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Chemistry (Hons.) B.U. SEM-V Course Code: DSE-2H)_Ther… · Chemistry (Hons.) B.U. SEM-V; DSE-2:...
Transcript of Chemistry (Hons.) B.U. SEM-V Course Code: DSE-2H)_Ther… · Chemistry (Hons.) B.U. SEM-V; DSE-2:...
Chemistry (Hons.) B.U. SEM-V
Course Code: DSE-2
Course Title: Analytical methods in chemistry (Theo)
Chapter-3: Thermal methods of analysis
Structure
3.1 Introduction
Objectives
3.2 Instrumentation
3.3 Sources of Error in TGA
3.4 Interpretation of TG Curve
3.5 Factors Affecting TG Curve
3.6 Applications of Thermogravimetric Analysis
3.7 Composition determination from the TG curve ob binary compounds mixture
3.8 Glossary
3.9 References and Further Readings
3.10 Questions and Problems
Chemistry (Hons.) B.U. SEM-V; DSE-2: Analytical methods in chemistry
Chapter-3: Thermal methods of analysis
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Introduction:
The study of the effect of heat on materials obviously has a long history, from man's earliest attempts at
producing pottery, extracting metals (about 8000 BC) and making glass (about 3400 BC) through the
philosophical discussions of the alchemists on the elements of fire, air, earth and water, to early work on the
assaying of minerals (about 1500 AD), followed by the development of thermometry and calorimetry. Only in
the late 19th
century did experiments on the effect of heat on materials become more controlled and more
quantitative. Much of this work depended upon the development of the analytical balance which has its own
interesting history.
In very brief summary, heat is one of the forms in which energy can be transferred. Such transfer
requires a temperature difference and three mechanisms of transfer have been identified: conduction,
convection and radiation. The laws of thermodynamics deal with thermal equilibrium (zeroth), conservation of
energy (first), direction of spontaneous processes including heat transfer (second), and the reference point for
entropy measurements (third).
To provide useful qualitative and quantitative information on the effect of heat on materials, experiments have
to be carefully planned, and use is often made of sophisticated equipment. The following formal definition of
thermal analysis was originally provided by the International Confederation for Thermal Analysis and
Calorimetry (ICTAC).
Previous
Thermal analysis (TA) refers to a group of techniques in which a property of a sample is monitored
against time or temperature while the temperature of the sample, in a specified atmosphere, is
programmed.
A recent discussion of the above ICTAC definition by Hemminger and Sarge points out some of the difficulties
and suggests some modification to:
New
Chemistry (Hons.) B.U. SEM-V; DSE-2: Analytical methods in chemistry
Chapter-3: Thermal methods of analysis
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Thermal analysis (TA) means the analysis of a change in a property of a sample, which is related to an
imposed temperature alteration.
The current field of thermal analysis is both diverse and dynamic. Although not a new field, more
advanced instrumentation, techniques and applications are constantly appearing on the market and in the
literature. Theoretically, almost any substance whether solid, semi-solid or liquid can be analyzed and
characterized with thermal analytical techniques. The schematic diagram of a generalized thermal analysis
instrument is shown in the Figure 3.2. Common materials include foods, pharmaceuticals, electronic materials,
polymers, ceramics, organic and inorganic compounds, even biological organisms. In theory, all thermal
analytical techniques simply measure the change of a specific property of a material as a function of
temperature. This in turn allows researchers access to information regarding macroscopic theories of matter
including, equilibrium and irreversible thermodynamics and kinetics. While numerous techniques are available,
the primary differences in the techniques are the properties of the material being studied as listed in the bellow
(Figure 3.1).
• Dielectric thermal analysis (DEA): dielectric permittivity and loss factor
• Differential thermal analysis (DTA): temperature difference
• Differential scanning calorimetry (DSC): heat difference
• Dilatometry (DIL): volume
• Dynamic mechanical analysis (DMA): mechanical stiffness and damping
• Evolved gas analysis (EGA) : gaseous decomposition products
• Laser flash analysis (LFA): thermal diffusivity and thermal conductivity
• Thermogravimetric analysis (TGA): mass
• Thermomechanical analysis (TMA): dimension
• Thermo-optical analysis (TOA): optical properties
Chemistry (Hons.) B.U. SEM-V; DSE-2: Analytical methods in chemistry
Chapter-3: Thermal methods of analysis
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Dr. Goutam Hazra, Assistant Professor in Chemistry, Kalna College, E-mail ID: [email protected]
Figure 3.1 Flow chart of the different types of thermal analysis.
