Calibration Of Differential Scanning Calorimeters Research Articles

Method of enthalpy calibration for differential scanning calorimeters

The article describes a new method for enthalpy calibration of differential scanning calorimeters. The method allows you to remove the limitation on the temperature range in which this metrological procedure can be carried out. The proposed method does not require the use of standard samples of heats of fusion, therefore, it is not limited by the thermophysical properties of the certified substances. The developed method solves the problem of increasing the accuracy of measurements of specific enthalpy and heats of phase transitions of various substances for all types of differential scanning calorimeters. The effectiveness of the method in solving this problem is confirmed and illustrated by the authors by comparing the results of measuring the heats of fusion of a number of metals. The results obtained on differential scanning calorimeters calibrated by the new method and on the same instruments, but calibrated in accordance with the generally accepted standardized method of the International Union of Theoretical and Applied Chemistry, were compared. The authors experimentally revealed the advantages of the new method in terms of saving time and money required for calibration. The article will be of interest to those specialists who use differential scanning calorimeters in the development of new materials, control of technological processes, production, input and output control, as well as to those specialists who develop methods for research and measurement of various materials on differential scanning calorimeters.

Reference material for temperature calibration of differential scanning calorimeters above 1400 °C

Platinum crucible pans are widely used for differential scanning calorimetry (DSC) at temperatures above the aluminum melting point (about 660 °C) due to good signal quality, chemical inertness and easiness of cleaning. However, reference materials commonly used for the calibration of DSC equipment operating with platinum pans exhibit phase transition temperatures no higher than about 1000 °C. In this work, the solid–solid, reversible, $$\alpha ^{\prime }_{\mathrm{H}}\rightarrow \alpha$$ phase transition of calcio-olivine, $$\hbox {Ca}_{2}\hbox {SiO}_{4},$$ was found to produce a marked DSC peak with an onset temperature of $$1453^{+1}_{-2}\,^{\circ }\hbox {C}$$ and a relatively small hysteresis. This compound is compatible with platinum crucibles and fulfills the requirements to be used as reference material for high-temperature calibration of differential scanning calorimeters.

Crystallization of ionic salts for calibration of differential scanning calorimeters

This study explores the possibility of using inorganic salts for temperature calibration of Differential Scanning Calorimeters (DSC) on cooling. The crystallization and melting of sodium nitrate, potassium nitrate, and rubidium nitrate has been studied at several cooling and heating rates. It has been found that these salts crystallize at almost the same temperature, at which they melt, meaning that they do not practically supercool. The DSC data have indicated that the median value of the supercooling is around 0.4–0.8°C. The study suggests that the process of crystallization of these and possibly other inorganic salts can be considered for developing new standards to calibrate DSC on cooling.

A procedure for calibration of differential scanning calorimeters

A new approach to calibration of differential scanning calorimeters based on the difference in heat capacities of heavy and light water is described. The advantages of heavy water over salt solutions currently used for the same purpose are described. Sources of uncertainties in measurements and possible ways to reduce these are discussed.

Development of a certified reference material for calibration of DSC and DTA below room temperature: NMIJ CRM 5401-a, Cyclohexane for Thermal Analysis

For the quality assurance of performance of differential scanning calorimeters (DSC) and differential thermal analyzers (DTA) below room temperature, we have developed “NMIJ CRM 5401-a, Cyclohexane for Thermal Analysis” applicable to calibration of DSC and DTA in the low temperature. Adiabatic calorimetry was used to measure the temperatures and enthalpies of solid–solid phase transition and fusion as certified values, and to determine the purity in amount of substance fraction as information. The certified values are consistent with their corresponding literature values within expanded uncertainties and have traceability to the SI. Purity in amount of substance fraction was measured by fractional melting method based on freezing point depression method and was confirmed to be more than 0.9999. NMIJ CRM 5401-a was produced based on a quality system in compliance with ISO Guide 34: 2000. We demonstrate the usefulness of NMIJ CRM 5401-a in the calibration, quality control, and validation aspects of DSC and DTA

Optimization of DSC calibration procedure

The influence of calibration of differential scanning calorimeters on data precision could not be underestimated. The more precise the calibration procedure is the more time is required for proper calibration of the DSC instrument. Since for each change of the experimental conditions (pan material, heating rate, purge gas, etc.) a calibration is needed, time spent on calibration of one calorimeter is often measured in dozens of hours per year. This paper describes the strategy for speeding up temperature and heat flow calibrations of differential scanning calorimeters. The amount of time saved by different modifications of the procedure as well as the influence of the approach on accuracy of calibration data is estimated.

