Determination Of Enthalpy Changes Research Articles

Ultra-precise particle velocities in pulsed supersonic beams

We describe an improved experimental method for the generation of cold, directed particle bunches, and the highly accurate determination of their velocities in a pulsed supersonic beam, allowing for high-resolution experiments of atoms, molecules, and clusters. It is characterized by a pulsed high pressure jet source with high brilliance and optimum repeatability, a flight distance of few metres that can be varied with a tolerance of setting of 50 μm, and a precision in the mean flight time of particles of better than 10(-4). The technique achieves unmatched accuracies in particle velocities and kinetic energies and also permits the reliable determination of enthalpy changes with very high precision.

Recent trends and some applications of isothermal titration calorimetry in biotechnology

Isothermal titration calorimeters (ITCs) are thermodynamic instruments used for the determination of enthalpy changes in any physical/chemical reaction. This can be applied in various fields of biotechnology. This review explains ITC applications, especially in bioseparation, drug development and cell metabolism. In liquid chromatography, the separation/purification of specific proteins or polypeptides in a mixture is usually achieved by varying the adsorption affinities of the different proteins/polypeptides for the adsorbent under different mobile-phase conditions and temperatures. Using ITC analysis, the binding mechanism of proteins with adsorbent solid material is derived by elucidating enthalpy and entropy changes, which offer valuable guidelines for designing experimental conditions in chromatographic separation. The binding affinity of a drug with its target is studied by deriving binding enthalpy and binding entropy. To improve the binding affinity, suitable lead compounds for a drug can be identified and their affinity tested by ITC. Recently ITC has also been used in studying cell metabolism. The heat produced by animal cells in culture can be used as a primary indicator of the kinetics of cell metabolism, which provides key information for drug bioactivity and operation parameters for process cell culture.

Calorimetric determination of enthalpy changes for the proton ionization of N-tris(hydroxymethyl)methyl-4-aminobutanesulfonic acid (TABS), N-tris(hydroxymethyl)methyl-3-aminopropanesulfonic acid (TAPS) and 3-[ N-tris(hydroxymethyl)methylamino]-2-hyroxypropane sulfonic acid (TAPSO) in water–methanol mixtures

Enthalpies for the two proton ionizations of the biochemical buffers N-tris(hydroxymethyl)methyl-4-aminobutanesulfonic acid (TABS), N-tris(hydroxymethyl)methyl-3-aminopropanesulfonic acid (TAPS) and 3-[ N-tris(hydroxymethyl)methylamino]-2-hyroxypropane sulfonic acid (TAPSO) were obtained in water–methanol mixtures with methanol mole fraction ( X m) from 0 to 0.360. The ionization enthalpy for the first proton (Δ H 1) of all three buffers was small and exhibited slight changes upon methanol addition. The ionization enthalpy of the second proton (Δ H 2) of TABS increased from 39.6 to 49.8 kJ mol −1 and for TAPS from 40.1 to 43.2 kJ mol −1, with a minimum of 38.2 kJ mol −1 at X m = 0.059. For TAPSO the increase was from 33.1 to 35.6 kJ mol −1 at X m = 0.194, with measurements at higher X m precluded by low solubility of TAPSO in methanol rich solvents. The solvent composition was selected so as to include the region of maximum structure enhancement of water by methanol. The results were interpreted in terms of solvent–solvent and solvent–solute interactions.