Chemistry (Hons.) B.U. SEM-V; DSE-2: Analytical methods in chemistry
Chapter-3: Thermal methods of analysis
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Figure 3.2 A generalized thermal analysis instrument and resulting thermal analysis curve.
Objectives
After studying this unit, you should be able to:
• explain the principle of TGA,
• describe the experimental setup of TGA,
• interpret the analytical information from TGA curves, and
• describe the applications of TGA in qualitative and quantitative analysis of inorganic, organic and
polymer materials.
Chemistry (Hons.) B.U. SEM-V; DSE-2: Analytical methods in chemistry
Chapter-3: Thermal methods of analysis
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Dr. Goutam Hazra, Assistant Professor in Chemistry, Kalna College, E-mail ID: [email protected]
Thermogravimetric Analysis
Thermogravimetric analysis or thermal gravimetric analysis (TGA) is a method of thermal analysis in
which the mass of a sample is measured over time as the temperature changes. This measurement provides
information about physical phenomena, such as phase transitions, absorption and desorption; as well as
chemical phenomena including chemisorptions, thermal decomposition, and solid-gas reactions (e.g., oxidation
or reduction). TGA measurements are used primarily to determine the composition of materials and to predict
their thermal stability. The technique can characterize materials that exhibit weight loss or gain due to
sorption/desorption of volatiles, decomposition, oxidation and reduction.
Thermogravimetric analysis (TGA) is the most widely used thermal method. It is based on the
measurement of mass loss of material as a function of temperature. In thermogravimetry a continuous graph of
mass change against temperature is obtained when a substance is heated at a uniform rate or kept at constant
temperature. A plot of mass change versus temperature (T) is referred to as the thermogravimetric curve (TG
curve). For the TG curve, we generally plot mass (m) decreasing downwards on the y axis (ordinate), and
temperature (T) increasing to the right on the x axis (abscissa) as illustrated in Figure 3.3. Sometime we may
plot time (t) in place of T. TG curve helps in revealing the extent of purity of analytical samples and in
determining the mode of their transformations within specified range of temperature. In thermogravimetry, the
term ‘decomposition temperature’ is a complete misnomer. In a TG curve of a single stage decomposition, there
are two characteristic temperatures; the initial Ti and the final temperature Tf (see Figure 3.3). Ti is defined as
the lowest temperature at which the onset of a mass change can be detected by thermo balance operating under
particular conditions and Tf as the final temperature at which the particular decomposition appear to be
complete. Although Ti has no fundamental significance, it can still be a useful characteristic of a TG curve and
the term procedural decomposition temperature has been suggested. The difference Tf – Ti is termed as reaction
interval.
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Chapter-3: Thermal methods of analysis
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Figure 3.3 A Typical TG Curve.
In a dynamic thermogravimetry a sample is subjected to continuous increase in temperature usually linear with
time whereas in isothermal or static thermogravimetry the sample is maintained at a constant temperature for a
period of time during which any change in mass is noted. Now we will take up the instrumentation commonly
used to obtain TG Curve. The first derivative of the TGA curve (the DTG curve) may plotted to determine
inflection points useful for in-depth interpretations as well as differential thermal analysis. From a TGA curve
we can predict the following informations i.e.
• Thermal Stability of Materials
• Oxidative Stability of Materials
• Composition of Multi-component Systems
• Estimated Lifetime of a Product
• Decomposition Kinetics of Materials
• The Effect of Reactive or Corrosive Atmospheres on Materials
• Moisture and Volatiles Content of Materials
In most of the TGA curves there occurs either weight loss or weight gain (or both) due to
sorption/desorption of volatiles, decomposition, oxidation and reduction of the samples.
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Chapter-3: Thermal methods of analysis
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Dr. Goutam Hazra, Assistant Professor in Chemistry, Kalna College, E-mail ID: [email protected]
•The Weight Loss occurs for:
Decomposition: The breaking apart of chemical bonds.
Evaporation: The loss of volatiles with elevated temperature.
Reduction: Interaction of sample to a reducing atmosphere (hydrogen, ammonia, etc.).
Desorption: Desorption of gasses from the surface of the sample.
•Weight Gain occurs for:
Oxidation: Interaction of the sample with an oxidizing atmosphere.
