Calorimetry, Chemistry tutorial

Introduction:

The science of evaluating the heat of chemical reactions or physical changes by using a calorimeter is termed as calometry. Scottish physician and scientist Joseph Black, who was the first to identify the dissimilarity between heat and temperature, is stated to be the founder of the Calorimetry.

Calorimetry:

Calorimetry is the quantitative measurement of the heat evolved or absorbed throughout a chemical procedure. The Calorimetry as the name implies is derived from the two English loan terms: color (Latin word) meaning heat and Greek metry meaning to measure. All the calorimetric methods are mainly based on the measurement of heat which might be produced (that is, exothermic process), consumed (that is, endothermic) or simply dissipated via a sample.

There are many methods which have been developed for heat measurement ranging from simple thermometric (that is, temperature measurement) techniques to a more recently advance in electronics and control that enables users to collect data and maintain samples under conditions which were priory not possible.

Calorimeter:

The property of an object whenever heat is transferred to it is that of temperature rises; likewise if heat is eliminated from an object, the temperatures of these object reduces. The relationship between heat transferred and change in temperature can be appreciated via the equation below:

q = CDT = C(Tf - Ti)

Here,

q = heat which is transferred

DT = change in temperature

C = proportionality constant or else termed as heat capacity

Calorimeter is the instrument employed to measure the heat of reaction throughout a well defined process. This can be simple and cheep or sophisticated and expensive. The calorimeter comprises of well insulated container and reaction initiated and temperature difference before and after the reaction measured.

This is significant to note that a calorimeter can be operated under constant pressure or constant volume.

The heat capacity of a calorimeter is stated as the amount of heat needed to change the temperature of the whole calorimeter by one degree. The heat capacity of the calorimeter is found out by transferring an identified amount of heat to it and measuring its temperature rise. The temperature differences are generally extremely small as a result extreme sensitive thermometers are needed for such measurements.

Differential Scanning Calorimetry:

Differential scanning Calorimetry (DSC) is a thermo-analytical method in which the difference in the amount of heat needed to raise the temperature of a sample and reference is evaluated as a function of temperature. Both the sample and reference are maintained at almost the similar temperature all through the experiment. Usually, the temperature program for a DSC analysis is designed in such a way that the sample holder temperature rises linearly as a function of time. The reference sample must encompass a well-defined heat capacity over the range of temperatures to be scanned.

The primary adiabatic differential scanning calorimeter which could be employed in biochemistry was developed via P.L. Privalov and D.R. Monaselidze in the year 1964. The word DSC was coined to illustrate this instrument that measures energy directly and allows accurate measurements of heat capacity.

Detection of phase transitions:

The fundamental principle of this method is that if the sample experiences a physical transformation like phase transitions, more or less heat flow to it than the reference to maintain both at the similar temperature. The quantity of heat flows to the sample essentially based on whether the procedure is endothermic or exothermic. By comparing the difference in heat flow between the sample and reference, differential scanning calorimeters are capable to measure the quantity of heat absorbed or discharged during these transitions. For illustration, a melting solid sample needs more heat flow via it in such a way that its temperature rises at similar rate as the reference. This is an endothermic phase transition from solid to liquid therefore heat is absorbed. Likewise, as the sample experiences exothermic processes (like crystallization) less heat is needed to increase the sample temperature. This is broadly employed in industrial settings as a quality control instrument because of its applicability in measuring sample purity and for studying the polymer curing.

Differential thermal analysis (DTA):

The other method that is closely associated to DSC is differential thermal analysis (DTA). The heat flow to the sample and reference is similar instead of the temperature. Identically phase changes and other thermal methods bring about a difference in temperature between the sample and reference as soon as the sample and reference are heated. This is significant to note that DSC measures the energy needed to keep both the reference and the sample at similar temperature while DTA evaluates the difference in temperature between the sample and the reference whenever they are both placed under the similar heat.

A graph or curve of heat flux versus temperature or time is plotted by data generated from a DSC experiment. This curve is helpful in computing enthalpies of transitions via integrating the peak corresponding to a given transition. The enthalpy of transition is provided by the equation below:

ΔH = KA

Here,

Δ = enthalpy of transition

K = calorimetric constant

A = Area under the curve. The calorimetric constant can be found out by examining a characterized sample with recognized enthalpies of transition.