The use of organic calibration standards in the enthalpy calibration of differential scanning calorimeters

The LCG Limited organic DSC calibration standards have been used to carry out the enthalpy calibration of a power compensated DSC in a temperature range (42–147°C) for which suitable metal standards are not available. The mean calibration factor was in agreement with the value obtained using indium, thus demonstrating that there was no significant error in using indium as a single point calibrant for measurements on the fusion of organic materials at lower temperatures.

Calibration of differential scanning calorimeters: A comparison between indium and diphenylacetic acid

The close proximity in melting temperature of the LGC Limited DSC standards indium and diphenylacetic acid, has enabled a direct assessment to be made of any differences resulting from the use of a metal or an organic compound in the calibration of DSC equipment. Following calibration with indium, the equilibrium fusion temperatures for diphenylacetic acid, were determined by both the stepwise heating and extrapolation to zero heating rate methods. The results were in excellent agreement with the certificate values and established that indium may be used as a calibrant when making accurate DSC measurements on organic materials in the same temperature range and that it has the advantage that it is non-volatile and can be used a number of times without significant change. Similar agreement was obtained in the measurement of the enthalpy of fusion, although the larger heat capacity change on fusion of diphenylacetic acid resulted in a greater uncertainty than with indium.

Round Robin Test on Validation of Cyclohexane as a Reference Material for Calibration of Differential Scanning Calorimeters

Round Robin Test on Validation of Cyclohexane as a Reference Material for Calibration of Differential Scanning Calorimeters

4,4′-Azoxyanisole for temperature calibration of differential scanning calorimeters in the cooling mode—Yes or no?

By measuring the transition peak using differential scanning calorimetry in the heating and cooling modes, it has been confirmed that the nematic-to-isotropic liquid phase transition of 4,4′-azoxyanisole is of first order, but with some underlying additional phenomenon. As a consequence, this phase transition can only be used under certain precautions for temperature calibration of scanning calorimeters in the cooling mode.

Temperature, heat and heat flow rate calibration of differential scanning calorimeters

This contribution discusses basic problems and fundamental requirements for a universally applicable, unified and metrologically correct calibration of differential scanning calorimeters (DSC). The essential features of previously published recommendations of the German Society for Thermal Analysis (GEFTA) [see Höhne et al., Thermochim. Acta 160 (1990) 1–12; Cammenga et al., Thermochim. Acta 219 (1993) 333–343; Sarge et al., Thermochim. Acta 247 (1994) 129–168; J. Thermal Anal. 49 (1997) 1125–1134] developed for a correct and instrument-independent calibration of temperature, heat and heat flow rate of DSC, are outlined.

The temperature calibration on cooling of differential scanning calorimeters

The procedure for the calibration of differential scanning calorimeters (DSC) has been addressed by several authors. Most of the work published on this subject dealt mainly with the calibration on heating. For calibration on cooling, Menczel and Leslie suggested the use of thermally stable liquid crystals, for which the isotropic to nematic or cholesteric to isotropic transitions do not exhibit hysteresis on cooling. In this work, a new method is proposed to calibrate differential scanning calorimeters on cooling. It is suggested that the calibration on heating, with standard metals, may also be used to perform the calibration on cooling. The onset of the nematic to isotropic transition on heating (and vice-versa on cooling) will be used to support the method and to quantify the errors, which are of similar magnitude to those obtained by Menczel and Leslie with thermally stable high purity liquid crystals. Results of the change in the indium and lead crystallisation onset temperatures with the cooling rate are presented. Using as an example the non-isothermal crystallisation of polyethylene, the errors obtained with the standard calibration of the DSC and the new calibration on cooling are discussed.