Absorption: Absorption of the gasses on the surface of the sample.
3.2 Instrumentation
A typical thermogravimetric analyzer consists of a precision balance with a sample pan located inside a
furnace with a programmable control temperature. The temperature is generally increased at constant rate (or
for some applications the temperature is controlled for a constant mass loss) to incur a thermal reaction. The
thermal reaction may occur under a variety of atmospheres including: ambient air, vacuum, inert gas,
oxidizing/reducing gases, corrosive gases, carburizing gases, vapors of liquids or "self-generated atmosphere";
as well as a variety of pressures including: a high vacuum, high pressure, constant pressure, or a controlled
pressure. The basic components of a typical thermobalance are listed below:
i) Balance
ii) Furnace: heating device
iii) Unit for temperature measurement and control (Programmer)
iv) Recorder: automatic recording unit for the mass and temperature changes
These components may be represented by simple block diagram as in Figure 3.4 and different parts of the TGA
instrument are shown in the Figure 3.5.
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Chapter-3: Thermal methods of analysis
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Figure 3.4 Block diagram of a Thermobalance.
Figure3.5 Different parts of the TGA instrument
Balance
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Chapter-3: Thermal methods of analysis
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Dr. Goutam Hazra, Assistant Professor in Chemistry, Kalna College, E-mail ID: [email protected]
The basic requirement of an automatic recording balance are includes accuracy, sensitivity, reproducibility, and
capacity. Recording balances are of two types, null point and deflection type. The null type balance, which is
more widely used, incorporates a sensing element which detects a deviation of the balance beam from its null
position; a sensor detects the deviation and triggers the restoring force to bring the balance beam to back to the
null position. The restoring force is directly proportional to the mass change. Deflection balance of the beam
type involve the conversion of the balance beam deflection about the fulcrum into a suitable mass – change
trace by (a) photographic recoding i.e change in path of a reflected beam of light available of photographic
recording, (b) recording electrical signals generated by an appropriate displacement measurement transducer,
and (c) using an electrochemical device. The different balances used in TG instruments are having measuring
range from 0.0001 mg to 1 g depending on sample containers used.
Furnace
The furnace and control system must be designed to produce linear heating at over the whole working
temperature range of the furnace and provision must be made to maintain any fixed temperature. A wide
temperature range generally -150 °C to 2000 °C of furnaces is used in different instruments manufacturers
depending on the models. The range of furnace basically depends on the types of heating elements are used.
Temperature Measurement and Control
Temperature measurement are commonly done using thermocouples , chromal – alumel thermocouple are often
used for temperature up to 1100 °C whereas Pt/(Pt–10% Rh) is employed for temperature up to 1750 °C.
Temperature may be controlled or varied using a program controller with two thermocouple arrangement; the
signal from one actuates the control system whilst the second thermocouple is used to record the temperature.
Recorder
Graphic recorders are preferred to meter type recorders. X-Y recorders are commonly used as they plot weight
directly against temperature. The present instrument facilitate microprocessor controlled operation and digital
data acquisition and processing using personal computer with different types recorder and plotter for better
presentation of data. In Figure 3.4 we have shown a schematic diagram of the specific balance and furnace
Chemistry (Hons.) B.U. SEM-V; DSE-2: Analytical methods in chemistry
Chapter-3: Thermal methods of analysis
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Dr. Goutam Hazra, Assistant Professor in Chemistry, Kalna College, E-mail ID: [email protected]
assembly as a whole to better understand the working of a thermobalance. In this diagram you can clearly see
that the whole of the balance system is housed in a glass to protect it from dust and provide inert atmosphere.
There is a control mechanism to regulate the flow of inert gas to provide inert atmosphere and water to cool the
furnace. The temperature sensor of furnace is linked to the programme to control heating rates, etc. The balance
output and thermocouple signal may be fed to recorder to record the TG Curve.
3.3 Sources of Error in TGA
There are a number of sources of error in TGA, and they can lead to inaccuracies in the recorded temperature
and mass data. Some of the errors may be corrected by placing the thermobalance at proper place and handling
it with great care. For understanding we are discussing some common source of errors during operation of a
thermobalance.
i. Buoyance effect: If a thermally inert crucible is heated when empty there is usually an apparent weight
change as temperature increases. This is due to effect of change in buoyancy of the gas in the sample
environment with the temperature, the increase convection and possible effect of heat from the furnace
in the balance itself. Now, in most modern thermobalances, this effect is negligible. However, if
necessary, a blank run with empty crucible can be performed over the appropriate temperature range.
