Galvanic cell, definition

At this stage it should be pointed out that the original definition of pH = —log cH (due to Sorensen, 1909 and which may be written as pcH) is not exact, and cannot be determined exactly by electrometric methods. It is realised that the activity rather than the concentration of an ion determines the e.m.f. of a galvanic cell of the type commonly used to measure pH, and hence pH may be defined as... 

The electrode is considered to be a part of the galvanic cell that consists of an electronic conductor and an electrolyte solution (or fused or solid electrolyte), or of an electronic conductor in contact with a solid electrolyte which is in turn in contact with an electrolyte solution. This definition differs from Faraday s original concept (who introduced the term electrode) where the electrode was simply the boundary between a metal and an electrolyte solution. 

It has been emphasized repeatedly that the individual activity coefficients cannot be measured experimentally. However, these values are required for a number of purposes, e.g. for calibration of ion-selective electrodes. Thus, a conventional scale of ionic activities must be defined on the basis of suitably selected standards. In addition, this definition must be consistent with the definition of the conventional activity scale for the oxonium ion, i.e. the definition of the practical pH scale. Similarly, the individual scales for the various ions must be mutually consistent, i.e. they must satisfy the relationship between the experimentally measurable mean activity of the electrolyte and the defined activities of the cation and anion in view of Eq. (1.1.11). Thus, by using galvanic cells without transport, e.g. a sodium-ion-selective glass electrode and a Cl -selective electrode in a NaCl solution, a series of (NaCl) is obtained from which the individual ion activity aNa+ is determined on the basis of the Bates-Guggenheim convention for acr (page 37). Table 6.1 lists three such standard solutions, where pNa = -logflNa+, etc. 

When reaction 15.12 is allowed to come to equilibrium, AE (the EMF of a galvanic cell based on reaction 15.12) runs down to zero, all concentrations assume their equilibrium values, and so Q becomes K. To put it another way, if all the reagents are present at standard-state concentrations (unity, by definition), Q becomes unity and E is just E°. 

Driven by the energy produced in these reactions, electrons move spontaneously through the external electric circuit containing the load (i.e. a device powered by electric current). According to the definition, the system in the configuration of Figure 3.1.5 operates as a galvanic cell. The Ag electrode is termed the cathode because... 

The notional definition of pH given in the table above is in practice replaced by the following operational definition. For a solution X the emf E(X) of the galvanic cell... 

The electromotive force of a galvanic cell is a measure of the electrical work which can be obtained from the reaction in the cell. The total or maximum work which can be obtained from the cell reaction includes also the work which is done against the external forces owing to the changes in volume (formation of gas, etc.) of the reacting substances. From the definition of affinity (p. 318) it follows, therefore, that the electromotive... 

The correspondence between the names anode and cathode and the charge on the electrode is different for electrolytic cells than for voltaic (galvanic) cells. Students sometimes get confused by trying to remember which is which. Check the definitions of these two terms in Section 21-2. The surest way to name these electrodes is to determine what process takes place at each one. 

As already stated, the measured electromotive force of a galvanic cell does not necessarily correspond to a single definite reaction. For example, a powerful primary cell may be constructed by placing a porous cup containing a platinum or carbon electrode, surrounded by nitric acid, into another vessel containing an amalgamated zinc electrode in an electrolyte of sulphuric acid. This cell may be represented by... 

Examples of metals that are passive under Definition 1, on the other hand, include chromium, nickel, molybdenum, titanium, zirconium, the stainless steels, 70%Ni-30% Cu alloys (Monel), and several other metals and alloys. Also included are metals that become passive in passivator solutions, such as iron in dissolved chromates. Metals and alloys in this category show a marked tendency to polarize anodicaUy. Pronounced anodic polarization reduces observed reaction rates, so that metals passive under Definition 1 usually conform as well to Definition 2 based on low corrosion rates. The corrosion potentials of metals passive by Definition 1 approach the open-circuit cathode potentials (e.g., the oxygen electrode) hence, as components of galvanic cells, they exhibit potentials near those of the noble metals. 

By definition, electrolytes should be ionic conductors and electronic insulators. If they were not, then galvanic cells in which they were employed would self-discharge on standing because the cell reaction could proceed even on open circuit [106]. In practice many conventional solid electrolytes, especially those based on Cu systems, have a non-negligible electronic conductivity contribution from electrons and/or holes, but provided that this is small enough, a reasonable shelf-life can be obtained. 

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