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2: Thermogravimetry

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    Learning Objectives

    After completing this chapter, you should be able to:

    Thermogravimetry is a technique used to detect any physical or chemical transitions which are accompanied by a weight loss or weight gain as the sample is heated in a controlled manner.

    2.1 Effect of heat on matter

    We need to first understand the effects of heat on a matter. And for further explanation please see Introductory Chemistry in Libretexts

    https://chem.libretexts.org/Bookshelves/Introductory_Chemistry/Introduction_to_General_Chemistry_(Malik)/01%3A_Matter_energy_and_their_measurements/1.09%3A_Heat_and_changes_in_physical_states_of_matter

    We have now understood the effects of heat on a matter and also able to identify the processes involving change in weight on heating through Activity 1D. It is important to keep in mind that the change in weight could be due to physical or chemical transitions. To be able to distinguish between physical and chemical transition, let us go through the next sub-topic.

    2.2 Changes in matter: Physical and Chemical Changes

    For further explanation please go through the Introductory Chemistry in Libretexts

    https://chem.libretexts.org/Bookshelves/Introductory_Chemistry/Introductory_Chemistry/03%3A_Matter_and_Energy/3.06%3A_Changes_in_Matter_-_Physical_and_Chemical_Changes#:~:text=3.6%3A%20Changes%20in%20Matter%20%20Physical%20and%20Chemical,4%20Summary%20...%205%20Contributions%20%26%20Attributions%20

    Activity 2.1: The following table contains a list of transitions. Can you categorize them as physical transitions /chemical transitions?

    Phenomenon Physical Chemical
    Adsorption    
    Dehydration    
    Desorption    
    Fusion (melting)    
    Chemisorption    
    Vaporization    
    Decomposition    
    Redox reactions    
    Reduction in gaseous atmosphere    

    (Dodd & Tonge, 2008)

    2.3 Principle and Instrumentation of TGA

    The instrument used to carry out thermogravimetric analysis is known as “thermobalance”.

    This is a schematic diagram of a thermobalance, an instrument used for carrying out thermogravimetric analysis. The thermobalance is meant for continuous mass measurement while heating or cooling the sample. Instruments typically comprise a precision microbalance, a heating oven, and optionally a gas control system for atmosphere regulation.
    Figure 2.3 Schematic diagram of a thermobalance. (Source: https://images.app.goo.gl/uyRxxbkVNkxLF3nT7)

    Working: Please go through the Chemlibre link to understand the principle and working of a thermobalance.

    https://chem.libretexts.org/Bookshelves/Analytical_Chemistry/Instrumental_Analysis_(LibreTexts)/31%3A_Thermal_Methods/31.01%3A_Thermogravimetric_Methods

    2.4 Interpretation of thermogravimetric curve

    The graphical information obtained from thermogravimetric analysis is known as thermogram/pyrolysis curve. TG curve is a plot of weight (W) decreasing downwards on the y-axis (ordinate), and temperature (T) increasing to the right on the x-axis (abscissa). A typical thermogram for a single step decomposition is shown in Fig. 2.4.

    The plateau ‘AB’ indicates no change in weight or the temperature range over which the sample is thermally stable. At point ‘B’ the sample starts decomposing which is indicated by an inflexion.

    Please read the following text explaining the interpretation of thermogram and then attempt Activity 2.2

    https://chem.libretexts.org/Courses/Franklin_and_Marshall_College/Introduction_to_Materials_Characterization__CHM_412_Collaborative_Text/Thermal_Analysis/Thermogravimetric_analysis_(TGA)

    Activity 2.2: A typical thermogram is given below

    A simplified thermogram-style graph showing how mass changes as temperature increases. The horizontal axis is labeled ‘TEMP ’, and the vertical axis is labeled ‘MASS ↑’. The curve starts at a high, nearly flat plateau labeled A (in red), indicating an initially stable mass with rising temperature. The curve then gently slopes down through a region labeled B (in blue), representing gradual mass loss. Finally, it drops sharply into a lower, nearly flat plateau labeled C (in green), indicating a new stable mass at higher temperature. Overall, the graph conveys rapid mass decrease at a critical temperature point between B and C.”
    Figure 2.4 Typical TG curve

    Fill in the blanks:

    1. The part of the TG curve where the mass is essentially constant______________
    2. The temperature at which cumulative mass change reaches a magnitude that the thermobalance can detect ________________
    3. The temperature at which the cumulative mass change reaches a maximum_______________

    Choose from the following options.

    1. the initial temperature (B)

    2. the record of weight from temperature B to C

    3. the final temperature C

    4. the plateau (AB)

    (Dodd & Tonge, 2008)

    Activity 2.3: For the processes given in the following table, predict if there will be loss in mass or gain in mass

    Phenomenon Mass loss/Mass gain
    Melting  
    Adsorption of gas  
    Desorption of gas  
    Vaporisation  
    Dehydration  
    Decomposition  
    Sublimation  

    2.5 Need for Derivative Thermogravimetry (DTG)

    In the above example (Fig. 2.4), we have considered the thermogravimetric curve which represents a single stage decomposition. Figures ‘1.2.5a’ and ‘1.2.5b’ show two-stage and three-stage decompositions respectively. In both these figures (2.5a and 2.5b) there is an overlay of TGA and DTG thermograms, clearly depicting advantages of DTG over TGA thermogram in locating the exact decomposition temperature.