Calorimetric determination of enthalpy changes for the proton ionization of glycine, N, N-bis(2-hyroxyethyl)glycine (“bicine”) and N-tris(hydroxymethyl)methylglycine (“tricine”) in water–methanol mixtures

Enthalpies for the two proton ionizations of glycine, N, N-bis(2-hyroxyethyl)glycine (“bicine”) and N-tris(hydroxymethyl)methylglycine (“tricine”) were obtained in water–methanol mixtures with methanol mole fraction ( X m) from 0 to 0.360. With increasing methanol the ionization enthalpy for the first proton (Δ H 1) of glycine increased from 4.4 to 9.4 kJ mol −1 with a minimum of 4.1 kJ mol −1 at X m = 0.059. The ionization enthalpy of the second proton (Δ H 2) for glycine decreased from 46.3 to 38.1 kJ mol −1. Δ H 1 of bicine increased from 3.5 to 7.6 kJ mol −1 at X m = 0.273 before dropping to 4.1 kJ mol −1 at X m = 0.360. Δ H 2 of bicine increased from 24.9 to 29.4 kJ mol −1. For tricine, Δ H 1 increased from 6.7 to 9.8 kJ mol −1 at X m = 0.194 then dropped to 7.4 kJ mol −1 at X m = 0.360. Δ H 2 for tricine first dropped from 30.8 to 28.5 kJ mol −1 at X m = 0.059 before increasing to 33.3 kJ mol −1 at X m = 0.273. The solvent composition was selected so as to include the region of maximum structure enhancement of water by methanol. The results were interpreted in terms of solvent–solvent and solvent–solute interactions.

The Conventional Method of the Determination of Enthalpy Change at Ion Solvation in Solutions: Is It Correct?

It is shown that the conventional (using the Born−Haber cycle) method of the determination of enthalpy change at ion solvation in solutions gives incorrect values for this fundamental parameter. A new experimental method free from the defects of the conventional one is presented.

Thermodynamics of dipeptides in water. VI. Calorimetric determination of enthalpy changes of dissociation processes in water of the free α-carboxyl and α-amino groups in a series of dipeptides. Comparison of these processes for two series of dipeptides

A calorimetric study has been made on the dissociation processes in water of the free α-amino group of the amino terminal residue and the free α-carboxyl groups of the carboxyl terminal residue in a series of dipeptides having valine as first common term. These values have been compared with those of the proton dissociation processes related to the same groups of the corresponding single α-amino acids. The results of this series have also been considered in relation to those from another series of dipeptides, previously studied, in an effort to compare the effects of different chains on the dissociation processes of the free α-carboxyl and free α-amino groups.

Calorimetric determination of enthalpy changes in octylcyanobiphenyl (8CB) liquid crystal transitions

A specially designed differential scanning heat-flux calorimeter is used for the determination of enthalpy changes in the transitions between mesophases of the octylcyanobiphenyl (8CB) liquid crystal. Latent heats can be separated from pre-transitional effects in certain cases and higher-order transitions can be well identified experimentally.

Heats of formation and decomposition of nickel and Ni 0.8Cu 0.2 hydrides measured in high pressures of gaseous hydrogen

A new twin calorimeter for the determination of enthalpy changes in gas-solid state reactions under high pressure of the gaseous component is described. Formation and decomposition of hydrides in nickel and Ni 0.8Cu 0.2 alloy are investigated in high pressures of gaseous hydrogen. The decomposition enthalpy of nickel hydride measured agrees with the value determined previously under normal pressure.

Coupling of thermometric titrimetry and constant-current coulometry: Study and applications

Two systems involving the coupling of thermometric and coulometry are presented. In the first assembly, the two electrodes were in the same compartment. Bromocoulometry was used to test the possibilities of this coupling in the sequential titration of a vitamin C+ aspirin mixture about (10−4M) and the determination of enthalpy changes. In a second coupling system, the anodic and cathodic compartments were separated by a sintered-glass disc of porosity 3. The conditions for obtaining an electrochemical cell which is also a good calorimeter are discussed. Examples of applications are given, such as analytical measurements, the study of an electrochemically catalysed reaction and of the thermal effects of electrode reactions.