The resultant record can be used as a correction curve for subsequent experiment performed in the same
condition.
ii. Condensation on balance suspension: Condensation of the sample will also affect the mass of the
sample and consequently the shape of TG curve .This can be avoided by maintaining a dynamic
atmosphere around the sample in the furnace so that the entire condensable product may be driven by
the flowing gases.
iii. Random fluctuation of balance mechanism
iv. Reaction between sample and container
v. Convection effect from furnace
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Chapter-3: Thermal methods of analysis
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vi. Turbulence effect from gas flow
vii. Induction effect from furnace
Errors of type (iii) can be avoided by proper placing of balance in the laboratory and error (v) can be avoided
by sensible choice of sample container. Last three errors (v-vii) have to be considered in the design of the
furnace, the balance and its suspension system. By avoiding excessive heating rate and proper gas flow rate
some of above mentioned errors may be avoided. In the light of above discussion it is necessary to calibrate
thermobalance before to use.
Calibration of thermobalance for the measurement of mass. It can be done by adding known mass of the sample
container and noting the reading of the chart. Temperature calibration: ferromagnetic standards are used for this
purpose. In a magnetic field there substances have shown detectable mass changes. The ferromagnetic
standards are quite suitable for the temperature range from 242 to 771° C.
3.4 Interpretation of TG Curve
Three factors should be noted when you get a TG curve:
1. General shape.
2. The particular Temperature at which changes in mass occur.
3. The magnitudes of the mass changes.
From the different shapes of the TG curves we can get the different information. Some shapes of the TG curves
(Figure 3.6) with their interpretations are given bellow:
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Chapter-3: Thermal methods of analysis
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Figure3.6 Different shapes of the TG curves.
i. The sample undergoes no decomposition with loss of volatile products over the temperature range
shown but solid phase transformation, melting, etc cannot be detected by TG.
ii. The rapid initial mass loss is characteristic of desorption or drying. If it is true, then re-run the sample
should result in type (i) curves,
iii. Single stage decomposition,
iv. Multi-stage decomposition with relatively stable intermediates: provide information on the temperature
limit of stability of reactants and intermediate products and also stoichiometry,
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Chapter-3: Thermal methods of analysis
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v. Multi-stage decomposition with no stable intermediate product. However heating-rate effect must be
considered. At low heating rate, type (v) resembles type (iv). At high heating rate, type (iv) and (v)
resemble type (iii) and lose all the details,
vi. Gain in mass due to reaction with atmosphere, e.g. oxidation of metals,
vii. Oxidation product decomposes again at higher temperature; this is not often encountered.
3.5 Factors Affecting TG Curve
. There have the following main factors which affect the shape, precision and accuracy of the experimental
results in thermogravimetry.
1. Instrumental factors: The Instrumental factors are classified into the following parts
i) Furnace heating rate.
ii) Recording or chart speed
iii) Furnace atmosphere
iv) Geometry of Sample holder/ location of sensors
v) Sensitivity of recording mechanism.
vi) Composition of sample container.
2. Sample Characteristics: The Sample Characteristics are classified into the following parts
a) Amount of sample
b) Solubility of evolved gases in sample.
c) Particle size
d) Heat of reaction
e) Sample packing
f) Nature of sample
g) Thermal conductivity.
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Chapter-3: Thermal methods of analysis
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3.6 Applications of Thermogravimetric Analysis
Followings are the different applications of Thermogravimetric Analysis.
Thermal Stability: related materials can be compared at elevated temperatures under the required atmosphere.
The TG curve can help to elucidate decomposition mechanisms.
Material characterization: TG curves can be used to "fingerprint" materials for identification or quality
control.
Compositional analysis: by careful choice of temperature programming and gaseous environment, many
complex materials or mixtures may be analyzed by decomposing or removing their components. The weight
loss at a temperature is helpful to determine the composition of a compound and follow reactions in its
decomposition. It also enables to identify crystals of unknown composition. It is used to analyze e.g. filler
content in polymers; carbon black in oils; ash and carbon in coals, and the moisture content of many substances.
Simulation of industrial processes: the thermo balance furnace is thought as mini-reactor and has ability to
perform operations like some types of industrial reactors.
Kinetic Studies: by understanding the controlling chemistry or predictive studies, a variety of methods can be
used to analyze the kinetic features of weight loss or gain.