    Thermogravimetric Analysis (TGA) and Derivative Thermogravimetry (DTG) plot showing weight (%) vs. temperature (°C) for a sample. The black curve represents TGA data, indicating a significant weight loss starting around 300°C and ending around 500°C, with key temperature points labeled at approximately 84°C and 434°C. The blue curve represents the DTG data, showing the rate of weight change (dw/dt) with a major peak at the point of maximum decomposition rate. The x-axis ranges from 0°C to 1000°C, while the y-axis on the left indicates weight percentage, and the right y-axis shows the derivative (dw/dt in %/min).
    Figure 2.5a. Thermogravimetric (TG) and Derivative thermogravimetry (DTG) curves for PVP at a heating rate of 10°C/min. (Source: Al-Hada et al., 2014) https://images.app.goo.gl/fWChwNJi9uxfHZAJ7
    Thermogravimetric Analysis (TGA) and Derivative Thermogravimetry (DTG) plot showing weight (%) vs. temperature (°C) for a sample. Ca (COO)₂·H₂O decomposes below ~200°C, Ca (COO)₂ decomposes between ~200°C and ~400°C, CaCO₃ decomposes between ~600°C and ~800°C, Final residue is CaO after ~800°C. DTG graph shows peak minima for each decomposition reaction and helps in predicting precise decomposition temperature at every stage.
    Figure 2.5b. TGA test result of calcium oxalate monohydrate (Source: Chegg.com) https://images.app.goo.gl/1Hbp2vSV2rBjeFc4A

    Figure ‘2.5b’ is for the decomposition of calcium oxalate monohydrate, the weight loss commences just above 100°C and continues up to 200°C. Between about 400 °C and 500°C further decomposition occurs, to give a product which is stable up to 700°C before decomposing to give another stable compound at 800°C. Every process of decomposition continues over a range of temperature hence the DTG curve is useful in providing information regarding precise decomposition temperature at every stage.

    (James & Tonge, 2008)

    Activity 2.4: Complete the following reaction for the decomposition of calcium carbonate.
    • Calcium carbonate on heating undergoes one step decomposition. Identify the volatile and stable compound/s remaining in the crucible post decomposition and indicate these on a thermogram.

    A chemical diagram showing the thermal decomposition of calcium carbonate (CaCO₃) at 900°C. On the left, a bowl represents calcium carbonate. An arrow labelled with a heat symbol (Δ) and "900°C" points to the right, where a bowl with a question mark represents the unknown solid product, and an upward arrow with a question mark indicates the unknown gas released. The diagram implies a chemical reaction: CaCO₃ →? + ?.

    Activity 2.5: Magnesium oxalate monohydrate (MgC2O4.H2O) undergoes three step decomposition on heating.

    1. Identify the volatile product and the residue remaining in the crucible at each stage of decomposition.
    2. Write the decomposition reaction taking place at each stage.
    3. Predict nature of thermogram for the decomposition of magnesium oxalate monohydrate.

    • A bowl on the left labelled magnesium oxalate monohydrate is heated to 150°C (Δ). • This produces a bowl labelled with a question mark (?) and releases a gas labelled with a question mark (?), shown as an upward arrow. • The solid product is then heated to 420°C (Δ), producing a bowl labelled with a question mark (?) and releasing another gas (?). • In the final step, the solid is heated to 600°C (Δ), producing a bowl labelled with a question mark (?) and releasing two gases, each shown with question marks (?) and upward arrows. There is a total of 5 question marks to be filled in for products and 3 upward arrows representing unknown gases.

    Activity 2.6: Ammonium nitrate (NH4NO3) undergoes one step decomposition on heating to give two volatile products.

    1. Identify the volatile product/s and the residue remaining (if any) in the crucible after decomposition.
    2. Write the decomposition reaction.
    3. Predict nature of the thermogram for the decomposition of ammonium nitrate.

    • A bowl on the left labelled magnesium oxalate monohydrate is heated to 150°C (Δ). • This produces a bowl labelled with a question mark (?) and releases a gas labelled with a question mark (?), shown as an upward arrow. • The solid product is then heated to 420°C (Δ), producing a bowl labelled with a question mark (?) and releasing another gas (?). • In the final step, the solid is heated to 600°C (Δ), producing a bowl labelled with a question mark (?) and releasing two gases, each shown with question marks (?) and upward arrows. There is a total of 5 question marks to be filled in for products and 3 upward arrows representing unknown gases.

