Fluorine combustion "bomb

The AH° of reaction at 298.16° K. of oxygen difluoride with hydrogen was measured using a Parr fluorine combustion bomb modified to contain a metal ampoule employing a burst diaphragm. This modification permits heat of reaction measurements on systems where reaction occurs spontaneously upon mixing the reactants. The AH° of the reaction OFt - HtO + 2HF (infinite dilution) was found... 

Apparatus and Procedure. The thermochemical measurements were made using a Parr fluorine combustion bomb and a National Bureau of Standards (NBS) isothermal calorimeter (No. 63090) manufactured bv the Precision Scientific Co. The bomb cylinder and all internal parts or the bomb were Monel. A Monel ampoule was fitted into the top of the bomb to retain the OF2 sample. The ampoule apparatus reduced the internal volume of the bomb from 380 to 315 cc. The ampoule screws... 

The standard calorimeter system for this series of experiments was specified as the NBS calorimeter containing the Parr fluorine combustion bomb plus sample ampoule. The standard bomb was defined as the Parr fluorine combustion bomb plus sample ampoule minus the water, the benzoic acid pellet, the Hastelloy cup containing the pellet, the oxygen, and the nitrogen used to pressurize the ampoule. 

Only six other centers have operated similar metal bomb calorimeters, mainly modeled on the Hubbard design, and it is therefore of interest to note that Gross and co-workers have been intrepid enough to use a simple two-compartment glass apparatus separated by a break-seal for fluorine combustion (5 atm F2). Their results were in excellent agreement with those obtained in metal bombs (107). 

There have been far more thermochemical experiments carried out in fluorine than in any other halogen atmosphere, the large majority of them by fluorine bomb calorimetry [110-116]. Thus, only fluorine combustion calorimetry will be covered in this section with a strong emphasis on bomb calorimetry. Note, however, that many technical details and safety precautions mentioned here for fluorine combustion calorimetry also apply to combustion in other halogens. 

Most substances are attacked to some extent by F2. Therefore, combustion calorimetric samples must be isolated from fluorine until the start of the reaction period. However, it is virtually impossible to find protective materials that can be easily sealed around the sample, that are inert to fluorine, or that do not lead to complex mixtures of products on fluorination. The use of a two-compartment bomb, such as the one represented in figure 7.12, enabled these problems to be overcome and became the preferred option in most fluorine combustion calorimetric studies [111,113,114,122,123], Fluorine is contained in tank A, which surrounds the bomb body B. The bomb head C is attached to the bomb body by the cylindrical screw collar D. The bomb, in an inverted position (figure 7.12),... 

Figure 7.12 Scheme of a two-chamber bomb used in fluorine combustion calorimetry. 

The main auxiliary apparatus needed to perform fluorine combustion calorimetry consists of a fluorine still, a bomb charging and discharging manifold, and a glove box [110,111,113], The fluorine still and the manifold should... 

Much of the discussion of oxygen flame calorimetry presented in section 7.3 is directly applicable to fluorine flame calorimetry. As in the case of bomb calorimetry, however, the special properties of fluorine combustion systems and problems associated with handling fluorine require a somewhat different experimental method [109,115,116]. Thus, for example, a metal burner should be used. Also, the fact that the mixing of many gases with F2 may lead to spontaneous ignition hinders the use of a premixed flame. Fluorine combustion calorimetry has been used to study the thermochemistry of important reactions, such as... 

Determination of Mercury, Fluorine, Boron, and Selenium. The Determination of Mercury. A coal sample is decomposed by igniting a combustion bomb containing a dilute nitric acid solution under 24 atm of oxygen. After combustion, the bomb washings are diluted to a known volume, and mercury is determined by atomic absorption spectrophotometry using a flameless cold vapor technique. 

Fluorine bomb calorimetry is a development from the early 1960s. Before that time, reliable enthalpy data concerning fluorides were very scarce, principally because fluorine gas is so very reactive. Fluorine bomb calorimetry was extended to high-pressure (up to 15 atm of fluorine) metal combustion bombs by Hubbard and co-workers [2] at the Argonne National Laboratory (ANL) in the United States in 1961. The technique has been developed over the past 30 years, and is now comparable in precision and accuracy to the other types of calorimetry. Enthalpies of formation have been determined from direct fluorination experiments. 

The combustion of an organic fluorine compound in a bomb containing water (typically 10 cm3 for a bomb of 300 cm3 total volume) can be represented by the equation [61-63]... 

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 major differences between the fluorine and oxygen combustion calorimetry methods arise from the exceptional reactivity and toxicity of fluorine. The substances studied by oxygen combustion calorimetry are normally stable when kept inside a bomb at 298.15 K and under 3 MPa of O2. Oxygen- and moisture-sensitive compounds can also be studied because various types of containers are available to prevent their reaction with O2 prior to ignition. Common examples are glass ampules, which are inert toward the combustion process and, more commonly, Melinex bags or polyethene ampules, which burn cleanly to CO2 and H2O. As carbon dioxide and water are also generated in the combustion of the sample, no extra complexity is introduced in the analysis of the final state of the bomb process by the use of those plastic containers. 

The gaseous product (SF6) does not interfere with the analysis of solids that form in the main bomb process. Furthermore, the contribution of reaction 7.82 to A f/jgp can be accurately calculated lfom the mass of sulfur and the value of ArC/°(7.82) at 298.15 K [124], It may also be necessary, in some cases, to use an auxiliary substance to promote complete combustion of the compound under study [111,113]. Tungsten is a very common combustion aid it reacts cleanly with excess fluorine according to... 

As illustrated in this section, the problems associated with using fluorine in combustion calorimetry seem to have been largely overcome. The fluorine bomb and flame calorimetry methods have been perfected to such an extent that, provided the chemistry of the process under study is well characterized, results of very good accuracy and precision can be obtained routinely. 

W. D. Good, D. W. Scott, G. Waddington. Combustion Calorimetry of Organic Fluorine Compounds by a Rotating-Bomb Method. J. Phys. Chem. 1956, 60, 1080-1089. 

Total decomposition of the sample, the purpose of which is to release fluorine from inorganic or organic matrixes and convert it to fluoride ions, is usually a prerequisite for determining the amount of total fluorine. Commonly used procedures involve oxygen bomb combustion in a closed bomb [176,180], open ashing [181,182], alkali hydroxide or alkali carbonate fusion [151,183-187], pyrohydroly-sis [187-191], acid extraction [192,193] and microwave acid digestion [194-196]. 

Recently, Rudzitis et al. (6) measured the enthalpy of combustion of PF in fluorine in a two-chambered bomb calorimeter at... 

A beam type of vacuum microbalance constructed from aluminium, nickel, and fluorite parts has been described 13 it was used to study corrosion of Si by F2 and Si02 by HF. A simple, new two-chamber fluorine bomb calorimeter has the advantage that a high F2 concentration can be used during the combustion.14 The same group of Russian workers have measured the enthalpy of combustion of Cu and W in this way.15 Ader has determined the energy of the reaction ... 

Fluorine and chlorine were determined by ion selective electrodes after Parr bomb combustion, and sulfur was determined by Leco SC-32. 

In the case of fluorine, doubts were thrown on the reliability of the oxygen bomb combustion. This method might give correct results for the analysis of some coal matrices but fusion should be systematically used in the case of fly ash consequently. 

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