Temperature calibration of temperature-modulated differential scanning calorimeters

Calibration of differential scanning calorimeters (DSC) is essential because it is not possible to calculate heat-flow rate and sample temperature directly from the quantities measured. The question arises whether the calibration used in the conventional DSC mode is also valid for temperature-modulated DSC (TMDSC). To prove this, a well-defined transition is needed which does not disturb the temperature profile within the sample during the modulation, the response of the whole system (calorimeter, sample, transition) must be linear. Some low-energy liquid-crystal transitions behave in this way under certain conditions and can be used for temperature calibration in TMDSC. The procedure is demonstrated for a smectic-A—nematic transition in a cyanobiphenyl liquid crystal (8OCB). Limitations of the method are discussed — in particular, rate effects. The TMDSC measurements also give some information about the order of phase transitions (8OCB: SN — second-order; NI — first-order).

Accurate temperature calibration of differential scanning calorimeters

Power-compensated differential scanning calorimeters are often used in measurements on cooling. In this paper we discuss and present new results on different methods for calibration for cooling. Liquid-crystals have transitions characterized by very little supercooling and are here used for calibration purposes. A difficulty is that the transitions of liquid-crystals usually occur in a narrow temperature range. The temperature range of precise calibration is evaluated for some metals by a method based on reversing the heat flow immediately after the beginning of melting. Further, we investigate the crystalline-plastic-liquid transitions of some alkanes. Finally, we calculate temperature profiles inside two samples of different thermal diffusivity. The results highlight important differences in the temperature distributions in organic and metallic substances under different conditions.

Temperature calibration of differential scanning calorimeters

A procedure for calibrating the temperature scale of a DSC is described. A different calibration trend was obtained using the transition points of organic compounds compared to that found using the melting points of highly pure metals. The crystal-crystal transitions of three ammonium salts were studied by this method; ammonium dihydrogen phosphate and ammonium sulphate were found to be suitable reference materials in the region −150 to 0°C, but ammonium nitrate was found to be unsuitable due to poorly defined transition points and inconsistent thermal behaviour.

Calibration of differential scanning calorimeters

Abstract

Remarks on the calibration of differential scanning calorimeters

The development of methods for temperature and heat calibration of DSC and DTA supported by the German Society of Thermal Analysis (GEFTA) are presented. Reasons are given for the recommendation of the temperature calibration method, which refers to the valid international temperature scale (ITS 90) and takes into account the fact that the fixpoints of this scale are defined in two phase equilibrium. Furthermore the hard problems with the heat flow and peak area calibration of DSC are described and the methods are presented to come to realistic and sufficient results. Thus both traditional (with the aid ofC p — and enthalpy — reference materials) and electrical calibration are critically discussed. Today's best values of suitable materials for heat calibration are reported.

Biological thermodynamic data for the calibration of differential scanning calorimeters: dynamic temperature data on the gel to liquid crystal phase transition of dialkylphosphatidylcholine in water suspensions

Heat capacity measurements of the gel to liquid crystalline transition of phosphocholine lipid suspensions in buffer solutions were performed using a differential scanning calorimeter (DSC) over a range of experimental conditions. Since the temperatures at the transition peak maxima, i.e. transition temperatures, were observed to increase linearly with scan rate due to the response time of the DSC, they were corrected to zero scan rate and found to agree closely with the literature values. The transition temperatures and enthalpies of the seven dialkylphosphatidylcholine lipids investigated ranged from 286.6 ± 0.1 K and 15.9 ±0.6 kJ mol −1 for 1,2-ditridecanoyl- sn-glycero-3-phosphocholine to 353.7 ± 0.1 K and 69.1 ±3.3 kJ mol −1 for l,2-ditetracosanoyl- sn-glycero-3-phosphocholine. At the slowest scan rates of 3–5 K h −1, the transition widths at half the peak maximum were close to 0.2 K. The transition temperatures were found to be independent of the following: the ionic strength of the sodium phosphate buffer from 0.01–0.02 M, the pH of the buffer from 6 to 8, the concentration of the suspension from 0.1–1.0 mass%, the commercial source of the lipid, and storage of the lipid in a freezer for 4 years. Suspensions of the dialkylphosphatidylcholine lipids in sodium phosphate buffer solutions can be used as sources of dynamic temperature data for the calibration of DSCs.

