Electromotive force dependence

Thermocouples are primarily based on the Seebeck effect In an open circuit, consisting of two wires of different materials joined together at one end, an electromotive force (voltage) is generated between the free wire ends when subject to a temperature gradient. Because the voltage is dependent on the temperature difference between the wires (measurement) junction and the free (reference) ends, the system can be used for temperature measurement. Before modern electronic developments, a real reference temperature, for example, a water-ice bath, was used for the reference end of the thermocouple circuit. This is not necessary today, as the reference can be obtained electronically. Thermocouple material pairs, their temperature-electromotive forces, and tolerances are standardized. The standards are close to each other but not identical. The most common base-metal pairs are iron-constantan (type J), chomel-alumel (type K), and copper-constantan (type T). Noble-metal thermocouples (types S, R, and B) are made of platinum and rhodium in different mixing ratios. 

If electron flow between the electrodes is toward the sample half-cell, reduction occurs spontaneously in the sample half-cell, and the reduction potential is said to be positive. If electron flow between the electrodes is away from the sample half-cell and toward the reference cell, the reduction potential is said to be negative because electron loss (oxidation) is occurring in the sample halfcell. Strictly speaking, the standard reduction potential, is the electromotive force generated at 25°C and pH 7.0 by a sample half-cell (containing 1 M concentrations of the oxidized and reduced species) with respect to a reference half-cell. (Note that the reduction potential of the hydrogen half-cell is pH-dependent. The standard reduction potential, 0.0 V, assumes 1 MH. The hydrogen half-cell measured at pH 7.0 has an of —0.421 V.)... 

This effect appears to be of importance in the case of normal galvanic cells, the electromotive forces of which depend on the concentration of solutions in equilibrium with depolarising solids such as calomel or mercurous sulphate. The exact relationships are, unfortunately, not yet wholly elucidated. 

Similar considerations apply of course to the opposing electromotive forces of polarisation during electrolysis, when the process is executed reversibly, since an electrolytic cell is, as we early remarked, to be considered as a voltaic cell working in the reverse direction. In this way Helmholtz (ibid.) was able to explain the fluctuations of potential in the electrolysis of water as due to the variations of concentration due to diffusion of the dissolved gases. It must not be forgotten, however, that peculiar phenomena—so-called supertension effects—depending on the nature of the electrodes, make their appearance here, and com-... 

In this cell, the following independent phases must be considered platinum, silver, gaseous hydrogen, solid silver chloride electrolyte, and an aqueous solution of hydrogen chloride. In order to be able to determine the EMF of the cell, the leads must be made of the same material and thus, to simplify matters, a platinum lead must be connected to the silver electrode. It will be seen in the conclusion to this section that the electromotive force of a cell does not depend on the material from which the leads are made, so that the whole derivation could be carried out with different, e.g. copper, leads. In addition to Cl- and H30+ ions (further written as H+), the solution also contains Ag+ ions in a small concentration corresponding to a saturated solution of silver chloride in hydrochloric acid. Thus, the following scheme of the phases can be written (the parentheses enclose the species present in the given phase) ... 

The lUPAC Commission for Analytical Nomenclature defines the calibration curve [138] as the dependence of the electromotive force of the given ISE -reference electrode cell on the logarithm of the activity or concentration of the given substance. It is recommended that the potential be plotted on the ordinate (the vertical axis) and the logarithmic function of the activity or concentration on the abscissa (the horizontal axis), with the concentration increasing from the left to the right. 

The electromotive force (emf) of liquid membrane electrodes depends on the activity of the ions in solution and their performance is similar in principle to that of the glass electrode. To characterize the behavior of liquid membrane electrodes, the linearity of the emf measurements vs. concentration of a certain ion in solution is checked. Additional performance data are the Nernstian slope of the linear range and the pH range over which the potential of the electrode is constant. 

A unitless correction factor that relates the relative activity of a substance to the quantity of the substance in a mixture. Activity coefficients are frequently determined by emf (electromotive force) or freezing-point depression measurements. At infinite dilution, the activity coefficient equals 1.00. Activity coefficients for electrolytes can vary significantly depending upon the concentration of the electrolyte. Activity coefficients can exceed values of 1.00. For example, a 4.0 molal HCl solution has a coefficient of 1.76 and a 4.0 molal Li Cl has a value of... 

I = EIR where I is the current, E is the electromotive force, and R is the resistance. The SI units for each of these is amperes, volts, and ohms, respectively. Ohm s law is also expressed as / = AE/7 where AE is the difference in electric potential. The resistance is dependent upon the dimensions of the conductor. 

Fig. 8 The dependence of the electromotive force (EMF) ofCd(ll) ion-selective electrode on logarithm of Cd(ll) concentration in M NaNOs at pH 7. Solid lines, calculated response curves on the basis of Eq. (7) in Ref 399.

The electrode in the half-cell in which oxidation is occurring is said to be the anode (here, the zinc metal), whereas the other is the cathode (here, the platinum). In principle, we could connect any pair of feasible half-cells to form a galvanic cell the identity of the half-cells will determine which electrode will act as the anode, and which the cathode. The electromotive force (EMF, in volts) of the cell will depend on the identity of the half cells, the temperature and pressure, the activities of the reacting species, and the current drawn. An EMF will also be generated by a cell in which the two half cells are the chemically identical except for a difference in reactant activities (concentrations) this is called a concentration cell. 

