proximity to the steel. The latter technique enables remote long-term monitoring. 

Measurements of electrode potentials using reference electrodes are of two general types those that involve liquid junctions and those that do not. An example of a cell which does not have a liquid junction is ... 

However, most of the time, the corrective term to E° is unfortunately unknown and beyond the error associated with potential measurements on an absolute (NHE) scale, mainly because of junction potential and reference electrode potential drifts. Thus in actual experiments Ej/2 and E° (or E°) are generally considered identical [94]. 

The constant term depends on the environmental conditions such as temperature, pH, concentration of oxygen and the reference electrode offset. But the differential method without the term has advantages on them, in case that the same reference electrodes are used in the short-time measurement. This formulation easily eliminates the effect of open circuit corrosion potential and reference electrode offset. If the potential or current density are constant in two boundary conditions, the differential boundary conditions are zero according to Eqn. (12) or Eqn. (13). 

Electrochemical detectors are constructed with three electrodes. The electrolysis of interest takes place at the working (marked W in Fig. 4.5) electrode at a potential measured by the reference electrode. The auxiliary (or counter) electrode potential is controlled to maintain the reference potential. The reference electrode is usually saturated calomel or silver/silver chloride. Platinum or glassy carbon is generally used for the auxiliary electrode [67, 53]. 

The sum of the surface and concentration overpotentials is called the total overpotential or overvoltage r) of the electrode which can be measured by standard reference electrodes, as already shown in Figure 21.1. All electrode potentials are related to the standard hydrogen electrode (SHE), the potential of which is set by definition at zero at all temperatures. The saturated calomel electrode (SCE), Hg/Hg2Cl2/Cr, is the most widely used reference electrode for potential measurement. Other electrode systems used are Hg/Hg2S04/S04, Hg/HgO/OH , and Ag/AgCI/Cr. 

Figure 10.15 shows an example of a skin surface admittance measuring circuit. It is a quasi-mono-polar system with three electrodes M is the measuring electrode, R a potential recording reference electrode, and CC is a current-carrying control electrode. 

Another consequence of a non-negligible voltage drop across the electrolyte is a deviation of the measured reference electrode potential, real ... 

Choice of reference electrodes is one of the most important points in electrochemical measurements in ILs. The reference electrodes are required to show stable electrode potentials, which are usually determined by an equilibrium between reversible redox couples. The redox reaction between silver and silver cation, Ag/Ag(I), is often used as the redox couple for reference electrode in conventional nonaqueous electrolytes. The reference electrode based on Ag/Ag(l) has been also used in various ILs. However, the potentials of Ag/Ag(l) reference electrodes are different in different ILs since the Gibbs energy for formation of Ag(I) depends on the ions composing the ILs. Therefore, it is necessary to calibrate the potentials of reference electrodes against a conunon standard redox potential. A redox couple of ferrocenium (Fc" ) and ferrocene (Fc) is often used for this purpose although its redox potential is considered slightly dependent on BLs. Platinum or silver electrodes immersed in ILs are sometimes used as quasi-reference electrodes. The potentials of these quasi-reference electrodes may seem to be stable in the ILs without any redox species. However, their potentials are unstable and unreliable since they are not determined by any redox equilibrium. Thus, use of quasireference electrodes should be avoided even when the potentials are calibrated by Fc /Fc couple. 

The sign convention for potentiometry is consistent with the convention described in Chapter 22 for standard electrode potentials. In this convention, the indicator electrode is the right-hand electrode and the reference electrode is on the left. For direct potentiometric measurements, the potential of a cell is then expressed in terms of the indicator eleetrode potential, the reference electrode potential, and the junction potential as shown in Equation 23-1 ... 

Hydrogen electrodes are difficult to use in many pH determinations and either Ag-AgCl or calomel reference electrodes are usually used A typical pH-measurement reference electrode is shown in Figure 5.12c. Thus we establish a cell in which glass-electrode potential is compared with the potential of a nonhydrogen reference electrode which in turn has an electrode potential determined against a hydrogen reference. The cell which is formed by the two electrodes used in a pH measurement is then described by... 

Unformnately it is not possible to measure this electrode potential in practice therefore, the so called reference electrodes are used which feature a stable electrode potential. The reference electrode must be kept thermodynamically in equilibrium, i.e. very little or ideally no current must flow into it, otherwise the electrode potential deviates from what is expected from the equilibrium state. There are different kinds of reference electrodes like the standard hydrogen electrode and the silver-silver chloride (Ag AgCl) electrode. Silver-silver chloride electrode is the most prevalent in the physiological studies and is shown in Fig. 1.4. 

