Temperature emf and

The thermoelectric properties were measured at 300 K for the FGM and its component layers separated from the FGM. The electrical conductivity ((T) and Hall coefficient (Rjj) were measured by the 6-probe method for the FGM and by the van der Pauw configuration for the components cut from the FGM using Pt-wire electrodes. The carrier concentration (n) and Hall mobility (/ h) calculated using the equation n=lleR (e electric charge) and / H= h respectively. The thermoelectric power (a) at 300 K was estimated from the linear relationship between thermoelectromotive force (EMF) and temperature difference within 5 K. 

Derive a relationship between the emf and temperature differential of a Type K thermocouple (see Table Q3.19). 

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. 

As in the 1,2-dichloroethane case too, transient EMF and SHG responses to KSCN were observed for the nitrobenzene membranes without ionic sites. This suggests that here too not only SCN but also K ions are transferred into the nitrobenzene phase. Salt extraction into the bulk of the organic phase, in analogy to similar observations previously reported for neutral ionophore-incorporated liquid membranes without ionic sites [55], was indeed independently confirmed by atomic absorption spectrometry. Figure 15 shows the concentration of K in nitrobenzene equilibrated at room temperature with a 10 M aqueous solution of KSCN as a function of equilibration time. The presence of the ion exchanger TDDMA-SCN efficiently suppresses KSCN extraction into the organic phase but in its absence a substantial amount of KSCN enters the nitrobenzene phase. The trends of the EMF and the SHG responses are therefore very similar in spite of the different polarities of the plasticizers. 

Basically, the calomel electrode consists of mercury, mercurous chloride (calomel), and chloride ion. The concentration of potassium chloride is 0.1 M in an aqueous-organic solvent (50 50) of the same nature as that contained in the solution to be investigated. The junction with the test solution is realized either with a capillary or a porous stone. When the capillary is used, a small hydrostatic pressure is maintained inside it in order to avoid any electrode contamination by the test solution. In the main part of our investigation, the porous stone junction was used. Moreover, the calomel electrode is thermostatted at 20°C, and temperature variations of this electrode giving appreciable emf variations involve uncertainty on the pon determination on the order of 0.2-0.3 poH unit/ 10°C. 

EMF and vapour pressure measurements are dependent on the temperature, the number of phases involved and, importantly, the reference state of the component in question. The problem with the reference state is important as experimentally stated values of partial Gibbs energies will be dependent on this value. The standard states are fixed before optimisation and may actually have values different from those used by the original author. Therefore, as far as possible like should be compared with like. 

The emf developed by a thermocouple depends upon the temperature of both the measuring and reference junctions. Thus, to determine temperature, the following data musi be known (1) the calibration data for the particular thermocouple (2) the measured emf and (3) the temperature of the reference junction. In laboratoiy cases, the reference junction can be maintained at the freezing temperature of water. However, in most modem instruments, the ambient temperature of the reference junction is sensed, and the correction is incorporated in the measurement circuitry. 

In Figure 18.8, K for the acid-base reactions (the first two) were determined by both cell EMF and conductivity measurements. Only conductivity measurements could be used to get K for the ion-pairing in NaCl. We note from this figure that association, even in NaCl, becomes appreciable at high temperatures. It has been said that it is difficult to find ionic solutions where ion-pairing is not important at high temperatures. 

An interesting aspect about temperature measurement is that calibration is consistent across different types of physical phenomena. Thus, once you have calibrated two or more established points for specific temperatures, the various physical phenomena of expansion, resistance, emf, and other variable physical properties of temperature will give the same temperature reading. ... 

E Chromel - Con-stantan (Cu-Ni) -200 to 900 0 to 200 Recommended for vacuum use or for inert, mildly oxidizing or reducing atmospheres. These thermocouples produce the greatest amount of emf and therefore can detect small temperature changes. 