=> Applications:

Differential scanning Calorimetry can be employed to monitor fusion and crystallization events and also glass transition temperatures. DSC can as well be employed to study the oxidation and other chemical reactions.

As the temperature of the amorphous solid is increased glass transition might take place and is in the real sense not a phase change however essentially because of change in heat capacity.

The amorphous solid tends to become less viscous as the temperature rises. At a specific temperature (or else termed as the crystallization temperature, Tc) the molecules become more mobile that as a result led to the spontaneous arrangement into crystalline form. 

This transition from amorphous solid to crystalline solid is the exothermic method, and yields in a peak in the DSC signal. As the temperature rises the sample ultimately reaches its melting temperature (Tm). The melting process yields in an endothermic peak in the DSC curve. The capability to find out the transition temperatures and enthalpies makes DSC a worthy tool in producing phase diagrams for different chemical systems.

603_Schematic DSC curve of amount of energy input.jpg

Fig: Schematic DSC curve of amount of energy input

Top: Schematic DSC curve of amount of energy input (y) needed to maintain each and every temperature (x), scanned across a range of temperatures.

Bottom: Normalized curves setting the preliminary heat capacity as the reference. Buffer-buffer baseline (dashed) and protein-buffer variance (solid).

=> Polymers:

The composition of a polymer can be examined by using DSC.

Melting points and glass transition temperatures for most of the polymers are available from standard compilations. The degradation of a polymer is pointed out by the lowering of the expected melting point, Tm, and therefore the experimental melting point and glass transition temperature for any polymer is compared by those available in a chart of standard compilations. Tm mainly depends on the molecular weight of the polymer that means that lower grades will have lower melting points than expected. The percentage crystallinity of a polymer can be determined from the crystallization peak of the DSC graph as the heat of fusion can be computed from the area under the absorption peak. Impurities in polymers can be found out by observing thermograms for anomalous peaks and plasticizers can be identified at their characteristic boiling points.

=> Liquid Crystals:

Matter might change from solid to liquid via a third state in which the properties of both phases are well displayed. This anisotropic liquid is termed as a liquid crystalline or mesomorphous state. The energy changes (although small) that take place as matter coverts from a solid to liquid crystal and from a liquid crystal to the isotropic liquid can be monitored by using DSC. Liquid crystals thus are a state of matter having properties between those of conventional liquid and those of solid crystals. Liquid crystals depend noticeably on temperature, concentration and also inorganic-organic composition ratio. 

=> Oxidative Stability:

Differential scanning Calorimetry can be employed to study the stability to oxidation of samples in an airtight sample chamber. The sample is first brought to the desired test temperature under the inert atmosphere, generally nitrogen. Consequently, oxygen is added to the system and oxidation is expected to take place. If this occurs, it is viewed as a deviation in the baseline. This analysis has been employed to find out the stability and optimum storage conditions for the material or compound.

=> Drug Analysis:

DSC is widely employed in the polymer and pharmaceutical industries. DSC is a versatile tool for the polymer chemist in studying curing processes that is deliberate efforts aimed at fine tuning of the polymer properties. The polymer molecules cross-linked exothermically throughout the curing method resultant in a positive peak in the DSC curve that generally appears soon after the glass transition. 

This is desirable to have well-characterized drug compounds in order to define the processing parameters in the pharmaceutical industry. For illustration, drugs must be delivered in the amorphous form, it is essential to process the drug at temperatures beneath those at which the crystallization can take place. 

=> General Chemical Analysis:

Freezing-point depression can be employed as the purity analysis tool if analyzed by Differential scanning Calorimetry. This is possible as the temperature range over which a mixture of compounds melts is based on their relative amounts. As a result, less pure compounds will show a broadened melting peak that starts at lower temperature as compare to a pure compound.

=> Food Science:

In food science research, DSC is employed in conjunction by other thermal analytical methods to find out water dynamics. Changes in water distribution might be correlated with changes in texture. Identical to materials science studies, the effects of curing on confectionery products can as well be examined.

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