Reference electrodes choice

On the other hand, an electrode pair has the advantages of flexibility in reference electrode choice and reduced replacement cost. Different junction types, including a remote salt bridge, may be used with the same glass electrode in order to determine which reference provides the best response. Normally, the selection of junction types of a combination electrode is limited. There is also more flexibility of internal elements since most combinations are made with silver-silver chloride internals. An application which must exclude silver ion contamination may require an electrode pair. If one of the electrodes of a pair is broken, its replacement cost is less than replacing a combination electrode. 

Choice of an appropriate reference electrode remains an enigma for non-corrosion-aware personnel-although commercially available polarisation-resistance probes can be adapted. An interesting aspect concerns localised corrosion in that, for some materials, localised corrosion only occurs within characteristic potential ranges. 

Accordingly, the standard potential of the ISE depends on the specific choice of the internal reference electrode (e.g. Ag-AgCl) and also that of the fixed A" concentration of the filling solution, so that simply... 

For the drain current IA of the ISFET, again eqn. 2.101 is valid there is a freedom of choice as to the drain-bulk voltage Vjb> but once its value has been chosen one has to calibrate with buffers. In fact, a reference electrode is not essential, but it contributes to more stable results. 

The SHE is experimentally inconvenient, so potentials are often measured and quoted with respect to reference electrodes other than the SHE. By far the most common reference is the saturated calomel electrode (SCE). We will usually make our choice of reference on the basis of experimental convenience. 

Junction potentials are absent or minimal, depending on the choice of reference electrode... 

One of the most common choices of reference electrode is the saturated calomel electrode (SCE) ... 

We have briefly encountered the solid-state fluoride electrode, which has a fully nemstian response down to c. 10 mol dm . The fluoride electrode is iiiunersed in a test solution of fluoride ion (usually aqueous), and the emf is then determined. At its heart is a single crystal of lanthanum fluoride doped with erbium fluoride, (see Figure 3.10). Like the pH electrode, a full fluoride electrode also contains a small reference electrode, meaning that a fluoride electrode is in reality a cell. The fluoride electrode does not suffer from interference from CP, so an AgCl Ag reference is the normal choice owing to its convenience and compact size. 

In some nonaqueous solvents, it is necessary to use a standard reaction other than the oxidation of molecular hydrogen. At present, there is no general choice of a standard reaction (reference electrode). Although in some cases... 

Fig. 30 Cyclic voltammograms (scan rate 2 mV s ) observed in 0.2 M Na2S04 + H2SO4 (pH 3) the working electrode was glassy carbon and the reference electrode was SCE. (a) M a2-P2Wi 5 M02CU. The choice of the reversal potentials help to distinguish the various steps, (b) dioxygen alone and 10 M q 2-P2Wi5Mo2Cu in the presence of different concentrations of dioxygen. For further details, see text (taken from Ref 161).

The value of the constant V, and hence the values of standard potentials, depend on the choice of the reference electrode and on the character of electrode reaction, which takes place on it With the reference electrode potential conventionally taken as zero, we can choose, for example, the normal hydrogen electrode (NHE), i.e., an electrode, for which the equilibrium at the interface is attained due to the reversible redox reaction H+ + e = H2, provided the activity of H+ ions in the solution is 1 mol/liter and the pressure of gaseous hydrogen above the solution is 1 atm. Many of the measured potentials are given below relative to the saturated calomel electrode (SCE) its potential relative to the NHE is 0.242 V. 

The electrochemical window of pure molten cryolite has not been expressly stated, but a voltammogram of purified cryolite recorded at a graphite working electrode exhibits very little residual current over the range of potentials extending from 0.4 to -1.9 V vs. a nickel wire quasi-reference electrode [7]. Physical property data for molten cryolite and phase equilibria for the AlF3-NaF melt system have been summarized [31,32]. The extremely high temperature of cryolite places severe constraints on the materials that can be used for cells. Platinum and boron nitride are the materials of choice. 