A thermocouple manometer is a glass vessel with a tube for connection to a vacuum system. A heater is installed on two inlets to the vessel, while a thermocouple is welded to two other inlets. Its junction, in turn, is welded to the heater, to which a constant amount of heat is supplied. But since the heat conductivity of the residual gas in the system depends on the pressure, the withdrawal of heat from the heater is different at different pressures. Therefore, the temperature of the heater, which is measured by the thermocouple, also varies. The electromotive force (e.m.f.) produced will be proportional to the pressure. The millivoltmeter measuring the e.m.f. of the thermocouple is graduated in mmHg. Such a manometer can be used to measure a pressure from 10 to 10 mmHg. 

The photoconductivity spectrum after preliminary irradiation of polymer is given by curve 2. The observed redistribution of the peaks is partly reversed on prolonged exposure to air. The bathochromic shift of the shorter wavelength peak depends on the exposure time. Ultraviolet irradiation produces a slight change in the polymer colour. Such irradiation increases, likewise, the photo-electromotive force a 1.5 h irradiation increases it 10 times. The photoconductivity spectrum is situated at longer wavelengths than the photo-electromotive force spectrum. 

Dependence of Electromotive Force on Concentrations Calculate the electromotive force (in volts) registered by an electrode immersed in a solution containing the following mixtures of NAD+ and NADH at pH 7.0 and 25 °C, with reference to a half-cell of E ° 0.00 V... 

Electrolytes, depending upon their strength, dissociate to a greater or less extenl in polar solvents. The extent to which a weak electrolyte dissociates may be determined by electrical conductance, electromotive force, and freezing point depression methods. The electrical conductance method is the most used because of its accuracy and simplicity. Arrhenius proposed that the degree of dissociation, a. of a weak electrolyte at any concentration in solution could be found from the rutio of the equivalent conductance. A. of the electrolyte at the concentration in question to (he equivalent conductance at infinite dilution A0 of the electrolyte. Thus... 

In 1821 Seebeck discovered that, in an electric circuit consisting of two different materials X and Y in the form of wires, when the two junctions are at different temperatures 0, and a potential exists at the terminals on open circuit and, if the circuit is closed, a current flows 271. At each junction there exists a contact potential E r and E x respectively which depends on the type of metal employed and the temperature of the junction. When the system is on closed circuit the electromotive force (emf) is given by ... 

The cell potential E (also called the cell voltage or electromotive force) is an electrical measure of the driving force of the cell reaction. Cell potentials depend on temperature, ion concentrations, and gas pressures. The standard cell potential E° is the cell potential when reactants and products are in their standard states. Cell potentials are related to free-energy changes by the equations AG = —nFE and AG° = —mFE°, where F = 96,500 C/mol e is the faraday, the charge on 1 mol of electrons. 

The constitution of alloys can be deciphered by several processes. One depends on measurement of the electromotive force of a battery consisting of the alloy and a plate of some resistant metal—for instance, platinum—compared with that of a similar cell made with one of the constituents of the alloy. To take a concrete example. Suppose a cell were constructed of a plate of copper and a plate of platinum dipping in some appropriate liquid, a certain electromotive force would result. Imagine a plate of tin riveted to the face of the copper plate, the electromotive force would now be that of the more electropositive metal, tin. If a plate of bronze be substituted, supposing it to contain free tin not in chemical combination with the copper, then the electromotive force will still be that of... 

What is wrong with the following argument If the terminals of an electrochemical cell are constructed from the same metal, the chemical potential of electrons [species i in Eq. (36)] at the terminals, which depends only on T, P and concentrations, are the same. From Eq. (36), the electromotive force of the cell is therefore zero ... 

A single cell delivers a cell voltage between 0.5 and 0.9 V (instead of the theoretical electromotive force [emf] of 1.23 V under standard equilibrium conditions) depending on the working current... 

Abstract. It is shown that reinforcement of PTFE by 15% of multiwall carbon nanotubes (MWNT) results in more than 2 times increase of strength parameters compared to starting PTFE matrix. Non-trivial temperature dependences of electrical resistance and thermal electromotive force were observed. Percolation threshold determined from dependence of the composite specific resistance on MWNT concentration was near 6% mass. Concentration and nature of oxygen-containing MWNT surface groups influence the strength parameters of the composite material. 

Figure 6. Dependence of thermo-electromotive force on extent of deformation for MWNT -PTFE composite materials containing 15% (curve 1) and 20% (curve 2) of MWNT.

When tho potential on the cell terminals is higher than the electromotive force of the cell the process proceeds in a reverse direction The surplus of solid salts (Hg2S04 and CdS04.8/3 HgO) ensures that the solutions at both electrodes are always saturated, so that the electrode potentials are constant at a constant temperature. The dependence on the temperature of the EMF of the described Weston cell is expressed by the equation ... 

The electromotive force of a given cell apart from temperature and pressure also depends on the concentration of the active substances in the system. This dependence for a common reaction... 

Both these reactions have their own characteristic potentials which depend only on the nature of each separate half cell. The electromotive force of the... 

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