Corrosion studies: TG provides a means of studying oxidation or some reactions with other reactive gases or
vapours.
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Chapter-3: Thermal methods of analysis
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Example of some TGA curves with their explanation:
Figure3.6 TGA thermal curve for CuSO4.5H2O
Figure3.7 Comparison of TG and DTG curves.
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Chapter-3: Thermal methods of analysis
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Figure3.8 TGA thermal curve for pure anhydrous calcium carbonate, CaCO3. The loss in mass is due to the
loss of CO2 (g) and the compound remaining is CaO.
Figure 3.9 TGA thermal curve of hydrated calcium oxalate, Ca(COO)2.nH2O, with both absorbed water from
precipitation process and water of crystallisation. There is a loss of absorbed water starting at about 90oC, and
loss of bound water at about 150oC. The stable compound above 225
oC is anhydrous calcium oxalate,
Ca(COO)2. This loses CO at about 450oC to form CaCO3. The calcium carbonate is stable until approximately
600oC, when it loses CO2to form CaO.
Chemistry (Hons.) B.U. SEM-V; DSE-2: Analytical methods in chemistry
Chapter-3: Thermal methods of analysis
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Dr. Goutam Hazra, Assistant Professor in Chemistry, Kalna College, E-mail ID: [email protected]
Composition determination from the TG curve ob binary compounds mixture
Figure 3.10 TGA thermal curves for (top) pure A compound, (middle) pure B compound and (bottom dotted
line) a mixture of A and B compounds. Because A and B compounds have unique temperatures at which mass
is lost, the composition of the mixture may be determined.
Chemistry (Hons.) B.U. SEM-V; DSE-2: Analytical methods in chemistry
Chapter-3: Thermal methods of analysis
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Dr. Goutam Hazra, Assistant Professor in Chemistry, Kalna College, E-mail ID: [email protected]
The decomposition of pure compound A and B occur at T1, and labelled as β1β2 and temperature T2, and
labelled as β3β4 respectively, as illustrated in Figure 3.10. The TG curve of mixture of A and B together is also
shown in Figure 3.10. You can see in this figure, the plateaus (corresponding to the regions of constant mass)
commence at about the same temperature as they do in the TG Curves for the pure compounds A and B. You
can also notice that the mass loss overall up to T1 is x mg and from T1 to T2 it is an additional y mg. By
measuring these two quantities x and y from the TG curves of Figure 3.10, we can determine the relative
quantities of A and B in the original binary mixture. To understand further consider the mixtures of calcium and
magnesium carbonates.
Estimation of Magnesium Oxide in a mixture of Magnesium Oxide [MgO] and Magnesium Oxalate
[Mg(COO)2.2H2O]
The following data were obtained from our TGA curve:
Original weight of sample = 25.00 mg
Weight of the sample after heating to 500oC = 10.40 mg
Loss in weight = 14.60 mg
Figure 3.11 TGA thermal curve of Magnesium oxalate.
Chemistry (Hons.) B.U. SEM-V; DSE-2: Analytical methods in chemistry
Chapter-3: Thermal methods of analysis
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Dr. Goutam Hazra, Assistant Professor in Chemistry, Kalna College, E-mail ID: [email protected]
But we have already calculated that Mg(COO)2.2H2O losses 73% of its mass upon heating to 500oC; therefore
the weight of the Mg(COO)2.2H2O in the original sample was:
mg Mg(COO)2.2H2O = �14.60��� ��������������.����
��������= 20�
Then the concentration of Mg(COO)2.2H2O in the original sample was:
% Mg(COO)2.2H2O =�������������.����
!��"��"�#�$%$&�'�&(��)100
% Mg(COO)2.2H2O =����
�*.����)100 = 80%
Thus the other 20% of the starting mixture is MgO.
Analysis of a mixture of calcium and magnesium carbonates from TGA curve
A typical TG curve of a mixture of calcium and magnesium carbonates is shown in Figure 3.12 You can notice
that a significant mass loss occurs before 210° C. This is due to the moisture present in the mixture. Another
mass loss at about 480° C is due to the following reaction:
Earlier we mentioned that CaCO3 decomposes at about 800 °C. In Figure 3.9 mass loss between about 600 °C
and 900 °C can be interpreted due to decomposition of CaCO3.
