Drop calorimetry

Heat capacities at high temperatures, T > 1000 K, are most accurately determined by drop calorimetry [23, 24], Here a sample is heated to a known temperature and is then dropped into a receiving calorimeter, which is usually operated around room temperature. The calorimeter measures the heat evolved in cooling the sample to the calorimeter temperature. The main sources of error relate to temperature measurement and the attainment of equilibrium in the furnace, to evaluation of heat losses during drop, to the measurements of the heat release in the calorimeter, and to the reproducibility of the initial and final states of the sample. This type of calorimeter is in principle unsurpassed for enthalpy increment determinations of substances with negligible intrinsic or extrinsic defect concentrations... 

Johnson et al. [143] used low-temperature adiabatic calorimetry and high-temperature drop calorimetry to obtain the heat capacity of both forms of mordenite as a function of the temperature. These results and the results of the reaction-solution calorimetric studies discussed herein, enabled the tabulation of the thermodynamic properties (C°, S°, Af H°, and Af G°) of mordenite from 0 K to 500 K and dehydrated mordenite from 0 K to 900 K. 

Figure 6.3. Levitation of a molten metal in a radio-frequency field. The coil consists of water-cooled copper tubes. The counter winding above the sample stabilizes levitation. The same coils (and possibly additional ones) act as the induction heater. This technique has been applied to container-less melting and zone refining of metals and for drop calorimetry of liquid metals. It can be also used to decarburize and degas in ultrahigh vacuum mono-crystalline spheres of highly refractory metals (adapted from Brandt (1989)). The arrows indicate the instantaneous current flow directions in the inductors.

Marchidan, D.I. and Ciopec, M. Relative enthalpies and related thermodynamic functions of some organic compounds by drop calorimetry, J. Therm. Anal. Calorlm., 14(1-2) 131-150, 1978. 

Another method to obtain enthalpies of formation of compounds is by solute-solvent drop calorimetry. This method was pioneered by Tickner and Bever (1952) where the heat formation of a compound could be measured by dissolving it in liquid Sn. The principle of the method is as follows. If the heat evolved in the dissolution of compound AB is measured, and the equivalent heat evolved in the dissolution of the equivalent amount of pure A and B is known or measured, the difference provides the enthalpy of formation of the compound AB. Kleppa (1962) used this method for determining enthalpies of formation of a number of Cu-, Ag- and Au-based binaries and further extended the use of the method to high-melting-point materials with a more generalised method. 

There are only a few measurements of the heat capacity of the liquid tribromides and triiodides and most of them have been made by Dworkin and Bredig (1963a, 1963b, 1971) using drop calorimetry. Rycerz and Gaune-Escard (1999a) measured the heat capacity of LaBr3(l)... 

The enthalpies of fusion of some hexagonal tribromides and orthorhombic triiodides have been measured by drop calorimetry (Dworkin and Bredig, 1963a, 1971). The en-... 

Pulse calorimeters pass electrical current through an electrically conducting sample to force a temperature increase, which is measured along with the voltage drop across the sample. If the heat loss from the sample is known (or estimated by calibration), the energy input divided by the temperature increase determines the true heat capacity, if the temperature change is small. Pulse calorimetry eliminates many of the drawbacks of drop calorimetry. It is fast, reproducible, and, with proper calibration, accurate. However, its use is limited to conductive materials. 

High-temperature enthalpies of the B-phase have been determined by drop calorimetry by O Brien and Kelley (1 )... 

High temperature enthalpies were measured by drop calorimetry by Holm (5, 1046.6-1172.9 K). 

McDonald ( ) measured high temperature enthalpy data of the liquid from 1118 to 1707 K by drop calorimetry. The adopted heat capacities are derived from his observed data. The average deviation of the observed enthalpy data from the adopted values Is about 0.1%. A glass transition Is assumed at 745 K below which the Cp Is assumed to be the same as that of the crystal. 

Southard (5) and Krasovltskaya et al. (6) determined enthalpy changes by drop calorimetry in the temperature regions 381.7-1776.8 K and 1015-2154 K, respectively. 

