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3: Differential Thermal Analysis

  • Page ID
    527187
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    Learning Objectives

    After completing this chapter, you should be able to:

    • Explain the principle and working of a differential thermal analyser.
    • Draw and interpret DTA thermogram.
    • Compare and contrast TG and DTA results.
    • Explain the applications of DTA and simultaneous TG-DTA analysis.

    When a molecule undergoes a physical or chemical transition, heat is either absorbed or liberated. Two thermal methods, DTA and DSC are particularly useful for investigating these physical and chemical changes.

    3.1 Principle and Instrumentation:

    In DTA, difference in temperature between the sample and an inert reference (∆T) is measured as the sample and the reference are heated or cooled in a controlled manner.

    \[∆T = T_S – T_R\]

    with \(T_S\) as the temperature of sample and \(T_R\) is temperature of reference which is thermally stable.

    Please go through the Chemlibre link to understand the principle and working of DTA.

    https://chem.libretexts.org/Bookshelves/Analytical_Chemistry/Instrumental_Analysis_(LibreTexts)/31%3A_Thermal_Methods/31.02%3A_Differential_Thermal_Analysis

    Instrumentation and working: A typical DTA set up is shown in the figure given below.

    Schematic diagram illustrating a thermal analysis setup with a single heat source at the bottom of an orange block. The top surface has alternating raised and recessed sections. Two recessed compartments are labelled: •	S: Sample •	R: Reference Arrows from the base to the top of the sample and reference compartments represent heat flow. A horizontal double-headed arrow labelled ΔT spans across the top, indicating the temperature difference between the sample and reference. The setup demonstrates differential heating behaviour in thermal analysis (e.g., Differential Thermal Analysis (DTA)).
    Figure 3.1 DTA set up

    In differential thermal analysis (DTA), the difference in temperature between the sample and a thermally inert reference material is measured as a function of temperature (usually the sample temperature). Any transition that the sample undergoes results in the liberation or absorption of energy by the sample with a corresponding deviation of its temperature from that of the reference. A plot of the differential temperature, ΔT, versus the programmed temperature, T, indicates the transition temperature(s) and whether the transition is exothermic or endothermic. DTA and thermogravimetric analyses (measurement of the change in weight as a function of temperature) are often run simultaneously on a single sample.

    3.2 DTA curve and its interpretation

    \[∆T = T_S – T_R\]

    TS= temperature of sample

    TR = temperature of thermally stable reference

    A typical DTA curve is represented below

    A typical DTA grpah is given below (Fig 3.2) where, the y-axis is marked with "+" and "−" signs representing the direction of ΔT (temperature difference between sample and reference).

    A schematic Differential Thermal Analysis (DTA) curve showing temperature difference (ΔT) on the vertical axis and an implied time or temperature progression on the horizontal axis. The curve features: •	A positive peak labelled EXOTHERM (in green), indicating an exothermic reaction, where the sample releases heat. •	A negative peak labelled ENDOTHERM (in red), indicating an endothermic reaction, where the sample absorbs heat.
    Figure 3.2 DTA curve

    Brain Teaser:

    Can you suggest why we adopt ∆T > 0 for exothermic and ∆T < 0 for endothermic transition?

    Activity 3: The following figure represents thermal investigation. Fill in the blanks by choosing the correct option. (James & Tonge, 2008)

    It is a schematic Differential Thermal Analysis (DTA) curve with temperature (T) on the horizontal axis and temperature difference (ΔT) on the vertical axis. The y-axis is labelled with positive and negative directions, indicating thermal deviations between sample and reference. The curve shows a single downward (negative) peak, representing an endothermic event, where the sample absorbs heat. After the peak, the curve rises and levels off, suggesting stabilization after the thermal event. .

    The record shown is that of a_________________ experiment since the_________ plot ∆T which is a ______________temperature. The ___________direction of the peak indicates that a __________________reaction has occurred. This in turn implies that the corresponding _________________ change ∆H must have been _________ ie the value of enthalpy ________________the thermal effect was ____________ than its value _____________.This means that the sample ____________ heat during the reaction. Furthermore, there is evidence of a change _______________in the temperature is increased beyond the thermal transition. This is shown by the ___________of the just beyond the end.

    Answer
    Select from the following list

    [upward/downward, free energy/heat capacity, greater/less, DTG/DTA, base-line/background, derivative/differential, took in/gave out, negative/positive, enthalpy/entropy, before/after/during, exothermic/endothermic/isothermal, abscissa/ordinate, distortion/displacement.]

    3.3 Comparison of DTA with TGA

    TGA DTA
    1 It measures change (loss or gain) in weight as the sample is subjected to controlled heating program It is a technique in which the difference in temperature between the sample and an inert reference material, is measured as a function of temperature
    2 It will detect only those physical and chemical transitions which are accompanied by change in weight. It can detect all physical and chemical transitions.
    3 This does not require an inert reference. This requires an inert reference material.
    4 It is a quantitative method It is a semi-qualitative method.
    5 This technique is generally used to study decomposition reactions. This technique is used to study phase transitions

    Query \(


    This page titled 3: Differential Thermal Analysis is shared under a CC BY-NC 4.0 license and was authored, remixed, and/or curated by Prabha Shetty (Open Education for a Better World - OE4BW) .

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