There are two procedures for doing this. The first makes use of a metal probe coated with an emitter such as polonium or Am (around 1 mCi) and placed above the surface. The resulting air ionization makes the gap between the probe and the liquid sufficiently conducting that the potential difference can be measured by means of a high-impedance dc voltmeter that serves as a null indicator in a standard potentiometer circuit. A submerged reference electrode may be a silver-silver chloride electrode. One generally compares the potential of the film-covered surface with that of the film-free one [83, 84]. 

Wlien an electrical coimection is made between two metal surfaces, a contact potential difference arises from the transfer of electrons from the metal of lower work function to the second metal until their Femii levels line up. The difference in contact potential between the two metals is just equal to the difference in their respective work fiinctions. In the absence of an applied emf, there is electric field between two parallel metal plates arranged as a capacitor. If a potential is applied, the field can be eliminated and at this point tire potential equals the contact potential difference of tlie two metal plates. If one plate of known work fiinction is used as a reference electrode, the work function of the second plate can be detennined by measuring tliis applied potential between the plates [ ]. One can detemiine the zero-electric-field condition between the two parallel plates by measuring directly the tendency for charge to flow through the external circuit. This is called the static capacitor method [59]. 

Electrochemical methods may be classified into two broad classes, namely potentiometric metiiods and voltannnetric methods. The fonner involves the measurement of the potential of a working electrode iimnersed in a solution containing a redox species of interest with respect to a reference electrode. These are equilibrium experiments involving no current flow and provide themiodynamic infomiation only. The potential of the working electrode responds in a Nemstian maimer to the activity of the redox species, whilst that of the reference electrode remains constant. In contrast, m voltannnetric methods the system is perturbed... 

Figure C2.8.3. A tliree-electrode electrochemical set-up used for the measurement of polarization curves. A potentiostat is used to control the potential between the working electrode and a standard reference electrode. The current is measured and adjusted between an inert counter-electrode (typically Pt) and the working electrode.

Potentiometric measurements are made using a potentiometer to determine the difference in potential between a working or, indicator, electrode and a counter electrode (see Figure 11.2). Since no significant current flows in potentiometry, the role of the counter electrode is reduced to that of supplying a reference potential thus, the counter electrode is usually called the reference electrode. In this section we introduce the conventions used in describing potentiometric electrochemical cells and the relationship between the measured potential and concentration. 

The concentration of Ca + in a sample of sea water is determined using a Ca ion-selective electrode and a one-point standard addition. A 10.00-mL sample is transferred to a 100-mL volumetric flask and diluted to volume. A 50.00-mL aliquot of sample is placed in a beaker with the Ca ion-selective electrode and a reference electrode, and the potential is measured as -0.05290 V. A 1.00-mL aliquot of a 5.00 X 10 M standard solution of Ca + is added, and a potential of -0.04417 V is measured. What is the concentration of Ca + in the sample of sea water ... 

Electrochemical Detectors Another common group of HPLC detectors are those based on electrochemical measurements such as amperometry, voltammetry, coulometry, and conductivity. Figure 12.29b, for example, shows an amperometric flow cell. Effluent from the column passes over the working electrode, which is held at a potential favorable for oxidizing or reducing the analytes. The potential is held constant relative to a downstream reference electrode, and the current flowing between the working and auxiliary electrodes is measured. Detection limits for amperometric electrochemical detection are 10 pg-1 ng of injected analyte. 

Two methods are used to measure pH electrometric and chemical indicator (1 7). The most common is electrometric and uses the commercial pH meter with a glass electrode. This procedure is based on the measurement of the difference between the pH of an unknown or test solution and that of a standard solution. The instmment measures the emf developed between the glass electrode and a reference electrode of constant potential. The difference in emf when the electrodes are removed from the standard solution and placed in the test solution is converted to a difference in pH. Electrodes based on metal—metal oxides, eg, antimony—antimony oxide (see Antimony AND ANTIMONY ALLOYS Antimony COMPOUNDS), have also found use as pH sensors (8), especially for industrial appHcations where superior mechanical stabiUty is needed (see Sensors). However, because of the presence of the metallic element, these electrodes suffer from interferences by oxidation—reduction systems in the test solution. 

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