Cell potential Galvanic cells

Galvanic Cells, Cell Potentials, and Standard Reduction Potentials... 

Electromotive Force of a Galvanic Cell, Cell Potential Difference, Maximum Work Available from a Chemical Reaction, and Nernst s Equation ... 

Electromotive Force of a Galvanic Cell, Cell Potential Difference... 

In this discussion we have emphasized the difference of electrical potential across a membrane without reference to a galvanic cell. Cells can be devised by which such differences can be experimentally determined. 

Here we investigate some of the properties of galvanic cells, cells used to produce an electric potential. Luigi Galvani discovered the first such cell by accident in 1791. Following Galvani s discovery, Alessandro Volta developed a practical cell in 1800, and it was Volta s cell that led to the work of Davy and Faraday. 

The electric potential difference of a -> galvanic cell (cell voltage) is the difference of electric potential between a metallic terminal attached to the right-hand electrode in the -> cell diagram and identical metallic terminal attached to the left-hand electrode. E includes the condition when current flows through the cell. The value of E measured when the left-hand electrode is at virtual equilibrium, and hence acting as a -> reference electrode, may be called the potential of the (right-hand) electrode with respect to the (left-hand) reference electrode. 

A gas-sensing probe is a galvanic cell whose potential is related to the concentration of a gas in a solution. Often, these devices are called gassensing electrodes in instrument brochures, which is a misnomer. 

To understand this phenomenon, we observe that a redox reaction away from equilibrium plus a pair of electrodes constitutes a galvanic cell. The potential difference between the platinum and the reference electrode can cause a current to flow. The current will cause an electrochemical reaction at the platinum electrode, locally changing the ratio of oxidized to reduced ions. The concentration at the electrode now differs from that of the bulk solution, though homogeneity will be restored by stirring and/or diffusion. An erratic signal resulls because the mixing is neither perfect nor instantaneous. 

The typical galvanic cell, the potential of which is controlled or measured in impedance measurements, can be represented by ... 

The electric potential difference of a -> galvanic cell (cell voltage) is the difference of electric potential between a metalhc terminal attached to the right-hand electrode in the cell diagram and identical metalUc ter-... 

Galvanic cells in which stored chemicals can be reacted on demand to produce an electric current are termed primaiy cells. The discharging reac tion is irreversible and the contents, once exhausted, must be replaced or the cell discarded. Examples are the dry cells that activate small appliances. In some galvanic cells (called secondaiy cells), however, the reaction is reversible that is, application of an elec trical potential across the electrodes in the opposite direc tion will restore the reactants to their high-enthalpy state. Examples are rechargeable batteries for household appliances, automobiles, and many industrial applications. Electrolytic cells are the reactors upon which the electrochemical process, elec troplating, and electrowinning industries are based. 

Potential differences leading to galvanic-type cells can also be set up on a single metal by differences in temperature, velocity, or concentration (see subsection Crevice Corrosion ). 

Conditions necessary for the onset of corrosion are quite often provided by heterogeneities. These heterogeneities may very well exist within the metal or alloy or may be imposed by external factors. These heterogeneities can give rise to variations in potential on a metal surface immersed in an electrolytic fluid. The galvanic cell thus formed gives rise to flow of current that accompanies corrosion [188]. 

Test methods available are the free-running test (galvanic cell), galvano-static test (constant current) and potentiostatic test (constant potential). These are always run in conjunction with visual examinations with particular emphasis on dissolution pattern. 

An interesting application of electrode potentials is to the calculation of the e.m.f. of a voltaic cell. One of the simplest of galvanic cells is the Daniell cell. It consists of a rod of zinc dipping into zinc sulphate solution and a strip of copper in copper sulphate solution the two solutions are generally separated by placing one inside a porous pot and the other in the surrounding vessel. The cell may be represented as ... 

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