Microcalorimeters solution

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. 

The densities and volumetric specific heats of some alkali halides and tetraalkylammonium bromides were undertaken in mixed aqueous solutions at 25°C using a flow digital densimeter and a flow microcalorimeter. The organic cosolvents used were urea, p-dioxane, piperadine, morpholine, acetone, dime thy Isulf oxide, tert-butanol, and to a lesser extent acetamide, tetrahydropyran, and piperazine. The electrolyte concentration was kept at 0.1 m in all cases, while the cosolvent concentration was varied when possible up to 40 wt %. From the corresponding data in pure water, the volumes and heat capacities of transfer of the electrolytes from water to the mixed solvents were determined. The converse transfer functions of the nonelectrolyte (cosolvent) at 0.4m from water to the aqueous NaCl solutions were also determined. These transfer functions can be interpreted in terms of pair and higher order interactions between the electrolytes and the cosolvent. 

Standard heat capacities of transfer can be derived from the temperature dependence of standard enthalpies of solution (8). While this technique can give general trends in the transfer functions from water to mixed solvents (9), it is not always sufficiently precise to detect the differences between similar cosolvents, and the technique is rather laborious. Direct measurements of the difference between heat capacities per unit volume of a solution and of the solvent a — gq can be obtained with a flow microcalorimeter (10) to 7 X 10 5 JK 1 cm-3 on samples of the order of 10 cm3. A commercial version of this instrument (Picker dynamic flow calorimeter, Techneurop Inc.) has a sensitivity improved by a factor oi about two. 

The use of Eq. (5) to fit data recorded using a microcalorimeter was first demonstrated by Bakri (6), who studied the acid hydrolysis of methyl acetate in hydrochloric acid. In that experiment, 1 mmol of methyl acetate was added to 2mL of 1 N hydrochloric acid solution in a glass ampoule. The experimental data were fitted to Eq. (5) using a least squares analysis which gave k = 0.116 x 10-3 sec-1 and AH= 1.98 kJmol-1. In this paper, Bakri also shows how the method may be applied to both second-order, solution phase A+B x reactions and to flow calorimetry. 

Calorimetric forms of the Ng equation were used by Willson et al. (7) to analyse the solid-state degradation of L-ascorbic acid. Known amounts (0.5 g) of dry L-ascorbic acid were placed in ampoules along with known quantities of water and the heat changes in the samples were recorded using an isothermal microcalorimeter. The power-time data obtained were analyzed using the calorimetric form of the Ng equation and the parameters obtained are shown in Table 7. It was shown that, at low added quantities of added water, the reaction could satisfactorily be described by solid-state kinetics, but at higher added quantities of water (more than 500 pL) the reaction was best described by solution phase kinetics. 

Willson RJ, Beezer AE, Mitchell JC. A kinetic study of the oxidation of L-ascor-bic acid (vitamin C) in solution using an isothermal microcalorimeter. Thermochimica Acta 1995 264 27-40. 

Ross, P.D. Goldberg, R.N. (1974). A scanning microcalorimeter for thermally induced transitions in solution. Thermochim. Acta 10,143-151. 

FIGURE 7.7 Thermostat of the heat-flow microcalorimeter showing the sample cell with the solution and the ion exchanger separated by a membrane to prevent ion exchange and to guarantee the initial state of the experiment. 

An efficient heat exchanger so that the doses of added solution are, say, within 10-3 K of the temperature of the microcalorimeter. 

Two operational arrangements fulfilling the above requirements are represented in Figures 5.16b and 5.16c. For convenience, both are incorporated in a Tian-Calvet microcalorimeter with large cells (i.e. c. 100 cm3). The first device uses a disc stirrer (up and down movement) and cancels any temperature difference between the added solution and the adsorbent by placing both the adsorbent and the solution reservoir in the top part of the microcalorimetric cell (Rouquerol and Partyka, 1981). The second device uses a propeller which is given very fast half-turns (c. 10 per minute) by means of a hindered magnetic transmission which serves to damp the vibrations from the motor. 

In the former case, the solid remains suspended in the liquid in the microcalorimeter cell. Then a mother solution is added, either in one step (to obtain an integral heat. A ffUnt)) or in several steps, leading to differential heats, A H(dlff)l). In the latter case one could also speak of titration calorimetry. some commercial microcalorimeters are especially constructed for such titrations. Since, with these techniques, part of the added adsorptive remains in solution, the enthalpy of dilution A yH must be subtracted it is dependent on composition and can be determined in a blank without adsorbent. The difference between A y H(int) and A y H(dlff) has been discussed before, see sec. 1.3c. 

In flow microcalorimetry the solid is placed in the microcalorimeter cell between, say two filters, and is successively brought in contact with solvent and mixtures (or solutions) of various compositions that flow through the cell or percolate over a small column of the adsorbent ). 

