Thermal Methods
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Transcript of Thermal Methods
CHM 342
Thermal Methods of Analysis
Background and Continued Evolution into Hyphenated Methods of Chemical Analysis
CHM 342
Thermal Methods of Analysis
Properties are measured as a function of temperature, time, or both Heat flow – direction and magnitude Mass change – loss / gain Mechanical properties
Sheer Strain Dynamic loading
Gas evolution
CHM 342
Traditional Thermal Analysis Calorimetric Methods of Analysis
Coffee cup calorimetry (constant P) Bomb calorimetry (constant V)
Gravimetric Methods of Analysis Heating until constant weight loss Traditional C / H analysis
Differential Thermal Analysis (DTA) Analysis of heat flow direction (endo vs. exo)
as a function of temperature as a function of time at a given temperature
CHM 342
Heat to constant mass Loss of waters of hydrationCuSO45H2O(s) CuSO4(s) + 5 H2O(g)
Decomposition of Oxalates
CaC2O4(s) + ½ O2(g) CaCO3(s) + CO2(g)
Carbonates
CaCO3(s) CaO(s) + CO2(g)
CHM 342
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 Cand 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
CHM 342
Differential thermal analysis (DTA) 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.
CHM 342
Evolution of Thermal Analysis ThermoGravimetric Analysis (TGA)
Analysis of mass change as a function of temperature as a function of time at a given temperature
Differential Scanning Calorimetry (DSC) Quantification of heat flow
as a function of temperature as a function of time at a given temperature
Dynamic Mechanical Analysis (DMA) ThermoMechanical Analysis (TMA) and more . . .
CHM 342
TGA – Principle of Operation
Thermogravimetry (TG) determines the mass change of a sample as a function of temperature or time.
A good tool for: quality control and assurance failure analysis of complex polymer mixtures and
blends study of a variety of chemical processes accompanied
by mass changes
CHM 342
TGA – Equipment The heart of the instrument
is the balance . . . . Rigorous demands for
microbalance in variable
temperature environ.
Data – massloss as a functionof temperatureor timeSometimes derivativeplot used to find pts. of inflection
CHM 342
Differential Scanning Calorimetry Differential Scanning Calorimetry (DSC) is one of the
most frequently used techniques in the field of thermal characterization of solids and liquids
melting/crystallization behavior solid-solid reactions polymorphism degree of crystallinity glass transitions cross-linking reactions oxidative stability decomposition behavior purity determination specific heat
CHM 342
Differential Scanning Calorimetry – Principle of Operation 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.
CHM 342
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
•Kinetic and thermodynamic information •Vary ramp rate to extract infoon activation energy barriers
CHM 342
DSC with TGA
Combine the thermo/kinetic data of DSC with the stoichiometric data from TGA
Increases complexity, cost, and information obtained
Precursor Bi(tmhd)3Molecular formula (C11H19O2)3BiVapor pressure 0.1 Torr at 160°CPhase & Color Colorless crystallineMelting point 112-116°C
CHM 342
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
CHM 342
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.
TGA-QMS measurement on FEP
Evolved Gas Analysis (EGA) using TGA and Mass Spectrometry
CHM 342
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
CHM 342
Pulse Thermal Analysis
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:
CHM 342
Pulse Thermal Analysis
Injection of gas which reacts chemically w/solids: 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
CHM 342
Pulse Thermal Analysis
Injection of gas which adsorbs on the solid: 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.
CHM 342
Pulse Thermal Analysis
Injection of inert gas for calibration of the MS - direct calibration for MS quantitation 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.