    Interactive Element

    i. Compare ammonium nitrate decomposition curve obtained in Activity 2.6 with calcium carbonate thermogram obtained in Activity 2.5

    ii. Can you suggest why ammonium nitrate thermogram shows zero mass post decomposition?

    2.6 Applications of Thermogravimetric analysis

    Thermogravimetric analysis of plaster for safety screening:

    Graph depicting the thermal gravimetric (TG) analysis of plaster, showing weight loss versus temperature with chemical reactions.

    Graph depicts thermogravimetric (TG) curve for plaster showing percentage weight loss as a function of temperature (°C). The x-axis ranges from 0°C to over 800°C, and the y-axis shows % weight loss (decreasing upward).
    Figure 2.6b. TG curve for plaster

    The curve has three distinct steps, each corresponding to a chemical decomposition reaction:

    1. CaSO₄·2H₂O → CaSO₄ + 2H₂O↑ around ~100–200°C
    2. Ca (OH)₂ → CaO + H₂O↑ around ~400–500°C
    3. CaCO₃ → CaO + CO₂↑ around ~600–800°C

    Each step corresponds to a weight loss due to the release of water or carbon dioxide.

    Plaster contains following ingredients,

    Gypsum--- CaSO4.2H2O

    Lime--- Ca(OH)2

    Chalk--- CaCO3

    From the weight loss at each step on the curve, the quantity of each ingredient can be determined in the original sample. In the manufacture of Portland cement, 5% gypsum is added to reduce the rate of setting. The gypsum is added to the fused clinker during processing, and the two components are subsequently milled to obtain uniform mixing and the required particle size. During milling, the thermal energy generated may cause partial dehydration of gypsum to hemihydrate CaSO4 .1/2 H2O which adversely affects (increases) the rate of setting of the cement. Hence it is important to monitor the presence of each hydrate in the final cement. In order to provide quantitation at the required levels, this problem can be solved by TGA and DTA or DSC.

    The dehydration of gypsum occurs as a two-stage endothermic process.

    CaSO4.2H2O → CaSO4.1/2 H2O → CaSO4

    So, if there is conversion of gypsum to hemihydrate, the TG curve in Fig. 2.6b will show two step decomposition for gypsum instead of one.

    Interactive Element

    Predict TG curve for plaster in which the gypsum is converted into a hemihydrate form.

    Activity 2.7: (James &Tonge, 2008)

    A manufacturer wishes to incorporate a plastic coating on the inside of a utensil. One factor to be evaluated is the stability of the following polymer.

    a. Polyethylene

    b. Polypropylene

    c. PVC

    d. Polytetrafluoroethylene

    Figure below gives TG curves for the above polymers.

    Thermogravimetric (TGA) plot showing four curves (a, b, c, d) representing weight loss (%) versus temperature (°C). The x-axis ranges from 0°C to over 600°C, and the y-axis (W%) shows weight loss from 0% (top) to 80% (bottom). Curve a (red) shows a single-step decomposition starting around 300°C. Curve b (green) shows an earlier and sharper weight loss beginning just below 300°C. Curve c (blue) has a two-step weight loss, with transitions near 300°C and 400°C. Curve d (black) shows a delayed and broader decomposition starting after 400°C and extending to ~600°C.

    Predict the following:

    1. Which is the most stable polymer?
    2. Which is the least stable polymer?
    3. Which polymer/s would be stable below 200°C ?

    So far, we learnt about qualitative applications of TGA. Some quantitative applications of thermogravimetric measurements are given below:

    Activity 2.8: Solve the following numerical problems.

    1) Calculate the percent weight changes W% for each of the following reactions which occur on heating the parent material.

    a) Ca(OH)2(S) →CaO (s) +H2O(g)

    b) 6PbO(s) + O2(g) → 2Pb3O4(s)

    [Ca=40.1, H=1.0, O=16.0, Pb=207.2]

    2) A mixture of calcium oxide and calcium carbonate is analysed by thermogravimetry. The resultant curve indicates one decomposition only between 600-900° C during which the weight of sample decreases from 250.6 mg to 190.8 mg. What is the percentage of calcium carbonate in mixture by weight?

    [Atomic mass: H=1.0, Pb=207.2, C=12.0, O=16.0, Ca= 40.1]

    3) The thermogram given below shows the mass of a sample of calcium oxalate monohydrate, CaC2O4.H2O, as a function of temperature. The original sample of 17.61 mg was heated from room temperature to 1000°C at a rate of 20°C per minute. Calculate the % weight loss at each step.

    After increasing the temperature to 200 Celsius, the sample lost 2.17mg. AT 550 Celsius, another drop of 3.38mg occurred. Finally, at 750 Celsius, a drop of 5.3mg occurred.

    Interactive Element

    For the TG curve in activity 2.8

    i. Write the decomposition reaction at each step.

    ii. Identify the volatilization product and the solid residue that remains at every step.

    Activity 2.9: Solve the puzzle

    https://thewordsearch.com/puzzle/6923734/thermal-analysis/

    This page titled 2: Thermogravimetry 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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