Titration Calorimetry System

The dissipation of a known amount of electrical energy inside the calorimetric cell by means of a calibration coil (i.e., the Joule effect) is used to relate the area of the thermal peaks recorded to the enthalpy effects which this represents. The difficulty with this type of calibration in the titration calorimetry systems is related to the fact that the mass of solution in the calorimetric cell is constantly increased by successive injections, thereby changing the calorific capacity of the cell. Therefore, thermal calibration should be regularly repeated after each series of injections in the same calorimetric run. 

When a micellar stock solution (i.e., its concentration is 10 times the CMC) is injected into a more dilute solution in the calorimetric cell, the constant value of partial molal enthalpy h2 in the premicellar region is due to estruction of micelles and dilution of unmiceUized species (Fig. 6.30a) the constant I12 value in the postmicellar region is ascribed to dilution of micelles. 

The molar change in partial molal enthalpy of the surfactant when monomers associate into a micelle at the cmc represents the standard enthalpy of micellisation rnich per mole of surfactant monomers [98]. In accordance with the variations of /12 as a function of m in Fig. 6.30a, the enthalpy of micellisation for C12N3C at 298 K is positive, indicating that the micellisation process is endothermic. 

The experimental curves describing the adsorption of C12N3C onto Spherosil XOB015 from aqueous solution at 298 K (i.e.. Figs. 6.30b and 6.30c) suggest that the phenomenon generally occurs in two stages. At very small quantities of adsorp- 

Surface-active molecules or ions with an amphiphilic structure are known to have low solubility in water and self-assemble into large aggregates [61], According to the enthalpy curves presented in Fig. 6.30, surfactant aggregation may occur not only in aqueous solution but also at the Solid-Liquid interface. Since the topic of surfactant aggregation is a well-developed field of research [62, 71, 73, 93, 99, 100], only a brief review of the broad principles is proposed in the present paragraph to better illustrate the contribution of titration calorimetry. 

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FIGURE 8.4 Simpli ed schematic of the major components of an isoperibol titration calorimetry system. 

Ways are discussed of measuring both compositions and heats of formation fi.e.. excess enthalpies) of two conjugate phases in model amphiphile/water systems by isoperibol titration calorimetry. Calorimetric and phase-volume data are presented for n-C H OH/water at 30... 

It may prove possible to apply titration calorimetry data in one further direction. If AG can be estimated for SAL-goethite complexation and reaction enthalpies can be obtained under equilibrium conditions, then an entropy change for this reaction can also be derived. This can only be done, however, if the adsorption reaction can be shown to be reversible. Since this has not been proven as yet in our systems, such thermodynamic extensions of titration calorimetry can only be speculative at this time. 

J. J. Christensen. Techniques and Theory of Titration Calorimetry. In Thermochemistry and Its Applications to Chemical and Biochemical Systems, M. A. V Ribeiro da Silva, Ed. NATO ASI Series C, Riedel Dordrecht, 1984 1-16. 

E D. J. Eatough, R. M. Izatt, J. J. Christensen. Determination of Equilibrium Constants by Titration Calorimetry Part III, Application of the Method to Several Chemical Systems. Thermochim. Acta 1972, 3, 233-246. 

The enthalpy change associated with formation of a thermodynamically ideal solution is equal to zero. Therefore any heat change measured in a mixing calorimetry experiment is a direct indicator of the interactions in the system (Prigogine and Defay, 1954). For a simple biopolymer solution, calorimetric measurements can be conveniently made using titra-tion/flow calorimeter equipment. For example, from isothermal titration calorimetry of solutions of bovine P-casein, Portnaya et al. (2006) have determined the association behaviour, the critical micelle concentration (CMC), and the enthalpy of (de)micellization. 

All enthalpy of solution measurements were carried out with an LKB 8700-1 precision calorimetry system. The experimental procedure and tests of the calorimeter have been reported previously (3, 4, 5). The purification of the solvent DMF (Baker Analyzed Reagent) and of all solutes used has been described in the same papers. The solvent mixtures were prepared by weighing and the mole fraction of water in the DMF-water mixtures was corrected for the original water content of the amide as measured by Karl Fischer titration. 

Titration calorimetry or thermometric titration calorimetry is a technique in which one reactant is titrated continuously into the other reactant, and either the temperature change or heat produced in the system is measured as a function of titrant added. In isoperibol titration calorimetry, the temperature of a reaction vessel in a constant-temperature environment is monitored as a function of time (Figure 8.4) (Hansen et al., 1985 Winnike, 1989). A single titration calorimetric experiment yields thermal data as a function of the ratio of the concentrations of the reactants. 

Titration calorimetry depends on calculation of the extent of reaction from the quantity of heat evolved. Its successful application to a given system depends on (a) the equilibrium constant and the reaction conditions being such that the reaction occurs to a moderate extent (i.e., not to completion) and (b) the enthalpy of reaction being measurably different from zero. 

Recently there has been considerable interest on the subject of chemical test reactions for isothermal microcalorimeters. Chemical test reactions allow a user to check if an instrument is functioning correctly because they reflect more accurately the processes under study in a real experiment. Indeed, the ideal case would be to have a universally accepted chemical test reaction for each type of experiment (perfusion, titration, etc.) one may wish to investigate. Examples of systems that have been proposed as chemical test reactions include 18-crown-6/barium sulfate (2) for titration calorimetry... 

The enthalpies of solution were measured with a LKB 8700-1 precision calorimetry system. The experimental procedure and test of the instrument have been given before (6,7). EC (Fluka, purissimum) was distilled under reduced pressure and the middle fraction was stored over molecular sieves (4 A) for at least 48 hr. ACN (Merck, pro analysis) was dried over molecular sieves and used without further purification. The purity of both solvents (determined shortly before use), as deduced from GLC, was always better than 99.8%. The volume fraction of water, determined by K. Fischer titration (8) was always less than 3.10-4. The mixed solvents were prepared by weight as shortly as possible before the measurements. AH° of Bu4NBr in W-ACN mixtures have been measured at 25°C while those in W-EC are at 45°C, which is above the melting point of pure EC. 

Because of the considerable potential of titration calorimetry as an analytical technique for characterization of the acidic functional groups of humic substances, our studies have been extended to river water humic substances. In this paper, results are presented for the thermochemical characterization of the acidic functional groups of river water humic substances from two quite different river systems 1) the Satilla River in southeastern Georgia, and 2) the Williamson River in southern Oregon. 

Bloor, D.M. HoUwarth, J.F. Wyn-Jones, E. Polymer/ surfactant interactions the use of isothermal titration calorimetry and EMF measurements in the sodium dodecyl sulphate/poly(A-vinylpyrrolidone) system. Langmuir 1995, 11, 2312-2313. 

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