Biological thermodynamic data for the calibration of differential scanning calorimeters: heat capacity data on the unfolding transition of β-lactoglobulin in solution

Differential scanning calorimetry (DSC) measurements of the unfolding transition of β-lactoglobulin in HCl-glycine-buffered solutions have been performed over a temperature range from 353.0 K at pH 2.3 to 362.2 K at pH 3.5. Three values for the van't Hoff transition enthalpies were calculated as follows: (1) from the fit of a two-state transition model to the heat capacity measurements (Δ H vf); (2) from the van't Hoff plot of ln(1/ K) 1/ T where K is the transition equilibrium constant; and (3) from the equation Δ H ve = 4.00 RT 2 m C max p/ A p where R = 8.31451 J mol −1 K −1, T m is the transition temperature, C max p is the peak maximum, and A p is the peak area. The best linear fit of the van't Hoff enthalpies to the transition temperatures, T m, was obtained with Δ H vf and is Δ H vf (kJ mol −1) = (438.6 ±2.8) +(1.8±1.0)( T m −358.0). Calorimetric transition enthalpies were determined from the transition peak area using an extrapolated sigmoidal baseline and using an extrapolated straight baseline. The best linear dependence of the calorimetric enthalpy on T m was obtained with the sigmoidal baseline (Δ H s) and is Δ H s(kJ mol −1) = (427.1 ± 4.2) + (0.5 ±1.5)( T m-358.0). Linear least-squares fits of Δ H vf and Δ H s to T m were independent of the DSC scan rate, the source of β-lactoglobulin, the buffer concentration from 0.1 to 0.2 M, and the concentration of the protein from 0.03 to 0.45 mM. The transition temperature exhibits a linear dependence on pH and a slight dependence on concentration. Cooperativity of the transition is 0.974 ± 0.007 while the average heat capacity change of the solution accompanying the transition is 13.5±7.8 kJ mol −1 K −1 over this temperature range.

Biological thermodynamic data for the calibration of differential scanning calorimeters: heat capacity data on the unfolding transition of lysozyme in

Differential scanning calorimetry measurements of the unfolding transition of lysozyme in HCl-glycine buffer solutions were performed over a temperature range from 326 K at pH 2.3 to 349 K at pH 3.9. Van't Hoff transition enthalpies were calculated from the fit of a two-state transition model to the heat capacity measurements (Δ H vf, from the van't Hoff plot of ln(1/ K) vs. 1/ T where K is the transition equilibrium constant, and from the ratio of the transition peak height to the area under the transition peak. The best linear fit of the van't Hoff enthalpies to the transition temperatures T m was obtained with (Δ H vf and was (Δ H vf (kJ mol −1) = 432.7±1.7+5.81±0.24 ( t m-337.2). Calorimetric transition enthalpies were determined from the transition peak area using an extrapolated sigmoidal baseline ((Δ H s) and an extrapolated straight linear baseline. The best linear dependence of the calorimetric enthalpy on T m was obtained with (Δ H s (kJ mol −1) = 434.7±4.1+6.39 ±0.60 ( T m− 337.2). Linear least-squares fits of Δ H vf and Δ H s to T m were independent of the DSC scan rate, the source of the lysozyme, the buffer concentration from 0.1 to 0.2 M and the concentration of lysozyme from 0.26 to 2.8 mM. The transition temperature exhibited a linear dependence on pH and concentration. Cooperativities of the transition ranged from 0.988±0.007 at 326 K to 1.012±0.007 at 349 K. The average heat capacity change of the solution accompanying the transition was 5.94±3.10 kJ mol −1 K −1 over this temperature range.