Transcript Thermal Methods of Analysis

Thermal Methods of Analysis
Background and Continued Evolution into Hyphenated
Methods of Chemical Analysis
CHM 342
Thermal Methods of Analysis
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Properties are measured as a function of
temperature, time, or both
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Heat flow – direction and magnitude
Mass change – loss / gain
Mechanical properties
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Sheer
Strain
Dynamic loading
Gas evolution
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Traditional Thermal Analysis
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Calorimetric Methods of Analysis
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Gravimetric Methods of Analysis
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Coffee cup calorimetry (constant P)
Bomb calorimetry (constant V)
Heating until constant weight loss
Traditional C / H analysis
Differential Thermal Analysis (DTA)
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Analysis of heat flow direction (endo vs. exo)
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as a function of temperature
as a function of time at a given temperature
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Coffee Cup Calorimetry
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Bomb Calorimetry
Adiabatic vs. Isoperibol
No heat flow vs. corrected
for heat flow . . .
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Heat to constant mass
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Loss of waters of hydration
CuSO45H2O(s)  CuSO4(s) + 5 H2O(g)
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Decomposition of
Oxalates
CaC2O4(s) + ½ O2(g)  CaCO3(s) + CO2(g)
 Carbonates
CaCO3(s)  CaO(s) + CO2(g)
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Combustion Analysis
a known mass of a compound (with an unknown formula but
known elemental makeup) is burned in an excess of O2
 CuO oxidizes traces of C
and CO into CO2. It also
ensures that all of the H2
is oxidized completely to H2O
 H2O is collected in
an absorber filled Mg(ClO4)2
 CO2 is collected in a separate
absorber filled with NaOH
 The change in mass of the absorbers is used to determine the
amount of CO2 and H2O produced and thus the initial
amount of C and H in the compound
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Differential thermal analysis (DTA)
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DTA involves heating or
cooling a test sample and an
inert reference under identical
conditions, while recording
any temperature difference
between the sample and
reference.
This differential temperature is
then plotted against time, or
against temperature.
Changes in the sample which
lead to the absorption or
evolution of heat can be
detected relative to the inert
reference.
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Evolution of Thermal Analysis
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ThermoGravimetric Analysis (TGA)
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Analysis of mass change
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Differential Scanning Calorimetry (DSC)
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Quantification of heat flow
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as a function of temperature
as a function of time at a given temperature
as a function of temperature
as a function of time at a given temperature
Dynamic Mechanical Analysis (DMA)
ThermoMechanical Analysis (TMA)
and more . . .
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TGA – Principle of Operation
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Thermogravimetry (TG) determines the mass
change of a sample as a function of temperature
or time.
A good tool for:
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quality control and assurance
failure analysis of complex polymer mixtures and
blends
study of a variety of chemical processes accompanied
by mass changes
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TGA – Equipment
The heart of the instrument
is the balance . . . .
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Rigorous demands for
microbalance in variable
temperature environ.
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Data – mass
loss as a function
of temperature
or time
Sometimes derivative
plot used to find
pts. of inflection
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Differential Scanning Calorimetry
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Differential Scanning Calorimetry (DSC) is one of the
most frequently used techniques in the field of thermal
characterization of solids and liquids
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melting/crystallization behavior
solid-solid reactions
polymorphism
degree of crystallinity
glass transitions
cross-linking reactions
oxidative stability
decomposition behavior
purity determination
specific heat
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Differential Scanning Calorimetry –
Principle of Operation
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a sample is placed inside a crucible which is then
placed inside the measurement cell (furnace) of
the DSC system along with a reference pan which
is normally empty (inert gas may be used).
By applying a controlled temperature program
(isothermal, heating or cooling at constant rates),
phase changes can be characterized and/or the
specific heat of a material can be determined.
Heat flow quantities are calculated based on
calibrated heat flow characteristics of the cell.
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Differential Scanning Calorimetry –
Equipment
Two pans
 Heat transfer disk (almost always made of
Constantan – an alloy of 60% Cu and 40% Ni)
 Data on endo or exo transitions at constant
temperature or during a temperature ramp
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•Kinetic and thermodynamic
information
•Vary ramp rate to extract info
on activation energy barriers
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DSC Data
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DSC with
TGA
Precursor
Molecular formula
Vapor pressure
Phase & Color
Melting point
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Bi(tmhd)3
(C11H19O2)3Bi
0.1 Torr at 160°C
Colorless crystalline
112-116°C
Combine the thermo/kinetic data of DSC with the
stoichiometric data from TGA
Increases complexity, cost, and information obtained
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Evolved Gas Analysis (EGA) using TGA
and MS
Attach a reasonably priced
(Quadrupole?) MS to a TGA
 While monitoring mass loss
with the TGA also examine
the gases present in the inert
background gas stream
 Allows the chemistry proposed based on mass
loss data to be confirmed via gas analyses
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Evolved Gas Analysis (EGA) using TGA
and Mass Spectrometry
TGA-QMS
measurement on FEP
A fluorinated ethylene-propylene copolymer (7.9 mg) was heated at 10
K/min in He atmosphere. Decomposition occurs in two steps. Tetrafluor
-ethylene (100 amu) and hexafluor-propylene (150 amu) were detected.
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Evolved Gas Analysis with FT-IR
Attach a reasonably
priced FT-IR to a TGA
 While monitoring
mass loss with the
TGA also examine
the gases present in
the inert background
gas stream w/FT-IR
 Allows the chemistry proposed based on mass loss
data to be confirmed via gas analyses
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Evolved
Gas
Analysis
with
FT-IR
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Pulse Thermal Analysis
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Developed within
the last decade to
allow analysis of
reaction products in
various gases
Pulse gases in . . .
Monitor products at
various temperatures
Depending on the type of gas
injected, the method offers three
primary options for the
investigation of gas-solid reactions:
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Pulse Thermal
Analysis
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Injection of gas which reacts
chemically w/solids:
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Investigation of changes in the
solid phase & gas composition
resulting from the injected gas
pulse.
Chemical reactions such as
reduction, oxidation, or
catalytic processes between
solid catalyst and gaseous
reactant(s) can be investigated
at desired temperatures.
See Figure for redox sequence
in the zirconia-supported PdO
catalyst: reduction of PdO by
methane and subsequent
reoxidation of Pd by oxygen at
500°C
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Pulse Thermal
Analysis
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Injection of gas which adsorbs
on the solid:
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Investigation of adsorption
phenomena occurring under
atmospheric pressure at required
temperatures.
Figure depicts the adsorption of
ammonia at 200°C on ZSM-5
zeolite.
Exothermal effect (section A) is
related to weight gain resulting
from NH3 chemisorption (allows
determination of the heat of
reaction per mole of adsorbed
NH3).
Section B presents the reversible
physisorption process.
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Pulse
Thermal
Analysis
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Injection of inert gas for calibration of the
MS - direct calibration for MS quantitation
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introduce a known amount of the analyzed gas
into the carrier gas
determine the relationship between the amount
of the gas and the intensity of the MS signal.
Ex. During the calcination of CaCO3,
two pulses of the reaction product CO2
were injected before and after
the MS signal (m/z = 44) resulting
from the decomposition.
The stoichiometric weight loss for
the 4.62 mg of CaCO3 is 2.03 mg,
the amount of evolved CO2 measured
by the TG curve was 2.02 mg.
The CO2 calculated from thecalibrated MS data corresponds to 2.01 mg.
CHM 342