Drop calorimetry method

Data for the temperature dependence of the enthalpy of UO2, obtained using drop calorimetry methods, showed a clear peak in the specific heat at 2610 K [87]. However, the structural origin of this feature was questioned by some groups, who proposed an alternative explanation in terms of electronic disorder (small polarons of... 

The heat capacity of CSe2(cr, 1) was studied within the temperature range of 78 to 328 K with the drop calorimetry method, and the data were reproduced by the equations ... 

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. 

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. 

In indirect methods, heat content measurements are performed over a large temperature range, for instance by drop calorimetry, and Cp is derived by the analytic derivation of heat content plots versus temperature. 

As a matter of fact, drop calorimetry has been largely superseded by differential scanning calorimetry (DSC) ( 5.6.2), but 1 include a description here because it illustrates the acquisition of high temperature enthalpies and heat capacities more intuitively than does DSC, and because much of presently used data were obtained by this method. 

Calorimetry is generally a high-accuracy method for determination of thermodynamic properties. A particular variant is the high-temperature drop calorimetry where the oxide phase is dissolved in a suitable melt and enthalpies of formation from oxides are evaluated from the thermal effect. Drop-calorimetry data (Rian 1992, Zhou and Navrotsky 1992, Lamberti et al. 1997) obtained for phases fiom the R(0)-Ba(0)-Cu(0) phase diagram are summarized in table 26. Similar, but much more scattered data were obtained in numerous early studies by various aqueous-solution calorimetry techniques for recent more precise data by the latter technique on RBa2Cu306.9 (R = Gd, Ho, Y), see Matskevich and McCallum (1999). Only a few adiabatic calorimetry studies have focused on determination of thermodynamic properties of the superconducting phases. 

The available experimental data on Cp(RCl3,liq) were obtained either by drop calorimetry measurements of H° T) — H°(298) or by differential scanning calorimetry with the use of suitable references. Both methods give fairly high instrumental accuracy (1-3%) 0acobson et al., 1999). Nevertheless, analysis of the whole series of results obtained does not reveal patterns in the variation of heat capacity for the series of lanthanide trichlorides unless additional data on Cp(RF3,liq), the structure of the liquid phase formed in melting, and the influence of the excess contribution of the excited electronic states of molecules on the heat capacity values are taken into account. 

The most widely used method for the measurement of the constant pressure molar heat capacity of molten salts, Cp, was drop calorimetry, in which a sample preheated to some definite temperature imparted its heat to a fluid in the calorimeter, raising its temperature. The expected accuracy of the measurements was 1 %. An alternative method introduced more recently is differential scanning calorimetry, but it has a somewhat lower accuracy (uncertainty >2 %), due to the smallness of the samples employed. It turned out that Cp of molten salts varied generally hardly at all with the temperature as it was reported in [2], [175], and [202] and shown in Table 3.11. 

In the laboratory, values are determined by measuring the change in heat content between room temperature (or calorimeter temperature - calorimeter is the measuring apparatus for heat) and a number of higher temperatures. The calorimeter commonly used for this purpose is the isothermal ( isoperiboF is a more appropriate term) room temperature calorimeter, which uses the drop method. The substance is heated to the desired temperature and is dropped into the calorimeter and the heat effect is measured. This technique is commonly referred to as the mixture method in calorimetry. 

Accurate experimental enthalpies of formation of iV-suhstituted imidazoles were measured using static bomb combustion calorimetry, the vacuum suhlimation drop method, and the Knudsen-effusion method <1999PCA9336>. 

Mixing of liquids with solids. The simplest method to measure the heat at mixing a certain amount a of a liquid salt A, considered as the solvent and kept at the experimental temperature 7e, with the weighted amount b of a solid salt B kept at the room temperature To, is the drop method. This method is very easy to operate and has been described already by Kubashewski and Evans (1964) and was applied to micro-calorimetry. The schematic description of this method is shown in Eigure 4.7a. The measured enthalpy corresponds to the enthalpy of mixing according to the equation... 

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... 

In 1972 Pittam and Pilcher [24] remeasured the enthalpy of combustion of methane using the same method (flow calorimetry) but with a much purer sample and a better analytical method (weighing the CO2). Their six determinations are shown on the right side of Figure 1. This is abetter measurement, but the scatter in the data is similar to that in the study by Rossini methane is very difficult to bum completely in a calorimeter [33]. When the two data sets are considered together neither stands out, and it is no longer certain that the point rejected by Rossini can be dropped. In the new assessment, the recomended value is an average of all twelve points. 

NMR, up to 180 °C by Shinoda et al. [52]. Some work has been reported involving erne determination by calorimetry (measuring heats of dilntion or specific heats). Archer et al. [53] nsed flow calorimetry to determine the erne s of several sulfonate surfactants at up to 178 °C. NoU [5J] determined erne s for dodecyltrimethylammonium bromide and commercial surfactants in the temperature range 25—200 °C using flow calorimetry. Surface tension is the classical method for determining erne s but many surface tension methods are not suitable for use with aqueous solutions at elevated temperatures. Exceptions include the pendant, sessile, and captive drop methods which can be conducted with high-pressure cells [54, 55]. 

Basically, the methods consist of a variety of calorimetric methods and a few non-calorimetric methods. In calorimetry the following methods are nsed adiabatic, isoperibol, isothermal, heat condnction, drop and differential scanning calorimeters, and differential thermal analysis. Cryoscopic, vapor pressure, and enthalpy of solution methods are considered to be non-calorimetric methods. 

The excess molar enthalpies of the BaF2-LiF-NdF3 system were measured in the temperature range 1220-1400 K principally by direct-reaction calorimetry with the direct drop method [16]. A high-temperature calorimeter operable up to 1800 K was used for excess molar enthalpies determinations it has been described several times [17]. 

---