Flame combustion calorimetry in oxygen

Flame combustion calorimetry in oxygen is used to measure the enthalpies of combustion of gases and volatile liquids at constant pressure [54,90]. Some highly volatile liquids (e.g., n-pentane [91]) have also been successfully studied by static-bomb combustion calorimetry. In general, however, the latter technique is much more difficult to apply to these substances than flame combustion calorimetry. In bomb combustion calorimetry, the sample is burned in the liquid state and must be enclosed in a container prior to combustion. Encapsulation may be difficult, because it is necessary to minimize the amount of vaporized compound inside the container as much as possible. In addition, volatile liquids tend to burn violently under a pressure of 3.04 MPa of oxygen, which leads to incomplete combustion. These problems are avoided in flame combustion calorimetry, where the sample is carried to the combustion zone as a vapor and burned under controlled conditions at atmospheric pressure. 

Particularly important compounds have been studied by flame combustion calorimetry. Methane [92-94], ethanol [95], diethyl ether [96], carbon monoxide [92,93,97], hydrochloric acid [98], and water [93,97,99] are representative examples. With a few exceptions (HC1, H2O, D2O [100], SO2 [101], cyanogen [102,103], and some lower chloroalkanes [104,105]), measurements by flame combustion calorimetry have been limited to substances of general formula CaHbOc. 

The study of reaction 7.73 is one of the most important thermochemical experiments ever made, and it will be briefly analyzed here to illustrate the flame combustion calorimetry method. The application of flame combustion calorimetry to hydrocarbons and other organic compounds containing oxygen, nitrogen, or chlorine is covered in detail in the review by Pilcher [90]. 

In flame calorimetry, it is not easy to measure directly with good accuracy the mass of reactants consumed in the combustion. Therefore, the results are always based on the quantitative analysis of the products and the stoichiometry of the combustion process. In the case of reaction 7.73, the H20 produced was determined from the increase in mass of absorption tubes such as M, containing anhydrous magnesium perchlorate and phosphorus pentoxide [54,99], When organic compounds are studied by flame combustion calorimetry, the mass of C02 formed is also determined. As in bomb calorimetry, this is done by using absorption tubes containing Ascarite [54,90]. 

For the combustion of hydrogen in an excess of oxygen the inlets D and F were connected to a H2 and a O2 cylinder, respectively. Inlet G was connected to a second O2 cylinder. This oxygen supply was used to flush the apparatus and vaporize the liquid water present in the burner vessel at the end of the experiment, so that it would be absorbed in the tube M and gravimetrically analyzed (see following discussion). Inlet E was not used in these experiments. 

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