Calomel electrode, potential measurement

The precision and accuracy of the measurement also depend strongly on the reference electrode, which affects the results through fluctuations in its own potential and through the liquid-junction potential at the test solution-liquid bridge interface. This subject is extensively treated in [158]. Common electrodes of the second kind have sufficiently stable potentials at a constant temperature, but difficulties can be encountered due to temperature hysteresis. Silver chloride electrodes are preferable to calomel electrodes, because their temperature hysteresis is substantially smaller with a calomel electrode, potential stabilization after a change in the temperature may even take several hours. Negligible temperature hysteresis is exhibited by the thallamide reference electrode [26,... 

Nevertheless, CO2 reduction does not take place easily, and the actual electrolysis potentials for CO2 reduction are much more negative in most cases than the equilibrium ones. The reason is that the intermediate species CO2, formed by an electron transfer to a CO2 molecule, proceeds as the first step at highly negative potential, such as -2.21 V vs. saturated calomel electrode (SCE) measured in dimethyl fonnamide (DMF), as discussed later in detail. 

At the heart of electrochemistry is the electrochemical cell. We will consider the creation of an electrochemical cell from the joining of two half-cells. When an electrical conductor such as a metal strip is immersed in a suitable ionic solution, such as a solution of its own ions, a potential difference (voltage) is created between the conductor and the solution. This system constitutes a half-cell or electrode (Fig. 15.1). The metal strip in the solution is called an electrode and the ionic solution is called an electrolyte. We use the term electrode to mean both the solid electrical conductor in a half-cell (e.g., the metal strip) and the complete half-cell in many cases, for example, the standard hydrogen electrode, the calomel electrode. Each half-cell has its own characteristic potential difference or electrode potential. The electrode potential measures the ability of the half-cell to do work, or the driving force for the half-cell reaction. The reaction between the metal strip and the ionic solution can be represented as... 

The normal hydrogen electrode (NHE) is the primary reference electrode and is used to define the accepted scale of standard potentials in aqueous media. It is also one of the most reproducible electrodes that are available. The hydrogen electrode has been successfully employed in dipolar aprotic solvents however, it is not frequently used. The aqueous saturated calomel electrode (SCE), connected to the electrolyte under study by a non-aqueous salt bridge, has become the reference electrode of choice for most investigators. Whether it is the SCE that is used, or any other suitable reference electrode for a given solvent, junction potentials will exist between the reference electrode and the electrolyte under study. These junction potentials will affect electrode potential measurements and will vary from one solvent/electrolyte system to another. In addition, the instability of the SCE in non-aqueous solvents has been noted. ... 

Fig. 5.3.3 Variation of calomel electrode potential with time (open symbols) and of mercuric species concentration measured spectrophotometrically (filled symbols) [126]. The upper series (open symbols) and concentration changes refer to a thick-skin type electrode, and the lower series to Hills and Ives type electrode (see details in [126])...

Measured against a sodium chloride saturated calomel electrode. Potential difference between the cathodic and anodic peaks determined at 100 mV/s. 

Example 13 The following data were recorded for the potential E of an electrode, measured against the saturated calomel electrode, as a function of concentration C (moles liter ). 

The following data were collected for the analysis of fluoride in tap water and in toothpaste, (a) For the analysis of tap water, three 25.0-mL samples were each mixed with 25.0 mL of TISAB, and the potential was measured with an F ISE relative to a saturated calomel electrode. Five 1.00-mL additions of a standard solution of 100.0-ppm F were added to each, measuring the potential following each addition. 

Reference Electrodes and Liquid Junctions. The electrical cincuit of the pH ceU is completed through a salt bridge that usually consists of a concentrated solution of potassium chloride [7447-40-7]. The solution makes contact at one end with the test solution and at the other with a reference electrode of constant potential. The Hquid junction is formed at the area of contact between the salt bridge and the test solution. The mercury—mercurous chloride electrode, the calomel electrode, provides a highly reproducible potential in the potassium chloride bridge solution and is the most widely used reference electrode. However, mercurous chloride is converted readily into mercuric ion and mercury when in contact with concentrated potassium chloride solutions above 80°C. This disproportionation reaction causes an unstable potential with calomel electrodes. Therefore, the silver—silver chloride electrode and the thallium amalgam—thallous chloride electrode often are preferred for measurements above 80°C. However, because silver chloride is relatively soluble in concentrated solutions of potassium chloride, the solution in the electrode chamber must be saturated with silver chloride. 

The polarographic half-wave reduction potential of 4-nitroisothiazole is -0.45 V (pH 7, vs. saturated calomel electrode). This potential is related to the electron affinity of the molecule and it provides a measure of the energy of the LUMO. Pulse radiolysis and ESR studies have been carried out on the radical anions arising from one-electron reduction of 4-nitroisothiazole and other nitro-heterocycles (76MI41704). 

Finally, calomel electrodes (and more especially hydrogen electrodes) are not suitable for field measurements because they are not sufficiently robust. The calomel electrodes are however essential for calibrating the field reference electrodes. Saturated KCI calomel electrodes are the most suitable because there is then no doubt about the reference potential of the calibrating electrode. Lack of adequate calibration is a common cause of cathodic protection system mismanagement. 

The mechanism of the action of metallic copper was investigated by Streicher who determined the potential of a Type 314 stainless steel, the redox potential of the solution (as indicated by a platinised-Pt electrode) and the potential of the copper. The actual measurements were made with a saturated calomel electrode, but the results reported below are with reference to S.H.E. In the absence of copper the corrosion potential of the stainless steel was 0-58 V, whereas the potential of the Pt electrode was... 