Chemistry (Hons.) B.U. SEM-V; DSE-2: Analytical methods in chemistry
Chapter-3: Thermal methods of analysis
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Dr. Goutam Hazra, Assistant Professor in Chemistry, Kalna College, E-mail ID: [email protected]
Portion of the curve ab represents a mixture of MgO and CaCO3 and cd represents a residue of the mixture of
MgO and CaO. Both these plateaus ab and cd represent weight m1 and m2, respectively. In fact mass m1 is due
to CaCO3 + MgO.
Figure 3.12 TG curve of mixture of calcium and magnesium carbonates.
Thus, m1 – m2 is the loss of CO2 between 500° C and 900° C due to the decomposition of CaCO3. Using TGA
curve we can relate the mass of different components formed during TGA experiment.
The mass of CaO (m3) formed can be calculated using following
m3 = 1.27 (m1 – m2)
This equation can be obtained as follows:
The CaO is formed by the evolution of CO2 on the decomposition of CaCO3,
Chemistry (Hons.) B.U. SEM-V; DSE-2: Analytical methods in chemistry
Chapter-3: Thermal methods of analysis
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From the above equation 1mole of CaCO3 gives 1 mole of CO2 and 1mole of CaO. Thus, moles of CO2 in the
given examples =��-–�/�
�0���/� and this is equal to moles of CaO formed.
Thus the amount of CaO must be, �� =��-–�/�
112M4�CaO�
Where, Mr(CaO) is the relative molar mass of CaO
�� =��-–�/�
11256g
�� = 1.27���–���g
We know the mass of residue left, i.e. m2, the mass of MgO (m4) can be calculated.
m4= m2-m3
Here m3 is the mass of CaO formed, which is equal to 1.27(m1-m2). Thus m4=m2-1.27(m1-m2)
Mass of the Ca (mCa) in the original sample can also be related to m1 and m2 by the following formula.
mCa = 0.91(m1-m2)
This can be obtained as follows:
We know amount of Ca in CaCO3 and CaO will be equal in moles, therefore amount of Ca is
�;" = ��2<0��=�
�0��=��= 1.27��� −���2
<0��=�
�0��=��
Where, Ar(Ca) and Mr(CaO) are the relative atomic mass and relative molar mass of Ca and CaO, respectively.
Thus,
= 1.27��� −���240
56= 0.91��� −���
Similarly, mass of magnesium in the original sample can be related to m1 and m2:
Chemistry (Hons.) B.U. SEM-V; DSE-2: Analytical methods in chemistry
Chapter-3: Thermal methods of analysis
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Dr. Goutam Hazra, Assistant Professor in Chemistry, Kalna College, E-mail ID: [email protected]
= �massofresidue − massofCaO�2IJ�K�
KJ�KL�
�M� = ��� −���2IJ�K�
KJ�KL�= ��� −���
24.3
40.3
�M� = 0.60��� −���
Analysis of a mixture of calcium oxalate and magnesium oxalate from TGA curve
Calcium oxalate monohydrate is unsatisfactory weighing form for determining calcium as oxalate because its
tendency to retain excess moisture even at 110°C, co-precipitated ammonium oxalate remains un-decomposed.
The anhydrous calcium oxalate is hygroscopic but CaCO3 is excellent weighing form if heated to 500 ± 25°C as
can be seen in TG curve of CaC2O4.H2O. Above 635oC the decomposition of CaCO3 commences to become
significant under usual laboratory conditions and completely converted to CaO at 900°C. Thus, calcium oxalate
monohydrate decomposes in three steps, first dehydration, second removal of CO and formation of CaCO3 then
third step CaCO3 decomposes to CaO (Figure 3.13).
CaC2O4.H2O → CaC2O4 + H2O at about 135-220 °C
CaC2O4 → CaCO3+CO at about 400-480 °C
CaCO3 → CaO+ CO2 at about 635-900 °C
But magnesium oxalate dihydrate decomposes in two steps instead of three steps. First, dehydration then
removal of CO and CO2 simultaneously and forming MgO, there is no horizontal corresponding to MgCO3 as it
is thermally unstable at this temperature.