Solution calorimetry (2) gave A H = 1.125 0.016 kcal mol for BeF2(low quartz) BeF2(vitreous) at 298.15 K. Confirmation of ths result came from transposed temperature-drop calorimetry (p and calorimetric conversion (8). These gave... 

Existence of two forms of SrBr has been shown by thermal analysis (15-18) and drop calorimetry (14). Values of reported are 915 K (15-16), 920 K (17), 919 K (18), and 918 K (14). We adopt the last value which is based on the drop calorimetry of Dworkin and Bredig (14). Several other Investigators (1, 19-21) have mistakenly interpreted the transition as due to melting. 

T g is that observed by Dworkin and Bredig ( ) from drop calorimetry. Other reported values for T are 926 K (2),... 

The low temperature heat capacity, 14.0-315 K was measured by Getting (7). Janz et al. (8) measured the heat content by drop calorimetry in the temperature range 630-1250 K, and gave enthalpy and heat capacity equations based on their measurements. The above information was used in a Shomate analysis in order to smooth the enthalpy and calculate heat capacity above 298.15 K. The values from the low and high temperature sources join smoothly at 298.15 K. The heat capacity was graphically extrapolated above the melting point. The entropy at 14.0 K was calculated from the extrapolated low temperature... 

The adopted melting point 1045 K was determined by Tokareva ( ). Dworkin and Bredig ( ) determined the enthalpy of fusion Afusf (1045 K) 6.78 0.1 kcal mol" by drop calorimetry. Chiotti et al. (27) also measured the enthalpy of fusion Ajus ( 55 6.79 0.2 kcal mol" in an adiabatic calorimeter. The adopted enthalpy of fusion, 6.822 kcal mol" (28.543 kJ mol" ), is calculated from the difference between the observed relative enthalpy of the liquid and the adopted value for the crystal at the melting point. 

Moore (8) measured high temperature enthalpy data from 670.5 to 941 K by drop calorimetry. The low temperature heat capacities and high temperature enthalpy data were smoothly joined at 298.15 K. The C values above 941 K were obtained by graphical extrapolation. Getting and Gregory ( ) measured high temperature heat capacities in the temperature range from 60 to... 

Todd (9) measured the low temperature heat capacities from 52.6 to 296.7 K, and made an extrapolation to 0 K which yielded an entropy of 8.12 cal mol" at 51 K. We adopt the measured heat capacities but make our own extrapolation to 0 K, based on the ratio of the measured heat capacities of ZrF (10), TiP (1 1) and TiCl ( ) from 6 to 50 K. This extrapolation gives S (50 K) = 6.758 0.7 cal K" mol which is adopted. Coughlin and King ( 3) measured high temperature enthalpy data from 335.9 to 566.8 K by drop calorimetry. Their data are smoothly joined with Todd s low temperature heat capacities. 

Welty (5) has measured the enthalpy changes for WClg(a, cr) in the temperature range from 406 to 502.4 K by drop calorimetry. 

Welty (1 ) has measured the enthalpy changes for fClg(p,cr) in the temperature range from 508 to 553 K by drop calorimetry. Because of the short temperature range, poor distribution of pints and lack of identification of the phase present at the conclusion of each drop, we feel that the enthalpy data are insufficient to define the heat capacity accurately. The adopted heat capacities are estimated so that they are consistent with the enthalpy data within their probable uncertainty. 

Westrum and Pitzer (1 ) measured high temperature enthalpy data by drop calorimetry in a narrow range of 510.6-523.2 K and derived a constant heat capacity of approximately 25 cal mol which is adopted in the tabulation. 

From their drop calorimetry. Powers and Blalock (6) selected a melting point of 783.15 K where they derived A. H = 5.23 —1 —1 rus... 

Tj g has been reported to be 953, 952, and 954 K by Ray and Dayal (1 ), Phipps and Partridge (1 ), and Johnson and Bredig (18. respectively. The latter value is adopted. The enthalpy of fusion was determined by Dworkin and Bredig (IJj ), using drop calorimetry... 

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