With this choice of standard reference electrode, the entire potential of the cell... 

It contains parameters related to the solid-state (i.e., Ex) as well as to the chemical part(i.e., namL, jcq) of the sensor. Because the reference electrode is physically separate from the ISFET, its potential is not included in the threshold voltage. However, that choice is rather arbitrary. Whether the actual output follows (6.65) or (6.66) depends on the externally applied gate voltage Eg, which has nothing to do with the... 

Because the electrical circuit is closed inside the sensor, no external reference electrode is necessary and the Severinghaus-type electrode can be used for measurement in either gaseous or liquid samples. It is important to remember, however, that the potential of the internal reference electrode must remain constant. In principle, it would be possible to use a liquid junction but it would add to the complexity of the design. Because the counterion resulting from the dissociation equilibrium is the only interfering ion, and because it is present in a very low concentration, it is possible to ascertain the constancy of the reference potential by careful choice of the internal electrolyte. Thus, for example, in the CO2 electrode the internal electrolyte is O.lMNaHCOs and 0.1 M NaCl and Ag/AgCl is used as an internal reference element. 

Most factors in parameter G (Chapter 4, Equations 4.55 and 4.56) and the potential of the reference electrode are temperature-dependent this is possibly also the case for factor x. Equation 4.56 also represents the concentration of the hydroxide ion. This means that a potential sensor based on the prewave will also have to contain a pH sensor. The hydroxide ion concentration derived from the output signal of this additional sensor needs to be introduced in the algorithm for the calculation of the hydrogen peroxide concentration. As an additional sensor, a glass electrode is an obvious choice, with a temperature-dependent potential, which is the case also for the potential of the reference electrode associated with the glass electrode and for the pH of the buffers. In this research work, the influence of... 

Choice of electrolyte for salt bridges and reference electrodes. Many of the difficulties encountered in potentiometric measurements can be attributed to erratic or drifting junction potentials caused by clogged junctions. Certain elementary rules should be observed in choosing the filling solution for a salt bridge or reference electrode, particularly when they will be used in organic solvents or solutions that are only partially aqueous. 

Values of (E° + Ej) for silver chloride and calomel reference electrodes are listed in Table 5.3. For reasons described previously, the unsaturated 3.5 M KC1 electrode appears to be the best choice for routine work. 

Until recently, the most popular reference half-cell for potentiometric titrations, polarography, and even kinetic studies has been the saturated aqueous calomel electrode (SCE), connected by means of a nonaqueous salt bridge (e.g., Et4NC104) to the electrolyte under study. The choice of this particular bridge electrolyte in conjunction with the SCE is not a good one because potassium perchlorate and potassium chloride have a limited solubility in many aprotic solvents. The junction is readily clogged, which leads to erratic junction potentials. For these practical reasons, a calomel or silver-silver chloride reference electrode with an aqueous lithium chloride or quaternary ammonium chloride fill solution is preferable if an aqueous electrode is used. 

Hg/HgO is often the electrode of choice in alkaline aqueous medium, silver/silver acetate in many nonaqueous medium such as acetic acid, etc. When the experiment requires large currents to flow between the working and the counterelectrodes, a particular attention must be paid to place the reference electrode at an equipotential line close to the working electrode or to make an appropriate ohmic drop correction. 

The choice of a suitable reference electrode for use in ionic liquids is a complex issue [28], which will be discussed in more detail elsewhere. There is also a range... 

The choice of the appropriate reference electrode and its separation from the rest of the cell may be critical. 

Mercury and solid s-p metal electrodes show stable electrochemical behavior in nonaqueous media, in particular in dipolar aprotic solvents. This knowledge was important for the advancement of electrochemical methodology, e.g., the special branch of polarography in nonaqueous systems has emerged. When performing electrochemical experiments in nonaqueous media, special attention should be paid to the reasonable choice of reversible reference electrodes. 

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