Drop calorimeters

A variation, which results in a more simple apparatus, is the drop calorimeter. The test piece is heated (or cooled) externally, dropped into the calorimeter and the resultant change in temperature monitored. For the simplest measurements, the calorimeter need not be surrounded by an adiabatic jacket but in that case, corrections for the heat exchange with the surroundings must be applied. A procedure using a drop calorimeter has been standardized for thermal insulation in ASTM C35l". It is possible to combine the adiabatic and drop calorimeter methods by dropping a heated sample into an adiabatic chamber and this has been used for plastics12. 

The drop calorimeter starts a sample at a high temperature (TH = 300°C to 1600°C) and then suddenly "drops" it into a bucket at room temperature the (small) rise in thebucket temperature is related to several initial values of TH. This is practical for high-temperature measurementsbuthas relatively lowprecision. 

Since the object of interest is dropped into the calorimeter, this type of calorimeter is often referred to as a Drop Calorimeter. Since the object of interest exchanges heat by mixing with the calorimeter object (liquid or solid) the technique is referred to as the Mixture... 

Tfor the determination of thermodynamic adsorption parameters, we used a drop calorimeter to measure specific heats and a conduction calorimeter to measure enthalpies of adsorption. 

In general, 2 types of calorimeters are used for measurement of specific heats, the adiabatic heated and the drop calorimeter. We chose the latter type for our study of specific heats because we needed the enthalpy function for our thermochemical calculations. This is the integral of the specific heat up to the experimental temperature, and cannot be measured directly by an adiabatically heated calorimeter. At higher temperatures the integration error will be considerable, so that... 

Direct measurements of the heat capacities of the system zeolite NaCaA-n-heptane have been made in the temperature range 25°-240°C with the drop calorimeter described above. 

McDonald and Stull (5) measured enthalpies in a copper block drop calorimeter, 20 points in the range 282.8 to 1667.8 K. The sample was reported to be the 0-rhombohedral form (based on X-ray data) with a metallic impurity of 0.59%. Stout et al. (6), also using a copper block drop calorimeter, measured enthalpies, five data points in the range 1820-2218 K. The sample was reported to have 0.04% metallic impurities and to contain 300 ppm 0 and 6300 ppm C. No information as to crystalline form was given. Wise et al. (9) measured the enthalpy of two crystalline boron samples (each reportedly containing some of the 0-modificatlon) in the temperature range 515-1103 K. 

Keneshea et al. ( ) measured the saturation enthalpy increments above 298.15 K for the condensed phases of NbClg in a drop calorimeter up to the critical point (804 3 K). A figure presented by Keneshea et al. (5) indicated roughly 30 data points, the lowest occurring at approximately 360 K. The differences between the saturation and standard enthalpy increments for the crystal phase are negligible, so that the heat capacity values which we adopt are those which are derived from the reported enthalpy equation, H (T) - H (298.15 K) - [-10.53 + 3.535 x 10 S] 0.07 kcal mol". This equation is reported to apply to the teraerature region 298.15 - 478.9 K. 

McDonald et al. ( ) measured the high temperature enthalpies of ZrF (cr) at temperatures 283.9-1225.8 K in a copper block drop calorimeter. Smith et al. (4) used a Bunsen ice calorimeter for the enthalpy measurements in the temperature range 273-1150 K. These two sets of enthalpy data are not in good agreement. It is possible that the discrepancies are due to the difference in crystal structure of the samples used (see "Transition Data" for more information). In order to join smoothly with the low temperature heat capacities at 298.15 K, the high temperature heat capacities derived from the enthalpy data of McDonald et al. 

In the same paper, Pankratz et al. ( ) reported measurements of the high temperature enthalpies (401-1794 K) for VN g which were obtained in a copper-block drop calorimeter. The subnitride sample was the same as that used in their combustion work and was contained in Pt-Rh alloy capsules during the "drop" experiments. Temperature measurements were based on the IPTS-68 scale. A technique employing orthogonal polynomials is used to fit their experimental enthalpies by computer. The curve is constrained to join smoothly with the low temperature C data near 298.15 K. The average deviation of the smoothed enthalpies from the experimental values is +0.46% the maximum deviation is +0.73% at 702 K. C data above 1800 K are obtained by graphical extrapolation. No anomalies are observed in either the low temperature C data or the high temperature enthalpies. 

Values for the enthalpies of fusion of alkali-metal cryolites may be obtained, using an aneroid, inverse-drop calorimeter with adiabatic shields.475... 

Reaction calorimeters are frequently calibrated using a known heat of a chemical reaction. No standard reaction is internationally accepted. For the measurement of heat capacities, drop calorimeters are frequently used and the calibration is made using a substance, the temperature dependence of which on heat capacity is known. As substances, metals like Cu, Ag, Au, and aluminum oxide in the form of sapphire are used. Calorimeters... 

