Temperature Solution Calorimetry

In a series of investigation, Arnett and coworkers have determined heats of ionization (AH ) of secondary and tertiary chlorides and alcohols in Sbt — SO2CIF solutions at low temperatures. Ihey have also measured heats of isomerizations of secondary to tertiary carbocations in superacid media. These measured thermodynamic data have been extremely useful in determining the intrinsic thermodynamic stabilities of secondary and tertiary carbocations and delocalized ions. 

---

Colinet C and Pasturel A 1994 High temperature solution calorimetry Solution Calorimetry, Experimental Thermodynamics vol IV, ed K N Marsh and PAG O Hare (Oxford Blackwell)... 

The solution experiments may be made in aqueous media at around ambient temperatures, or in metallic or inorganic melts at high temperatures. Two main types of ambient temperature solution calorimeter are used adiabatic and isoperibol. While the adiabatic ones tend to be more accurate, they are quite complex instruments. Thus most solution calorimeters are of the isoperibol type [33]. The choice of solvent is obviously crucial and aqueous hydrofluoric acid or mixtures of HF and HC1 are often-used solvents in materials applications. Very precise enthalpies of solution, with uncertainties approaching 0.1% are obtained. The effect of dilution and of changes in solvent composition must be considered. Whereas low temperature solution calorimetry is well suited for hydrous phases, its ability to handle refractory oxides like A1203 and MgO is limited. 

Low melting metals (Sn and also Bi, In, Pb, and Cd) are extensively used as solvents in calorimetric studies of metallic phases [35]. Transition metals do not, however, dissolve readily in tin [43] and other solvents such as Cu and A1 have been used. An experimental probe for high-temperature solution calorimetry is shown in Figure 10.8. 

Charlu T. V., Newton R. C., and Kleppa O. J. (1975). Enthalpies of formation at 970 K of compounds in the system MgO-AljOj-SiOj from high temperature solution calorimetry. Geochim. Cosmochim. Acta, 39 1487-1497. 

Circone S, Navrotsky A (1992) Substitution of " A1 in phlogopite high-temperature solution calorimetry, heat capacities, and thermodynamic properties of the phlogopite-eastonite join. Am Mineral 77 1199-1205... 

The enthalpy of formation of ZrSi04(s) from its constituent oxides has been determined at 977 K by Ellison and Navrotsky [92ELL/NAV] using high temperature solution calorimetry. They determined an enthalpy of reaction at this temperature of -(27.9 1.9) kJ-moP. A value for the enthalpy of reaction at 298.15 K was then determined from this value and the heat capacities of ZrSi04(s), Zr02(s) and Si02(s) which, in turn, was used to calculate the enthalpy of formation. The enthalpy of formation determined in the study and its associated uncertainty are selected by this review ... 

The book begins with a chapter on calorimetry (Navrotsky) followed by two chapters on the experimental determination of activity-composition relationships of mineral solid solutions by phase equilibrium experiments (Wood) and by high temperature solution calorimetry (Newton). After chapters on the nature of activity-composition relationships (Powell) and the expression of non-ideal behaviour using Margules equations (Grover), a review of experimental techniques available for determining site occupancy is given (Whittaker). 

Reviews of batch calorimeters for a variety of applications are published in the volume on Solution Calorimetry [8] cryogenic conditions by Zollweg [22], high temperature molten metals and alloys by Colinet andPasturel [19], enthalpies of reaction of inorganic substances by Cordfunke and Ouweltjes [16], electrolyte... 

Solution Polymers. Acryflc solution polymers are usually characterized by their composition, solids content, viscosity, molecular weight, glass-transition temperature, and solvent. The compositions of acryflc polymers are most readily determined by physicochemical methods such as spectroscopy, pyrolytic gas—liquid chromatography, and refractive index measurements (97,158). The solids content of acryflc polymers is determined by dilution followed by solvent evaporation to constant weight. Viscosities are most conveniently determined with a Brookfield viscometer, molecular weight by intrinsic viscosity (158), and glass-transition temperature by calorimetry. 

Differential scanning calorimetry (DSC) showed this to be prone to highly exothermic decomposition (100 J/g) at ambient temperatures. Solutions are a little more stable. 

One of the most powerful techniques available for the evaluation of the solubility attributes of polymorphic or solvate species is solution calorimetry. Each distinct crystal phase is characterized by a well-defined heat of solution as it dissolves into a given solvent system, and the difference between the heats of solution of each phase in the same defined solvent system equals the heat of transition between them at the temperature of measurement. Solution calorimetry has been used to complement or to investigate a wide variety of crystal properties,... 

When one polymorph can be thermally converted to another, differential scanning calorimetry (DSC) analysis cannot be used to deduce the heat of transition between the two forms, and so solution calorimetry represents an alternative methodology. This situation was encountered when evaluating the polymorphs of losartan [140], Enthalpies of transition were obtained in water (A(A//sol) = 1.723 kcal/mol) and in A A-dimethylformarnide (A(A//S0 ) = 1.757 kcal/mol), with the equivalence in results demonstrating the quality of the results. Although enthalpy does not indicate stability, the authors deduced from solution calorimetry that form I was more stable than form II at ambient temperature. 

The enthalpies of phase transition, such as fusion (Aa,s/f), vaporization (AvapH), sublimation (Asut,//), and solution (As n//), are usually regarded as thermophysical properties, because they referto processes where no intramolecular bonds are cleaved or formed. As such, a detailed discussion of the experimental methods (or the estimation procedures) to determine them is outside the scope of the present book. Nevertheless, some of the techniques addressed in part II can be used for that purpose. For instance, differential scanning calorimetry is often applied to measure A us// and, less frequently, AmpH and AsubH. Many of the reported Asu, // data have been determined with Calvet microcalorimeters (see chapter 9) and from vapor pressure against temperature data obtained with Knudsen cells [35-38]. Reaction-solution calorimetry is the main source of AsinH values. All these auxiliary values are very important because they are frequently required to calculate gas-phase reaction enthalpies and to derive information on the strengths of chemical bonds (see chapter 5)—one of the main goals of molecular energetics. It is thus appropriate to make a brief review of the subject in this introduction. 

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. 

Solution calorimetry, 24 11-15 in bromine ttifluoride, 24 12-14 in fluorosulfuric acid, 24 11-12 in water, 24 14-15 Solution-phase reactive intermediates flow systems, 46 159-160 low temperature, 46 131-136 Solution X-ray scattering measurements, transferring, 41 409-410 Solvation, ionic, 21 211-213 Solvents... 

It has been reported that the y-form, which melts at approximately 101°C, is the thermodynamically most stable form at room temperature and that the sorbitol system is monotropic [8]. Studies of the various forms show that upon standing or upon stress conditions, sorbitol will convert to the y-form [7]. However, a solution calorimetry study performed on selected crystal forms of sorbitol reports that sorbitol hydrate is the most stable form followed by the y-form [18]. 

The heat of solutior Hs, represents the heat released or absorbed when a mole of solute is dissolved in a large quantity of solvent. It can be determined by solution calorimetry or from solubility values forsaturated solutions equilibrated at controlled temperatures overthe range of interest. The working equation fordetermininekHs is... 

---