Standard emf

C (298.15 K) and 1 bar. standard cell potential See standard emf. standard emf ( °) The emf when the concentration of each solute taking part in the cell reaction is 1 mol-L 1 (strictly, unit activity) and all the gases are at 1 bar. The standard emf of a galvanic cell is the difference between its two standard potentials E° = E°(cathode) — °(anode). 

Standard emf standard equilibrium electrode potential equilibrium electrode rest potential Non-equilibrium electrode potential... 

Analyze We are given the equation for a redox reaction and asked to use data in Table 20.1 to calculate the standard emf (standard potential) for the associated voltaic cell. Plan Our first step is to identify the half-reactions that occur at the cathode and the anode, which we did in Sample Exercise 20.4. Then we can use data from Table 20.1 and Equation 20.8 to calculate the standard emf. 

The extensive use of a wide variety of electrical equipment in semiconductor manufacturing, coupled with the emergence of the published studies summarized above has recently led to EMF concerns within the industry. However, typical workplace exposures to EMF within the semiconductor industry are expected to be far below existing health-based exposure limits (see EMF Standards Guidelines for Magnetic Fields). 

Were the FlCl in its standard state, AC would equal where is the standard emf for the reaction. In general, for any reversible chemical cell without transference, i.e. one with a single electrolyte solution, not one with any kind of junction between two solutions. 

Thus, if the activities of the various species can be detennined or if one can extrapolate to infinite dilution, the measurement of the emf yields the standard free energy of the reaction. 

As seen in previous sections, the standard entropy AS of a chemical reaction can be detemiined from the equilibrium constant K and its temperature derivative, or equivalently from the temperature derivative of the standard emf of a reversible electrochemical cell. As in the previous case, calorimetric measurements on the separate reactants and products, plus the usual extrapolation, will... 

Two methods are used to measure pH electrometric and chemical indicator (1 7). The most common is electrometric and uses the commercial pH meter with a glass electrode. This procedure is based on the measurement of the difference between the pH of an unknown or test solution and that of a standard solution. The instmment measures the emf developed between the glass electrode and a reference electrode of constant potential. The difference in emf when the electrodes are removed from the standard solution and placed in the test solution is converted to a difference in pH. Electrodes based on metal—metal oxides, eg, antimony—antimony oxide (see Antimony AND ANTIMONY ALLOYS Antimony COMPOUNDS), have also found use as pH sensors (8), especially for industrial appHcations where superior mechanical stabiUty is needed (see Sensors). However, because of the presence of the metallic element, these electrodes suffer from interferences by oxidation—reduction systems in the test solution. 

Ideally a standard cell is constmcted simply and is characterized by a high constancy of emf, a low temperature coefficient of emf, and an emf close to one volt. The Weston cell, which uses a standard cadmium sulfate electrolyte and electrodes of cadmium amalgam and a paste of mercury and mercurous sulfate, essentially meets these conditions. The voltage of the cell is 1.0183 V at 20°C. The a-c Josephson effect, which relates the frequency of a superconducting oscillator to the potential difference between two superconducting components, is used by NIST to maintain the unit of emf. The definition of the volt, however, remains as the Q/A derivation described. 

Ion-selective electrodes are a relatively cheap approach to analysis of many ions in solution. The emf of the selective electrode is measured relative to a reference electrode. The electrode potential varies with the logarithm of the activity of the ion. The electrodes are calibrated using standards of the ion under investigation. Application is limited to those ions not subject to the same interference as ion chromatography (the preferred technique), e.g. fluoride, hydrogen chloride (see Table 10.3). 

ASTM E230-96el. Standard Specification for Temperature-Electromotive Force (EMF) Tables for Standardized Thermocouples. American Society for Testing and Materials, 1996. 

Drawbacks of ion-selective electrodes ° Selectivity not always sufficient ° Direct read-out is less accurate with higher valent ions EMF-Drift may require frequent standardization... 

Each metal or metal area will develop an electrode with a measurable electrical potential. This potential can be referenced to that of a standard hydrogen electrode, which by convention is set at zero. Thus, all metals have either a higher or lower potential compared to hydrogen, and a comparative list of metals can be produced indicating their relative nobility. This list is the galvanic or electrochemical series and measured as an electromotive force (EMF). 

NOTE The EMF of boiler steels with respect to the standard hydrogen... 

In this expression, E° is the standard emf of the cell, the emf measured when all the species taking part are in their standard states. In practice, this condition means that all gases are at 1 bar, all participating solutes are at 1 molT-1, and all liquids and solids are pure. For example, to measure the standard emf of the Daniell cell, we use 1 M CuS04(aq) and a pure copper electrode in one electrode compartment and 1 M ZnS04(aq) and a pure zinc electrode in the other. 

A problem with compiling a list of standard potentials is that we know only the overall emf of the cell, not the contribution of a single electrode. A voltmeter placed between the two electrodes of a galvanic cell measures the difference of their potentials, not the individual values. To provide numerical values for individual standard potentials, we arbitrarily set the standard potential of one particular electrode, the hydrogen electrode, equal to zero at all temperatures ... 

The standard potential of a couple is the standard emf of a cell (including the sign) in which the couple forms the right-hand electrode in the cell diagram and a hydrogen electrode forms the left-hand electrode in the cell diagram. 

The standard emf is measured as 0.76 V and the hydrogen electrode is found to be the cathode therefore, this emf is reported as —0.76 V. Because the entire emf is attributed to the zinc electrode, we write... 

SOLUTION We can determine the standard potential of an electrode by measuring the emf of a standard cell in which the other electrode has a known standard potential and applying Kq. 3. 

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