Cell potential, measurement

Group 12 (IIB) Perchlorates. The zinc perchlorate [13637-61 -17, cadmium perchlorate [13760-37-7] mercury(I) perchlorate [13932-02-0] and mercury(II) perchlorate [7616-83-3] all exist. Cell potential measurements show that zinc and cadmium perchlorates are completely dissociated in concentrations up to 0.1 molar in aqueous solutions (47—49). Mercurous perchlorate forms a tetrahydrate that can be readily converted to the dihydrate on heating to above 36°C (50). 

The activity coefficients of sulfuric acid have been deterrnined independentiy by measuring three types of physical phenomena cell potentials, vapor pressure, and freeting point. A consistent set of activity coefficients has been reported from 0.1 to 8 at 25°C (14), from 0.1 to 4 and 5 to 55°C (18), and from 0.001 to 0.02 m at 25°C (19). These values are all based on cell potential measurements. The activity coefficients based on vapor pressure measurements (20) agree with those from potential measurements when they are corrected to the same reference activity coefficient. 

R is the ideal gas constant, T is the Kelvin temperature, n is the number of electrons transferred, F is Faraday s constant, and Q is the activity quotient. The second form, involving the log Q, is the more useful form. If you know the cell reaction, the concentrations of ions, and the E°ell, then you can calculate the actual cell potential. Another useful application of the Nernst equation is in the calculation of the concentration of one of the reactants from cell potential measurements. Knowing the actual cell potential and the E°ell, allows you to calculate Q, the activity quotient. Knowing Q and all but one of the concentrations, allows you to calculate the unknown concentration. Another application of the Nernst equation is concentration cells. A concentration cell is an electrochemical cell in which the same chemical species are used in both cell compartments, but differing in concentration. Because the half reactions are the same, the E°ell = 0.00 V. Then simply substituting the appropriate concentrations into the activity quotient allows calculation of the actual cell potential. 

Unfortunately, in many nonaqueous solvents there is no completely unambiguous way of determining half-cell potentials. This ambiguity stems from the fact that the free energies of transfer (AG°) of individual ions from one solvent to another are not knowable. (It should be noted that no ambiguity necessarily exists if one is content with comparing whole-cell potentials in different solvents. This is true because the free energies of transfer of dissolved salts are know-able.) Some type of extrathermodynamic assumption is usually necessary to compare half-cell potentials measured in one solvent to those measured in some other solvent. Popovich has provided excellent... 

Finally, although the free energies of transfers found in Table 7 are quite useful in comparing cell potentials measured in various solvents, these values are insufiicient to adequately interpret potentials measured under many nonaqueous conditions. The primary reason for this difficulty stems from the manner in which AG°(M", w s) is defined. For example, AGj (Na, w CH3CN) is associated with the transfer of sodium ions... 

In this expression, E° is the standard cell potential, the cell potential measured when all the species taking part are in their standard states. In practice, that means all gases are at 1 bar and all ions are at 1 mol-L 1. For example, to measure the standard potential of the Daniell cell, we should use 1 M CuS04(aq) in one electrode compartment and 1 M ZnS04(aq) in the other. 

Equations (47)-(50) indicate how thermodynamic quantities can be obtained from cell potentials measured under standard conditions. However, standard states are hypothetical states (e.g., infinitely dilute behavior at 1.0 m concentration), which cannot be prepared in the cell. As a result, an extrapolation procedure is used to find 8° from measured cell voltages as a function of concentration. From Eq. (47), we write the dependence of 8 on the concentration of the electrolyte in the form... 

Cell potential Measured using a voltmeter connected to the two electrodes of a cell (see Fig. 12.4 in the text)... 

The standard cell potential, E°, is the cell potential measured when all reactants and products are in their standard states. 

Figure 2. displays cell potential measured from the electrodialysis cell as a function of operation temperature. As operation temperature increased from 35 to 90°C, the cell potential decreased from 0.68 to 0.41 V at the applied current of 2 A. This is because high temperatures reduce the amount of electrical energy required to concentrate HI molarity from a thermodynamic standpoint and hence electrical energy demand AG for electrodialysis of Hix solution decreases with increasing temperature... 