Temperature-induced ultraviolet difference absorption spectrometry for determination of enthalpy changes: Binding of 4-methylumbelliferyl glycosides to four lectins

Raising the temperature in a single mixture of a lectin and a chromophoric glycoside allows determination of the binding enthalpy. This is made possible by continuously monitoring the displacement of the complex from its equilibrium concentration with a sensitive difference absorption spectrophotometer. The method is illustrated with the following lectins: concanavalin A, soybean agglutinin, peanut agglutinin and Erythrina cristagalli agglutinin. The ligands are 4-methylumbelliferyl glycosides. The binding enthalpies found range from −60 kJ · mol −1 for the Galß1 → 3GalNAc-ßglycoside and peanut agglutinin to −30 kJ · mol −1 for a monosaccharide glycoside and the other lectins.

Thermodynamics of complex formation: DL-isoserine with nickel(II) and copper(II)

Researches in this laboratory have shown how in aqueous solution, the aminoacids, DL-4-amino-3-hydroxy-butanoic and DL-3-amino-2-hydorxypropanoic acid (isoserine) form complexes with divalent metals. The complexes contain the pentatomic chelate ring ▪ where the alcoholic group appears to be ionized even in acidic solution [1, 2]. On the other hand the corresponding isomers, threonine and serines bind the metal via the α-aminoacid moiety and only in alkaline solution the hydroxyl group dissociates and is involved in chelation to the metal [3]. In order to explain the different roles of aminoethanolate and aminocarboxylate moieties in complex formation, we have now undertaken the calorimetric determination of enthalpy changes in the reactions of DL-isoserine with Ni(II) and Cu(II) in aqueous solution at 25 °C and I = 0.1 mol dm −3 (KCl). The calorimetric measurements have been carried out by a modified LKB 8700-2 calorimeter. The values of the thermistor resistence in the reaction vessel were obtained by measuring the output voltage of the unbalanced Wheatstone bridge by a digital microvoltmeter HP 3455A. The e.m.f. values were printed at regular time intervals by a strip-printer HP 5150A with timer and converted to resistence values by an empirical relation. The ΔH 0 values obtained for the protonation reaction of isoserine, H 2NCH 2CH(OH)COOH, H 2L, are: ▪ The enthalpy changes of Ni(II) complexes are: ▪ The data for Cu(II) complexes are: ▪The corresponding entropy changes are rather high, particularly in the copper complexes. This stands for a remarkable entropy contribution to the stability of the complexes, coming from redistribution of water molecules involved in the process. Some hypotheses on the structure of the complexes can be put forward. They will be tested by extending the calorimetric studies to other ligands containing the aminoethanol moiety.

A computer method for the calculation of enthalpy changes for ion association in solution from calorimetric data

A general method for the determination of enthalpy changes in solution for systems containing an unlimited number of components, that can give rise to mononuclear, polynuclear, protonated, hydroxo and mixed species was set up. A computer program, DOEC, able to elaborate data on a calorimetric titration such as concentrations, changes to the mole number of the various species, solution ionic strength and reaction enthalpy was written. The program can also correct the ionic strength changes, that can occur during a titration. Thanks to the analogy of the mathematical expressions, the program can also be used for the treatment of spectrophotometric data.

The application of a new DTA technique to the measurement of the heats of fusion of bulk crystallized polyethylenes and polyethylene single crystals

AbstractDTA has been widely used for many years and it has always been assumed that the technique is intrinsically secondary in that the determination of enthalpy changes requires calibration of the equipment. Recently the present authors have shown that this is not so, and under certain conditions DTA becomes an absolute method. A modification of the new technique has now been applied to a polymer and the results are presented here. The heats of dissolution and hence of fusion of various polyethylene samples have been measured, together with their densities. The results are shown to be consistent with those of other workers. No special accuracy or importance is claimed for the actual results, however, because the purpose of the paper is to introduce a new technique in a familiar polymer context. There is scope for improvement and extension of the procedures described here, and the status of DTA in polymer science will be increased in consequence. The use of a liquid system greatly facilitates the preparation and handling of the material under test.