Graphite calorimeter

C06-0016. When 0.100 g of graphite (elemental carbon) bums in the calorimeter of Example, the temperature rises from 23.5 °C to 29.9 °C. Determine the molar energy of combustion of graphite. 

Safety studies of the graphite anode samples were performed using a Perkin-Elmer Differential Scanning Calorimeter (DSC, model Pyris 1) instrument. The temperature scanning rate was 10 C/min over a temperature range of 50 to 375°C. 

Calorimeters Temperature measurements Graphite, water, polystyrene lOi-lO i... 

The calibration is done by national or secondary standards laboratories. Calibration at the National Institute of Standards and Technology (NIST) involves exposure to either gamma ray source ( Co) over the dose range 10° to 10 Gy, or high-energy electron beam (1 to 28 MeV) combined with a graphite or water calorimeter. ... 

Thin film dosimeters are calibrated by a calorimeter. For example a totally absorbing graphite body calorimeter is reported to be suitable for measuring energy deposited from electron accelerators operating within the range 4-400 MeV.1CM2... 

Borosilicate glass is a range of glasses based on boric oxide, silica, and a metal oxide. It has excellent thermal shock resistance and chemical resistance. A recent patent claimed the use of borosilicate glass powder (50-100 phr) in conjunction with expandable graphite (100 phr) or vermiculite in polyolefin, epoxy, or elastomers to achieve good fire retardancy (as evidenced by the Cone Calorimeter test at 35 kW/m2).99... 

For a reaction studied under conditions of constant pressure, we can obtain the enthalpy change by using a calorimeter. However, this process can be very difficult. In fact, in some cases it is impossible, since certain reactions do not lend themselves to such study. An example is the conversion of solid carbon from its graphite form to its diamond form ... 

In the same vein, fig. 2.28 Illustrates the adsorption of long chain n-alkanes from n-heptane on different grades of graphite, to compare their surface properties. One of these adsorbents, Vulcan 3G, was a standard sample especially prepared under lUPAC auspices. The enthalpies were measured as displacement enthalpies in a flow calorimeter. All enthalpy and adsorption isotherms are S-shaped. In fig. 2.28a, right, is plotted as a function of for the... 

Calorimeters temperature measurement graphite, water, polystyrene... 

Burns, D.T. and Morris, W.T., Recent developments in graphite and water calorimeters for electron beam dosimetry at NPL, Proc. Int. workshop on Water Calorimetry, Report NRC-29637, National Research Council of Canada, Ottawa, Canada (1988) 25-30. 

H2(g) + C(cr) + 4HCl(g). They calibrated their calorimeter by means of the reaction H2(g) + Cl2(g) + 2HCl(g) for which they took AjH (293 K) - -44 kcal mol. Their data have been re-evaluated to give AjH (CC1, g, 298.15 K) = -25.68 kcal mol , but no correction is applied for the enthalpy of formation of carbon which was not the reference state "graphite" formed in the reaction. Domalski and Armstrong ( reviewed several cases of active carbon formed in similar reactions and quoted a enthalpy of formation of carbon from +1.9 to +3.95 kcal mol. If this correction is applied to the data of Bodenstein, their enthalpy of formation of CCl (g) should be 2 to 4 kcal mol less negative than -25.68 kcal mol . ... 

In principle, you can determine A// for any chemical reaction by using a calorimeter to measme the heat evolved or absorbed during the reaction. However, consider the reaction involving the conversion of carbon in its allotropic form diamond to carbon in its allotropic form graphite. 

FOLLOW-UP PROBLEM 6.5 A chemist burns 0.8650 g of graphite (a form of carbon) in a new bomb calorimeter, and CO2 forms. If 393.5 kJ of heat is released per mole of graphite and T increases 2.613 K, what is the heat capacity of the bomb calorimeter ... 

This reaction has three strikes against it as far as. its use in calorimetry is concerned. The combination of graphite with hydrogen does not occur readily if we did manage to get these materials to react in a calorimeter, the product would not be pure methane, but an exceedingly complex mixture of hydrocarbons. Even if we succeeded in analyzing the product mixture, the result of such an experiment would be impossible to interpret. 

Sometimes it is impossible or impractical to measure the AH of a reaction by using a calorimeter. Consider the reaction in Figure 15.12, the conversion of carbon in its aUotropic form, diamond, to carbon in its allotropic form, graphite. 

Calorimeters can be used as in-house standards for the calibration of other dosimeters. It is crucial to pay special attention to the irradiation geometry to ensure the same dose to the calorimeter and to the dosimeter being irradiated. The thickness of the calorimeter (i.e., the absorber) must be chosen so that, for unidirectional perpendicular electron beams, the absorbed dose measurement is the average dose on the ascending part of the depth-dose curve. Phantoms of, e.g., polystyrene have been built of similar size as the water, graphite, or polystyrene calorimeters allowing the secondary dosimeter to be placed at depths of interest to provide the same irradiation conditions both for the absorber and the reference or routine dosimeter. 

In medical applications, it is of basic significance that the radiation absorption characteristics of the medium to be irradiated and that of the calorimetric absorber should be similar. Thus, calorimeters to that purpose have been constructed of water (Domen 1982), graphite (Petree and Lamperti 1967), polystyrene, polyethylene-carbon mixture (Milwy et al. 1958), and A-150 tissue-equivalent plastic (McDonald et al. 1976 Smathers et al. 1977). 

Semi-adiabatic calorimeters have been designed for dosimetry at high-energy electron accelerators (1-10 MeV) both for calibration and for routine process control (Humphreys and McLaughlin 1989 Miller and Kovacs 1985 Burns and Morris 1988) and also for low energies between 100 keV and 500 keV (Janovsky and Miller 1987). The disc-shape absorber is either water or graphite containing thermistors for temperature measurement placed in the center of the absorber. The absorber is placed in polystyrene foam insulation. 

Calorimeters have been designed for use at industrial electron accelerators for dose measurement in the range of 1.5-50 kGy and 4-10 MeV electron energy. (ISO/ASTM 51631 2003b). These calorimeters are made with water, polystyrene (see O Fig. 49.6), or graphite calorimetric body and are calibrated by comparison with transfer reference dosimeters... 

Differential calorimeters consisting of two graphite discs (with separate thermocouples each) have been designed and used mainly for cahbration of routine dosimeters (Janovsky 1985). During irradiation, only one of the two graphite discs is irradiated, while the other one is used as reference. The temperature difference measured between the two discs serves to calculate the dose. Radak et al. (1973) designed similar instrument for calibration at low electron energies (400 keV). 

Later work with various binary liquid mixtures and different types of solids indicated that the flow calorimeter methods of specific surface determination can also be applied to homogenous surfaces represented by graphitised carbon blacks. In this case strong preferential adsorption of long-chain paraffins from n—heptane was used to produce a monolayer and the corresponding heat of adsorption measured the total graphitic basal plane in such carbons [11]. 

It would not be practical to measure the molar enthalpy of this last reaction by allowing graphite to react with oxygen in a calorimeter, because it would be difficult to prevent the formation of some CO2. From Hess s law, the standard molar enthalpy of formation of CO is the sum of the standard molar enthalpies of the reactions that have the formation reaction as the net result ... 

FIG. 6 Instrumental response curves (calorimeter signals and refractive index detector signals) of the step-by-step displacement of water (2) by n-octyltrimethylammonium bromide (1) on Vulcan 3G graphitized carbon black at 298.15 K. The adsorption path is followed by the desorption path. The concentration increments are indicated in the figure in mmol/dml... 

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