Question

Following thermal changes in a material's environment, name a method that may be used in conjunction with thermal analysis to give more insight of a material's chemical and physical response mechanism? Explain how it could be used.

Answer 1

Differential thermal analysis involves the technique of recording the difference in temperature between a substance and a reference material against either time or temperature. Thus, a differential thermo gram consists of a record of the differences in sample and reference temperature (differential temperature, ∆T) plotted as a function of time t, sample temperature (Ts), reference temperature (Tr) or furnace temperature (T_f).

the peaks in the figure gives idea about crystallinity or fusion process.  Physical changes give rise to endothermic curves whereas chemical reactions give rise to exothermic peaks.

both metallic and non metallic sample can be used n this technique.

FACTORS AFFECTING Differential thermal analysis CURVE

The Differential thermal analysis curve is affected by a larger number of factors than the TG curve. Majority of these factors are common to TG and hence will not be discussed but only their special influences on Differential thermal analysis curves will be considered. The various factors affecting die Differential thermal analysis curve are as follows:

a. Environmental factors.

b. Instrumental factors.

c. Sample factors.

Environmental factors: The Differential thermal analysis technique is more sensitive to the gaseous environment around the sample. The reaction of gaseous atmosphere with the sample may also produce extra peaks in the curve. For example

a. Oxygen in air which causes an oxidation reaction may give rise to an exothermic peak.

b. If we record the Differential thermal analysis of lignite in the dynamic nitrogen atmosphere, it pyrolyses and distills off volatile matter. However, in the dynamic oxygen atmosphere, the lignite oxidizes, resulting exothermic instead of endothermic peaks.

Instrumental factors

a. Sample holder: The geometry and the material used in the fabrication of the sample holder affect the resolution, shape and size of the Differential thermal analysis peaks. For better resolution, the size of holders and the amount of sample should be as small as possible.

b. Differential temperature sensing devices: The positions of the temperature measuring the differential therm-couples affect the intensity of the peaks, shapes of the peaks and the base line. However, one gets best results when the differential temperature is plotted against the sample temperature.

c. Temperature-programmer controller: One should be quite careful while selecting temperature- programmer controller because a constant heating rate is required in Differential thermal analysis .

d. Thermal regime: The heating rate has a great influence on the Differential thermal analysis curves. Higher the heating rates, higher the peak temperatures and sharper the peaks with greater intensity. Heating rates of 10 to 20 °C per minute are employed.

e. Recorder: Differential thermal analysis curve is greatly influenced by the type,chart-speed and pen-response of a recorder.

Sample characteristics

a. Physical: The degree of crystallinity of the sample also affects the Differential thermal analysis curves. The weight of the sample also influences the peak intensity and temperatures. Both these increase with increasing weight. Particle size.

b. Chemical: The chemical reactivity of the sample, the sample holder, therm-couple material, the ambient gaseous environment and added diluents greatly alter the Differential thermal analysis peaks

APPLICATION OF Differential thermal analysis

Physical chemistry

Heat of reaction: The derivation of expression relating area under a Differential thermal analysis peak to the heat of reaction has been worked out by Daniels. They concluded that the peak area ‘A’ in Differential thermal analysis is always a linear function of heat of reaction ΔH=K *A/NO.

Specific heat: Differential thermal analysis has been employed by David to determine specific heat of substances like naphthalene. The samples do not undergo any thermal effect other than the normal change in specific heat. C was determined by using the following formula: C =K*K’(b-a)/d*m*s d(T).

Thermal diffusivity: Differential thermal analysis has been used to determine the thermal diffusivities by measuring the temperature difference ΔT between the center and surface of the sample, heated at a uniform rate.The thermal diffusivity D was determined by using me following relation D=βr2 /б ΔTs .

Analytical chemistry

Identification of substances: We know that the DTA curve for two substances is not identical. Therefore, these serve as finger prints for various substances.

Identification of products: When a substance reacts with another substance, the products are identified by their specific DTA curves.

Melting points: It can be easily identified by DTA. So, that method is used directly to check the purity of the compound.

Quantitative analysis: The area of the peak is proportional to the total heat of the reaction. Therefore, the quantitative analysis is possible with the help of the standard curve of peak area vs. weight.

Quality control: DTA technique has been widely used for the quality control of a large number of substances like cement, glass, soil, catalysts, textiles, explosives, resins, etc. The characterization of limestone used in production of Portland cement has been done by DTA. The amount of magnesium carbonate in cements can be controlled by a quantitative analysis of the DTA curve.

Inorganic chemistry: DTA technique has been used to study the thermal stability of a large number of inorganic compound and complexes. DTA technique is preferred because it helps in distinguishing between reversible phase changes and irreversible decompositions. DTA techniques have been used to study oxalates, metal amine complexes, carbonates and oxides.

Organic chemistry: DTA investigations have been carried to help identification, purity determination and quantitative analysis including the evaluation of kinetic parameters of polymers, explosives, pharmaceuticals, oils, fats and other organic chemicals.