MgC2O4.2H2O → MgC2O4+2H2O at about 135-220 °C
MgC2O4 → MgCO3 + CO → MgO+CO2 at about 500 °C
The TG curve for the mixture of CaC2O4.H2O and MgC2O4.2H2O shows two mass losses up to 500 °C, first at
about 200 °C due to loss of water and the second mass loss occurs in the 390-500 °C range which is due to
Chemistry (Hons.) B.U. SEM-V; DSE-2: Analytical methods in chemistry
Chapter-3: Thermal methods of analysis
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decomposition of both calcium and magnesium oxalates. Thus, at 500°C, the composition of the mixture will be
CaCO3 and MgO. Third mass loss after 500 °C is due to only the decomposition of CaCO3. If m1 and m2 are the
mass of the mixture at 500°C and 900°C then similar to the previous example we can calculate the amount of
calcium and magnesium in the original sample.
Figure 3.13 TG curve of mixture of calcium and magnesium oxalates.
3.8 Glossary
Decomposition is the process by which substances are broken down into more simple ingredients.
Oxidation and combustion a combustion reaction occurred at a high-temperature between sample and an
oxidant (usually oxygen) into a thermogravimetric analyzer at normal conditions.t is a high-temperature
exothermic redox chemical reaction. The thermogravimetric analyzer would ion combustion the sample by
heating it beyond the ignition temperature of a sample.
TGA the weight loss of a sample as a function of temperature is determined. In most cases, the sample is
heated with a constant heat rate under air or nitrogen. Sometimes, the differential weight-loss curve is also
obtained (DTG).
Chemistry (Hons.) B.U. SEM-V; DSE-2: Analytical methods in chemistry
Chapter-3: Thermal methods of analysis
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Dr. Goutam Hazra, Assistant Professor in Chemistry, Kalna College, E-mail ID: [email protected]
TGA curve a plot of weight% vs. temperature, usually consisting of a series of steps, obtained by measuring
the mass of a sample during heating.
Thermal stability In a desired temperature range, if a species is thermally stable, there will be no observed
mass change. TGA can be used to evaluate the thermal stability of a material.
3.9 References and Further Readings
1. G. Lombardi, For Better Thermal Analysis, 2nd ed., ICTA, Rome (1980).
2. J. O. Hill, For Better Thermal Analysis and Calorimetry, 3rd ed., ICTAC (1991).
3. P.J. Haines, Thermal Methods of Analysis: Principles, Applications and Problems, Springer (1995),
ISBN 978-94-011-1324-3.
4. T. Hatakeyama, Liu Zhenhai. Handbook of Thermal Analysis Hardcover, Wiley-Blackwell
(1998)ISBN-13: 978-0471983637.
5. Paul Gabbott, Principles and Applications of Thermal Analysis, Blackwell Publishing Ltd (2008),
ISBN: 9781405131711.
6. Robert F. Speyer, Thermal Analysis of Uaterlals. Marcel Dekker, Inc. New York (1993), ISBN 0-8247-
8963-6.
7. 3.10 Questions and Problems
Q1. Calculate the percentage mass change (m %) for the following reactions.
Q2. What are common source of errors in thermogravimetric analysis?
Q3. What are the common instrumental factors affecting TG curves.
Chemistry (Hons.) B.U. SEM-V; DSE-2: Analytical methods in chemistry
Chapter-3: Thermal methods of analysis
Page 26
Dr. Goutam Hazra, Assistant Professor in Chemistry, Kalna College, E-mail ID: [email protected]
Q4. A thermogram of a magnesium compound shows a loss of 94.0 mg from a total of 180.0 mg
used for analyte. Identify the compound either as MgO, MgCO3, or MgC2O4.
Q5. A mixture of CaO and CaCO3 is analysed by TGA. The result indicates that mass of the
sample decreases from 250.6 mg to 190.8 mg only between 600°C and 900°C calculate the percentage
of calcium carbonate in the mixture.
Q6. What types of standard are required to calibrate the mass variation obtained with TGA
equipment?
Q7. What types of standards are required for temperature calibration of any TGA.
Q8. A mixture of CaCO3 and CaO is analysed using TGA technique. TG curve of the sample
indicates that there is a mass change from 175.3 mg to 125.4 mg between 500–900°C. Calculate the
percentage of CaCO3 in the sample.
Q9. A 250 mg hydrated sample of Na2HPO4 decreases to a mass of 165.7 mg after heating to 15
°C. What is the number of water hydration in Na2HPO4.
Q10. Draw a labeled diagram of the TG curve obtained by heating a mixture of 90 mg of
CaC2O4.H2O and 80 mg of MgC2O4.H2O to 1200 °C. Calculate the amount of all mass losses.
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