Standard potentials calomel

Expression (8) can be used to calculate the real energy of ions in any solvent, provided the standard potential of the calomel electrode, and the standard potentials of the elements and the A ° P° (Hg, Cl") under study... 

FIG. 3 Temperature dependence of the standard potential of the saturated calomel electrode (1) and Ag/AgCl electrode (2). 

In these relationships, og and qc are, respectively, the standard potentials of the glass and calomel electrodes, is the Faraday constant, and Aci is the activity of solvated Cl". 

The standard potential of the 02/02 pair is equal to -0.15 V in water and -0.60 V in DMF. Usually, dioxygen easily captures two electrons in the stepwise reaction O2 + e —> O2 , then O2 + e 02 . In DMSO, dioxygen reductions into the superoxide ion and then into the dioxygen dianion are characterized by Ey2 = -0.5 V and Ey = -1.5 V in regard to the saturated calomel electrode (Sawyer and Gibian 1979). The superoxide ion occupies an intermediate position in the following redox triad O2 —> 02 —> In accordance with such a position, the superoxide ion... 

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 standard potential (E°) for this reaction is +0.268 V. If the cell is saturated with KCI at 25°C, the potential is +0.241 V. A calomel electrode saturated with KCI is called a saturated calomel electrode, abbreviated S.C.E. The advantage in using saturated KCI is that [Cl ] does not change if some liquid evaporates. 

When other reference electrodes are used as, for example, the saturated Calomel electrode (SCE), a constant quantity needs to be added to the right-hand side of Nemst s equation which reflects the difference between the SCE and the SHE electrodes, i.e., in this case the standard potential is... 

The first two terms on the right hand side of the equation are constant at a given temperature and express the standard potential of the calomel electrode ii r, Hffsds i ci-> the activity of chloride ions equalling unity the last formula could, therefore, be written in the form ... 

Here Egl is the standard potential of the glass electrode. This quantity varies from specimen to specimen, it depends also on the age and on the pretreatment of the electrode. Within one set of measurements it can be regarded as constant. If we adapt the usual calibration process, described below, it is not necessary to measure the standard potential and to deduct the potential of the calomel electrode from the results, as the pH can be read directly from the pH-meter. 

The steady-state potential measured against a saturated calomel electrode (SCE) is negative in alkaline and positive in acidic solution, and even higher positive in the presence of oxidizing agents. Calculated standard potentials from thermodynamic data are around —0.1 V in acidic and around —0.9 V in alkaline solution against NHE. 

The Standard Potential of Chlorine. Measurements of the potentials of galvanic cells without liquid junctions from which the standard potential of chlorine may be deduced have been made by Lewis and Ruppert40 who used, as one electrode, platinum over which a mixture of chlorine and nitrogen was bubbled, and, as reference, a calomel electrode and hydrochloric acid as electrolyte. The arrangement may be represented by... 

In which 22 is the standard potential of the electrode. However iu a saturated calomel solution the relation ... 

The standard potential for the reduction of No + to No(Hg) was measured by a modified radiopolarographic technique (31). Usually, the half-wave potential is determined by measuring the distribution of an element between the mercury and aqueous phases as a function of applied voltage. The half-life of No is too short to allow time for the recovery of No from the Hg phase for assay, therefore Meyer et al. measured the depletion of No in the aqueous phase as a function of a controlled potential. They assumed that equilibrium was reached in 3 min of electrolysis and that the electrode reaction was reversible. A sharp drop in No concentration in the aqueous phase occurred between -1.8 and -1.9 V v . the saturated calomel electrode or -1.6 V vs. the standard hydrogen electrode. Thus, their best estimates are summarized in the following equation. 

Given that the standard potential of the calomel electrode is 0.268 V and that of the Hg/Hg2 electrode is 0.789 V, calculate for calomel (Hg2Q2). 

Unlike the table of the Electrochemical Series, which lists standard potentials, values for radicals are experimental values with experimental conditions given in the second column. Since the measurements leading to potentials for ion radicals are very dependent on conditions, an attempt to report standard potentials for radicals would serve no useful purpose. For the same reason, the potentials are also reported as experimental values, usually a half-wave potential in polarography) or a peak potential E in cyclic voltammetry). Unless otherwise stated, the values are reported vs. SCE (saturated calomel electrode). To obtain a value vs. 

Abbreviations are CV — cyclic voltammetry DMF — N,N-Dimethylformamide E swp — potential sweep E° — standard potential — peak potential E — half-peak potential E — half wave potential M — mol/L i eCN — acetonitrile pol — polarography rot Pt dsk — rotated Pt disk SCE — saturated calomel electrode TBABF — tetrabutylammonium tetrafluoroborate TBAl — tetrabutylammonium iodide TBAP — tetrabutylammonium perchlorate TEABr — tetraethylammonium bromide TEAP — tetraethylammonium perchlorate THF — tetrahydrofu-ran TPACF SO — tetrapropylammonium trifluoromethanesul-fite TPAP — tetrapropylammonium perchlorate and wr — wire. 

Effect of Temperature on the Standard Potential of Calomel Half Cell and Platinum Electrode... 

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. ... 

In the case of standard electrode potential, it is appropriate to have a standard electrode whose reversible potential is made arbitrarily zero and against which the potentials of other electrodes can be measured. The hydrogen electrode is an accepted standard. It is composed of a rod of platinum covered with platinum black saturated with hydrogen gas at atmospheric pressure. Electrode potential based on this zero are said to refer to the hydrogen scale. However, in experimental work, it is often more suitable to use another standard electrode. Calomel is a common example. It consists of a pool of mercury covered with calomel (mercurous chloride) and immersed in a solution of potassium chloride. 

In defining the formal potential in Eq. (1.2.23), the side reactions are acid-base equilibria. Of course, all other kinds of chemical equilibria, e.g., complex formation and precipitation, have similar consequences. In the case of an electrode of the second kind, e.g., a calomel electrode, the so-called standard potential of the calomel electrode is nothing but the formal potential of the electrode at flchioride = 1 The potential of the calomel electrode at various KCl concentrations is always the formal potential of this electrode at the specified concentration (see Chaps. II.9 and ni.2). 

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