Reference Electrode an equilibrium (reversible) electrochemical half-cell of reproducible potential against which an unknown electrode potential can be measured. Examples of those commonly used in corrosion are the Pt, H /H (the hydrogen electrode), Hg/Hg Clj/Cl" (the calomel electrode), Cu/CuS04/Cu, Ag/AgCl/Cl", all with fixed activities of the dissolved ions. 

If it is desired to use the biamperometric method for detecting the end point, then the calomel electrode and also the silver rod (if used) must be removed and replaced by two platinum plates 1.25 cm x 1.25 cm. The potentiometer (or pH meter) used to measure the e.m.f. must also be removed, and one of the indicator electrodes is then joined to a sensitive galvanometer fitted with a variable shunt. The indicator circuit is completed through a potential divider... 

The half-wave potentials of these steps are approximately — 0.1 and — 0.9 V (versus the saturated calomel electrode). Hie exact stoichiometry of these steps is dependent on the medium. Hie large background current accruing from this stepwise oxygen reduction interferes with the measurement of many reducible analytes. In addition, the products of the oxygen reduction may affect the electrochemical process under investigation. 

The Hg/dimethyl sulfoxide (DMSO) interface has been studied by electrocapillary and capacitance measurements in a range of temperatures.291,304 Eamo was measured using the streaming electrode method. All potentials were recorded in a nonisothermal cell against a 0.1 M NaCl calomel electrode (CE) in water at 25°C. The potential difference of the cell CE/0.1 M NaC104 (aq.)/0.1 M NaC104 (DMSO)/CE was -0.096 V. This value was used to recalculate the data.312... 

Figure 17. PMC behavior in the accumulation region, (a) PMC potential curve and photocurrent-potential curve (dashed line) for silicon (dotted with Pt particles) in contact with propylene carbonate electrolyte containing ferrocene.21 (b) PMC potential curve and photocurrent-potential curve (dashed line) for a sputtered ZnO layer [resistivity 1,5 x 103 ft cm, on conducting glass (ITO)] in contact with an alkaline electrolyte (NaOH, pH = 12), measured against a saturated calomel electrode.22...

A glass electrode, a thin-walled glass bulb containing an electrolyte, is much easier to use than a hydrogen electrode and has a potential that varies linearly with the pH of the solution outside the glass bulb (Fig. 12.11). Often there is a calomel electrode built into the probe that makes contact with the test solution through a miniature salt bridge. A pH meter therefore usually has only one probe, which forms a complete electrochemical cell once it is dipped into a solution. The meter is calibrated with a buffer of known pH, and the measured cell emf is then automatically converted into the pH of the solution, which is displayed. 

The standard Volta potential of the calomel electroded (Hg.Cl ) was determined for the first time by Klein and Lange, but the accepted and usually used value of this potential (equal to 0.207 V) was found by Randles.48 Later measurements by Parrel and McTigue et al.21 and Antropov et al. gave 0.212 V and 0.220 V, respectively. In the methanol a value equal to 0.25 has been found. 

It is also important to understand how the potential gradient between an electrode and the bulk solution is established and controlled. Because the potential difference between the electrode and the bulk solution is not measurable, a second electrode must be employed. Although in general the potential difference between an electrode and solution cannot be determined, the potential difference between two electrodes in that solution can be determined. If the solution electrode potential difference of one of the electrodes is held constant by maintaining a rapid redox couple such as silver-silver chloride or mercury-mercurous chloride (calomel), then the potential... 

As in normal potentiometry one uses and indicator electrode versus a reference electrode, the electrodes should, especially in pH measurements, be those recommended by the supplier of the pH meter in order to obtain a direct reading of the pH value displayed. In redox or other potential measurements any suitable reference electrode of known potential can be applied. However, a reference electrode is only suitable if a junction potential is excluded, e.g., an Ag-AgCl electrode in a solution of fixed Ag+ concentration or a calomel electrode in a saturated KC1 solution as a junction in many instances a direct contact of Cl" with the solution under test (possibly causing precipitation therein) is not allowed, so that an extra or so-called double junction with KN03 solution is required. Sometimes micro-electrodes or other adaptations of the surface are required. 

In potentiometric measurements the simplest approach to the liquid-junction problem is to use a reference electrode containing a saturated solution of potassium chloride, for example the saturated calomel electrode (p. 177). The effect of the diffusion potential is completely suppressed if the solutions in contact contain the same indifferent electrolyte in a sufficient... 

In addition to their use as reference electrodes in routine potentiometric measurements, electrodes of the second kind with a saturated KC1 (or, in some cases, with sodium chloride or, preferentially, formate) solution as electrolyte have important applications as potential probes. If an electric current passes through the electrolyte solution or the two electrolyte solutions are separated by an electrochemical membrane (see Section 6.1), then it becomes important to determine the electrical potential difference between two points in the solution (e.g. between the solution on both sides of the membrane). Two silver chloride or saturated calomel electrodes are placed in the test system so that the tips of the liquid bridges lie at the required points in the system. The value of the electrical potential difference between the two points is equal to that between the two probes. Similar potential probes on a microscale are used in electrophysiology (the tips of the salt bridges are usually several micrometres in size). They are termed micropipettes (Fig. 3.8D.)... 

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