After the cell has been in the thermostat bath for at least 10 min, measure the emf and repeat this measurement at least three times at 5-min intervals to verify that there is no systematic drift in the emf. The potentiometer circuit should be checked against the standard cell immediately before each reading. The emf should be determined as a function of temperature at four values in the range from 0 to 40°C, and an ice bath can be used for the 0°C measurement. 

Procedure. The furnace should be turned on the day before measurements are to be made so that it will achieve a steady temperature. Set the power supply to a predetermined voltage appropriate for the desired reaction temperature. The heating current should be measured with a series ammeter and recorded periodically. At the start of the experiment, place the reference thermocouple junction in a Dewar flask filled with ice and distilled water (see Chapter XVII) and connect the thermoconple to a potentiometer or digital voltmeter (see Chapter XVI). Measure the thermocouple emf and check to see if the furnace is at the proper... 

Because of the high resistance of the glass membrane (10 to 100 MO), it is not practical to measure the emf directly. Instead, pH meters either use a direct-reading electronic voltmeter or electronically amplify the small current that flows through the cell and detect the voltage drop across a standard resistor potentiometrically. Both battery-operated and ac line-operated pH meters are available connnercially from such firms as Beckman Coulter, Thermo Orion, and Coming. Such pH meters are calibrated to read directly in pH units, have internal compensation for the temperature coefficient of emf, and have provision for scale adjustments. 

Figure 2. Variation of emf [Ef = Eob8 + (2RT/F) In m] with molality and temperature in 8.68% monoglyme...

Isopiestic measurements showed that the water activity in H20/ NMA solvent mixture of x2 = 0.5 is the same, within experimental error, as that predicted by Raoult s Law. Hence, the emf was corrected to 1 atm partial pressure of hydrogen on the assumption that NMA is involatile and that all of the solutions obey Raoult s Law. The observed emf for each HBr solution at each solvent composition and temperature, corrected as described above, is listed in Table II. 

Pownceby M. 1. and O Neill H. St. C. (1994) Thermodynamic data from redox reactions at high temperatures IV. Calibration of the Re-Re02 oxygen buffer from EMF and NiO -I- Ni — Pd redox sensor measurements. Contrib. Mineral. Petrol. 118, 130-137. 

A proper description of electronic defects in terms of simple point defect chemistry is even more complicated as the d electrons of the transition metals and their compounds are intermediate between localized and delocalized behaviour. Recent analysis of the redox thermodynamics of Lao.8Sro,2Co03. based upon data from coulometric titration measurements supports itinerant behaviour of the electronic charge carriers in this compound [172]. The analysis was based on the partial molar enthalpy and entropy of the oxygen incorporation reaction, which can be evaluated from changes in emf with temperature at different oxygen (non-)stoichiometries. The experimental value of the partial molar entropy (free formation entropy) of oxygen incorporation, Asq, could be... 

Modem digital voltmeters allow the measurement of highly accurate emfs. If the emfs are measured over a range of ionic strengths this will allow the effect of ionic strength on the equilibrium constants to be studied in detail. Furthermore if the dependence of emf on temperature is measured, the thermodynamic quantities. A//, A5 and AC can be found and interpretations of the magnitudes made. If the experimental set-up can be adapted to a determination of the emfs at various pressures to be made, then AV values can also be found. 

The digital data were transferred from punched paper tape to cards and the EMF versus temperature tables for the compensated thermocouple were... 

The system is designed to collect up to six channels of analog information, as a function of time, from each thermobalance. Nominal collection rate is one data set logged alternately from each instrument every 5 sec for a per instrument rate of six sets per min. Data acquired from the two thermobalances is converted to actual units, such as temperature in °C. and so on. and stored in two arrays of 100 data sets, with one array being assigned to each instrument The conversion of the thermocouple EMF into temperature is based on two polynomials, one for the PtRhlO%-Pt system and the other for NiCr-Ni and stored in the data acquisition program. When the two arrays are filled, they are automatically recorded on tape. 

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