Washburn corrections

The nature of the Washburn corrections is illustratedby the thermochemical cycle in figure 7.4. It can be concluded from this cycle that... 

Figure 7.5 General scheme of the initial-state Washburn corrections for a combustion reaction involving a CaHbOcNd compound.

Table 7.1 Auxiliary data for the Washburn corrections associated with the combustion of 4-cyanopyridine N-oxide (see text and figures 7.5 and 7.6)... 

The Washburn corrections for the initial state, At/wi (figure 7.5) correspond to the energy changes for bringing the bomb contents from their standard state to the initial bomb conditions. The traces of N2 inevitably present as an impurity in the O2 are ignored in the computation. 

Step 9 completes the Washburn corrections for bringing the reactants to the initial bomb conditions ... 

The bomb process is then considered to occur isothermally at 298.15 K, with a corresponding energy change A(/ibp(298.15 K). In the final state the bomb contents are a gaseous mixture of 02, N2, C02, and H20, and an aqueous solution of 02, N2, C02, and HNO3 (figure 7.6). The Washburn corrections for the final state include the following steps. 

Step 26 completes the Washburn corrections that bring the products from the final bomb conditions to their standard states. For our example,... 

It is common practice to separate the term relative to the decomposition of nitric acid from all other Washburn corrections, usually represented by AUw-The net correction is denoted by AIJs ... 

The experiments are usually carried out at atmospheric pressure and the initial goal is the determination of the enthalpy change associated with the calorimetric process under isothermal conditions, AT/icp, usually at the reference temperature of 298.15 K. This involves (1) the determination of the corresponding adiabatic temperature change, ATad, from the temperature-time curve just mentioned, by using one of the methods discussed in section 7.1 (2) the determination of the energy equivalent of the calorimeter in a separate experiment. The obtained AT/icp value in conjunction with tabulated data or auxiliary calorimetric results is then used to calculate the enthalpy of an hypothetical reaction with all reactants and products in their standard states, Ar77°, at the chosen reference temperature. This is the equivalent of the Washburn corrections in combustion calorimetry... 

Standard procedures were followed, and experimental details will be published elsewhere. The mean value found for Al/C was —31.803 kjoules per gram, with a standard deviation of 0.015%. Washburn corrections and conversion to constant-pressure conditions yielded AHc° = —1506.5 0.5 kcal. per mole, and AHf° = —57.7 d= 0.5 kcal. per mole. 

COOH)2 (c). The heat of combustion of crystalline oxalic acid was measured by Stohmann, Kleber, and Langbein,1 Jahn,2 and Verkade, Hartman, and Coops.2 The data of the last named investigation were reviewed by Washburn,2 who showed that the correction to a pressure of 1 atmosphere (from the conditions of the bomb process) is —0.66 per cent. With the Washburn correction to 1 atmosphere, the data of Verkade, Hartman, and Coops2 yield 59.7 for the heat of the reaction, (COOH)2 (c)+102 (g) =2C02 (g)+H20 (liq.) at 18° and a constant pressure of 1 atmosphere. 

CHsCOOH (liq.). The heat of combustion of liquid acetic acid was measured by Berthelot and Matignon8 and Roth,2 and of gaseous acetic acid by Thomsen.15 Applying a Washburn correction of —0.10 per cent, we have computed Roth s2 value to be 206.7 for the heat of combustion of the liquid, and that of Berthelot and Matignon3 to be 209.2. Thomsen s15 data yield 227.4 for the heat of reaction of gaseous acetic acid at 111° with oxygen at 18° to form gaseous carbon dioxide and liquid water at 18°. 

W. N. Hubbard, D. W. Scott and G. Waddington, in Experimental Thermochemistry Measurement of Heats of Reaction (Ed. F. D. Rossini), Interscience, New York, 1956. These additional terms for an abitrary organic compound are customarily called the Washburn corrections , after E. W. Washburn, J. Res. Natl. Bur. Stand., 10, 525 (1933). 

In a typical experiment, if 500 mg of the compound is used, it must be weighed with an uncertainty of less than 50 ig. Usually the combustion process increases the temperature of the calorimeter by about 2 °C, which must be determined with an uncertainty of less than 0.2 millidegree. To ensure complete combustion, the contents of the bomb are slowly released through a carbon dioxide absorbent, which is then weighed. The ratio of the amoxmt of carbon dioxide found to that expected should be between 1.0001 and 0.9999. All the other experimental variables must be determined equally well. Finally, the data must be reduced to standard conditions (25° and 1 MPa) making use of the Washburn corrections [11]. It is important to emphasize that at least five experiments without detectable errors must be done. 

The net change is a decrease in the amount of each reactant in its standard state and an increase in the amount of each product in its standard state. The internal energy change of step 2 is AC/(IBP, T ef), whose value is fovmd from Eq. 11.5.8. The internal energy changes of steps 1 and 3 are called Washburn corrections. ... 

The Washburn corrections needed in Eq. 11.5.9 are internal energy changes for certain hypothetical physical processes occurring at the reference temperature T ef involving the substances present in the bomb vessel. In these processes, substances change from their standard states to the initial state of the isothermal bomb process, or change Ixom the final state of the isothermal bomb process to their standard states. 

In making Washburn corrections, we must use a single standard state for each substance in order for Eq. 11.5.9 to correctly give the standard internal energy of combustion. In the present example we choose the following standard states pure solid or liquid for the reactant compound, pure liquid for the H2O, and pure ideal gases for the O2 and CO2, each at pressure p° = Ibar. 

One of the chief difficulties associated with moving-bomb methods is that many of the physical properties necessary for rigorous evaluation of the Washburn correction to standard states are unknown. The relatively large volume of solution used in the bomb (10 to 60 cm ) means that the energy of solution of carbon dioxide is appreciable, thus in the combustion of organofluorine compounds in the presence of 10 cm of water it is typically... 

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