Electrical calibration, combustion calorimeter

Electrical calibration has the advantage of being more flexible. It can afford s0 through equation 7.23 ifitisdone on the reference calorimeter proper. Flowever, it can also be performed on the initial or final state of the actual experiment leading to (e0 + ecl) or (e0 + ecf), respectively. Twenty or 30 years ago the electrical calibration required very expensive instrumentation that was not readily available except in very specialized places, such as the national standards laboratories. Although the very accurate electronic instrumentation that is available today at moderate prices may change the situation, most users of combustion calorimetry still prefer to calibrate their apparatus with benzoic acid. 

The obtained A 7 a() value and the energy equivalent of the calorimeter, e, are then used to calculate the energy change associated with the isothermal bomb process, AE/mp. Conversion of AE/ibp to the standard state, and subtraction from A f/jgp of the thermal corrections due to secondary reactions, finally yield Ac f/°(298.15 K). The energy equivalent of the calorimeter, e, is obtained by electrical calibration or, most commonly, by combustion of benzoic acid in oxygen [110,111,113]. The reduction of fluorine bomb calorimetric data to the standard state was discussed by Hubbard and co-workers [110,111]. 

The value of s (e, or sr) is usually determined by electrical calibration (note that contrary to combustion calorimetry, it is not common practice to separate the initial and final energy equivalents of the calorimeter into the contribution of the reference calorimeter, e0, and those of the contents present in the initial, C1, and, final, ecf, states see section 7.1). In the case of the calorimeter in figure 8.1, a current I is passed trough the resistance F for a known period of time t and the potential change V across F is measured. Then ... 

In a typical experiment, 56.39 mg of technetium caused a temperature rise of 0.0572°C on combustion. Electrical calibration of the calorimeter shows its water equivalent to be 5847 J/K, so this experiment gives a value of ... 

The energy equivalent of the calorimeter, (calor) is defined as the amount of energy required to increase the temperature of the calorimeter by 1 K. The most precise determination of (calor) is based on the transfer of a determined quantity of electrical energy through a heater placed at the same location as the combustion crucible. Because most of the calorimeters used are of the isoperibol type and are not equipped for electrical calibration, a standard reference material, benzoic acid, is used. Its certified energy of combustion in O2 must have been measured in an electrically calibrated calorimeter. Because the conditions under which the specific energy of combustion reported on the certificate was determined usually differ from those ones used in combustion calorimeters, certain corrections must be applied [31]. Details of these corrections are given in the certificate. 

The whole apparatus is contained in a water calorimeter, in which the rise in temperature produced by the combustion is measured. In calculating the heat of combustion, we must correct for the heat evolved in the combustion of the iron wire. The calorimeter may be calibrated with a substance of which we know the heat of combustion, or by reproducing with a known electric current the rise in temperature produced by the combustion. The heat of combustion is then equal to the electrical energy produced inside the cylinder. 

Figure 4.30 illustrates the liquid calorimeter. It also operates in an isoperibol manner. The cross-section represents a simple bomb or reaction calorimeter, as is ordinarily used for the determination of heats of combustion [8]. The reaction is carried out in a steel bomb, filled with oxygen and the unknown sample. The reaction is started by electrically burning the calibrated ignition wire. The heat evolved during... 

One of the major sources of energy for the generation of heat, electricity, or power is natural gas. For any (large) consumer of natural gas, it is thus of commercial interest to determine the amount of energy contained in a certain quantity of gas with low uncertainty. This amount of energy is determined by multiplying the quantity of gas with its heat of combustion. The heat of combustion is usually measured by means of field calorimeters that rely on calibration gases for reliable and traceable measurements. The uncertainty of the heat of combustion realized by the calibration gas must thus be considerably smaller than the required measurement uncertainty, which is here approximately 1%. 

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