It is useful to think of the cell potential as the difference between the potentials associated with the two half-cell reactions, although these are not separately measurable. Standard electrode potentials are the half-cell potentials measured against a hydrogen electrode, where the half-cell reaction is... 

This value is in excellent agreement with that obtained from cell potential measurements. 

Our inability lo measure absolute potenliaHfyr half-cell processes is not a serious problem because relative half-cell potentials, measured versus a common reference electrode, arc just as useful. These relative potentials can be combined to give real cell potentials. In addition, they can be used toealculale equilibrium constants of oxidation-reduction processes. 

Organic cocktail oxidation experiments were carried out at various current densities, and the corresponding cell potentials were recorded as a function of time. Cell potential measurements for two different current densities are shown in Fig. 5. At the higher applied current density, the cell potential is higher partly because of increased electrode potentials at the anode (and cathode) solution interfaces. Since electrochemical reaction rates increase with increas-... 

Figure 16.6 Multiple-wheel electrode half-cell potential measuring instrument with computer-assisted data acquisition. Note the slight wetting of the concrete surface at the wheels in order to achieve a good electrolytic contact between reference electrode and concrete...

Interpretation. Half-cell potential measurements allow the location to be determined of areas of corroding rebars being the most negative zones in a potential field (Figure 16.7). However, the interpretation of the readings is not straightforward because the concrete cover and its resistivity in addition to the actual corrosion potential of the steel (Chapter 7) influence the readings at the concrete surface. 

F re 16.10 Cumulative probability of half-cell potential measurements on different bridge decks [14.16]... 

Procedure. The detailed procedure for the measurement of resistivity of concrete is described in a RILEM recommendation [17]. The measuring system should be calibrated on concrete with known resistivity. As with half-cell potential measurements, concrete shall be clean and a good electrolytic contact between the electrodes and the concrete surface is important, but complete wetting of the surface should be avoided. When using the 4-point method, measurements should be taken as far from the rebars as possible (e. g. diagonally inside the rebar mesh. Figure 16.13). 

J. Gulikers, R. Polder, M. Raupach, Recommendation on half-cell potential measurements . Materials and Structures, 2003, 36, 461-471. 

Half-cell potential measurement is the most frequently used technique on site to evaluate corrosion of steel in concrete, giving an indication of the corrosion prob-... 

Four steel wire probes coated with Lacomate insulator were planted in the repair mix at different depths, that is, 5,10,15, and 20 mm. Half-cell potential measurements were made between the corroded main bar and the four probes after wet and dry cycles using saline water. It can be seen clearly that the potentials of the probes did not increase sharply. It showed a trend of increasing corrosion activities in the steel wires and slight reduction in the corrosion activities in the bolts, connected to the main steel reinforcement bar. 

Several factors in the explanation given in this section are important and will be used later to explain how we measure and stop corrosion. The electrical current flow, and the generation and consumption of electrons in the anode and cathode reactions are used in half-cell potential measurements and cathodic protection. The formation of protective, alkaline hydroxyl ions is used in cathodic protection, electrochemical chloride removal and realkalization. The fact that the cathodic and anodic reactions must balance... 

Figure 2.6 Schematic of a reference electrode (half cell) potential measurement of steel in concrete.

Reference electrode (half cell) potential measurements... 

Cell Potentials Measuring Cell Potential Standard Reduction Potentials Nonstandard Conditions... 

The choice for the standard component in cell potential measurements is the standard hydrogen electrode (SHE), illustrated in Figure 13.9. A platinum wire or foil is the conducting source of electrons. Hydrogen gas is bubbled over the electrode at a pressure of 1 atm, and the electrolyte solution is 1 M HCl(aq). The resulting half-reaction is... 

By convention, this half-reaction is assigned a potential of (exactly) zero volts. To determine the half-cell potential of any other electrode/electrolyte system, it can be connected to the SHE and the cell potential measured. Because the SHE is assigned a value of 0.00 V, the observed cell potential is attributed to the other halfreaction. 

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