In the high-temperature region, the main method of measurement is the drop calorimetry, where the sample is heated to the chosen temperature outside the calorimeter in a furnace and the heat capacity is calculated from the temperature dependence of the enthalpy changes measured after dropping the sample into the calorimeter. The application of this technique affects, however, the behavior of the sample heated in the furnace (decomposition, reaction with the crucible, etc. should be avoided) as well as at the cooling from the furnace temperature to that of the calorimeter. Sometimes the sample does not complete its phase transition at cooling (e.g. at the temperature of fusion, a part of the sample crystallizes while the other part becomes glassy). In such a case, the drop calorimeter must be supplemented by a solution calorimeter in order to get the enthalpy differences of all the samples to a defined reference state. 

Measurement of the heat content. Heat content determinations at high temperatures are generally easier than direct heat capacity measurements. They were widely used in early stages of measurement to obtain Cp data of molten salts. The enthalpy increment of a substance between temperatures T and T2, Hp2 — Hp T2 > T ), is measured in general using a drop calorimeter. Two techniques are employed, depending on the way the measurements are carried out. 

Similarly, Holm et al. (1973) measured the enthalpy increments Hr — 7/298 of the congmently melting compounds of 2 1 and 1 1 alkali metal chlorides and magnesium chloride using a high-precision adiabatic drop calorimeter. From the results obtained at several experimental temperatures corresponding with solid and liquid samples, they determined the enthalpies of fusion of these compounds. Values for heat capacities for the molten salt mixtures were also derived. The heat capacities for the 2 1 and 1 1 compounds were estimated from those of the binary compounds according to the relation... 

The solution of the sample in a 2 1 mixture of concentrated hydrofluoric and nitric acids at Tref = 298 K was chosen as the reference state. The relative enthalpy, 7/rei(7m), was measured by indirect method of double calorimetry. This procedure enables us to determine Hiei(Tm) as the sum of enthalpy increase measured during the cooling of the system in a drop calorimeter (Acooi and during its dissolution in a solution calorimeter (Asoi//). Equation (4.34) can thus be written in the form... 

The //rei values determined by double calorimetry are independent of the magnitude of both Acooif and Asoi//. In silicate systems, they often vary when one composition is repeatedly measured. This may happen because the value of Acooi (and hence that of Asoif7 also) depends on the non-reproducible state of the sample after cooling down in the drop calorimeter. The samples may often consist of a mixture of glass and crystalline phases including not only the components of the system but also the products of their decomposition. 

After measurement in the drop calorimeter, the crucible with the sample was opened by carefully cutting off the lid. The entire sample was removed, ground to the particle size less than 0.04 mm and homogenized. A part of the sample was subjected to X-ray powder diffraction and IR spectroscopic analyses. The heat of solution, Asoi77, of the sample was then measured in the solution calorimeter, described in detail by Proks et al. (1967). Approximately 0.05 g of sample was dissolved in 100 ml of the 2 1 mixture of concentrated hydrofluoric and nitric acids. The measurement of the heat of dissolution was repeated 3 times on an average. 

The heat content of Ag2Se(cr) and Cu2Se(cr) were measured in the temperature range 350 to 1500 K using a drop calorimeter. Tables with thermodynamic properties at different temperatures were presented but no analytical expressions for the temperature dependence of the heat capacity were given. The following expressions were derived by this review from least-squares refinements using the data in Tables 1 and 3 of the paper,... 

BIN/STR] Binford Jr., J. S., Strohmenger, J. M., Herbert, T. H., A modified drop calorimeter. The heat content of aluminum carbide and cobalt(ll) fluoride, J. Phys. Chem., 71, (1967), 2404-2408. Cited on page 431. 

The heat capacities of metallic thorium (containing 6.04 mass% Th02) and Th02 were measured in what is assumed to be a drop calorimeter. Two samples of thorium were used a rod, which was found to contain 6.04 mass% Th02, and a powder, found to contain 26.8 mass% Th02. Even after correction for the presence of thoria, the results for... 

The enthalpy of thorium dioxide relative to 273.15 K was measured at ten temperatures from 323 to 1173 K. A Bunsen ice drop calorimeter was used to make the measurements on two samples of widely different bulk densities. The corresponding heat-capacity values for the higher density sample are represented within their uncertainty (estimated to be +0.3 to 0.5%) by the following equation ... 

The enthalpy of Th02, sealed in tungsten, was measured from 2415 to 3400 K, using a modified high-temperature drop calorimeter. Enthalpies of a sample of molybdenum (NBS reference material) agreed within 1% of the NBS data. No details are given of the Th02 sample used. 

Enthalpy increments of Th02(cr) were measured using a high temperature Calvet drop calorimeter from 376 to 940 K (20 measurements). The Th02(cr), prepared by the oxalate route, contained less than 1000 ppm impurity, presumably including the 500 ppm MgO deliberately added as a sintering aid. 

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