Microcalorimeters have the ability of directly measuring the order of the reaction (n), the rate constant (k), the reaction enthalpy (Ar//), and the equilibrium constant (ATeq). " For example, solution microcalorimetry may be used to determine the free energy of dissolution of a solid compound, which is particularly important in pharmaceutical research for dissolution studies and in the determination of the relative thermodynamic stability of polymorphs. " The change in the Gibbs-Helmholtz free energy, AGsoi, on dissolution is... 

The A//soi is the enthalpy change that occurs on dissolution of one mole of compound in a solvent. The solution microcalorimeter may be used to obtain the enthalpy of solution directly. The change of free energy can be calculated from the concentration using the enthalpy obtained. The change in the entropy of solution AAsoin can then be determined from the Gibbs-Helmholtz equation. " ... 

Microcalorimetry is a growing technique complementary to DSC for the characterization of pharmaceuticals. Larger sample volume and high sensitivity means that phenomena of very low energy (unmeasurable by DSC) may be studied. The output of the instrument is measured by the rate of heat change dq/dt) as a function of time with a high sensitivity better than 0.1 pW. Microcalorimery can be applied to isolated systems in specific atmospheres or for batch mode where reactants are mixed in the calorimeter. Solution calorimetry can be used in adiabatic or isoperibol modes in microcalorimeters at constant temperature. (See the corresponding article about calorimetry of this edition.)... 

Solution calorimetry allows us to investigate processes that involve enthalpy changes. Adiabatic microcalorimeters and isoperibol calorimeters used in batch modes or flow modes allow for the precise determination of the heat of solution. Mixing the reactants is accomplished by breaking a bulk allowing reactants to mix or by special chambers where the reactants are mixed together. 

There are two possible modes of operation for flow microcalorimeters used for solution-phase studies. 

The microcalorimeter used (LKB, Sweden, bartch 2107) was described in detail in Ref. 11. The mixing procedure was the same as that described in Ref. 11, i.e., the heat of dilution of a surfactant solution (2 mL) was measured on mixing with 2 mL of solvent. In the reference cell the heat of mixing of 2 mL solvent with the... 

Fig. 13. Operational stability of D-amino acid oxidase fixed in cells of Trigonopsis variabilis CCY 15-1-3 entrapped in standard (A) and hardened (a) calcium pectate gel and standard (O) and hardened calcium alginate gel ( ). The relative activity was monitored by continuous processing, with the substrate (cephalosporin C) solution in the flow microcalorimeter [39]...

Apparatus and Procedure The calorimeter which was used for the measurement of the heats of mixing of DNA and PF was a LKB batch type microcalorimeter (LKB-10700). For calorimetric measurement, the DNA sample was dissolved into the buffer solution, and equal volumes (about 1.2 cm ) of DNA and dye solutions were mixed. 

Angberg et al. (10) studied the hydrolysis of acetyl saliclyic acid solutions, for which it was shown that elevated temperatures were needed to follow this rapid hydrolysis process. Thus, the isothermal microcalorimeter was neither more accurate, nor quicker or easier than using a conventional analytical approach, such as titration or chromatography, for this hydrolysis reaction. 

They also obtained good agreement between heat of solution and thermal activity measured in an isothermal microcalorimeter over the range 0 to 100% crystallinity. Salvetti et al. (36) have demonstrated that different physical forms of carbohydrates can be differentiated by measuring heats of solution. Pikal et al. (1978) used heat of solution measurements to correlate the extent of crystallinity with the chemical stability of antibiotics. 

Raschella, D. L. "Solution Microcalorimeter for Measuring Heats of Solution of Radioactive Elements and Compounds" Ph.D. Dissertation, The University of Tennessee, Knoxville, December 1978 U.S. Department of Energy Document No. 0R0-4447-081, 1978. 

TTC is performed using a MicroCal titration microcalorimeter (Northampton, MA). Solutions are degassed under vacuum prior to use. Protein at a concentration of 1 mg/mL is poured in the calorimeter cell and lipid (2.5 mg/mL) is added automatically by aliquots of 7 /integrated using the Origin software supplied by MicroCal Inc. 

Calorimetric measurements at a salt concentration of 0.8 M (NaCl) were carried out with a differential scanning microcalorimeter microDSC III (Setaram, Caluire, France). The melting of naphthalene was used to calibrate the apparatus. The sample cell was filled with 850 mg carrageenan solution (0.2% w/w in 0.8M NaCl) and the reference cell with exactly the same amount of NaCl solution. Heating and cooling curves were recorded in the temperature range from 10 to 120°C at a rate of 1.0°C min-1. 

Another type of microcalorimeter is the continuous flow calorimeter, developed in 1967 by the American physical chemists P. R. Stoesser and Stanley J. Gill. This instrument permits two reactant solutions to be thermally equilibrated during passage... 

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