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SKA6014
ADVANCED ANALYTICAL CHEMISTRY
TOPIC 13Classical and Thermal Methods
Azlan Kamari, PhD
Department of ChemistryFaculty of Science and Mathematics
Universiti Pendidikan Sultan Idris
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Classical and Thermal Methods
Karl Fischer (moisture determination)
Representative of a wide variety of high-performance, modernanalytical titration methods
The only titration discussed in detail during this class
Thermal Methods
Thermogravimetry (TG)
Differential thermal analysis (DTA)
Differential scanning calorimetry (DSC)
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Karl Fischer Titration (KFT)
Applications
Food, pharma, consumer products
Anywhere where water can affect
stability or properties
Karl Fischer (German chemist)
developed a specific reaction for
selectively and specifically
determining water at low levels.
reaction uses a non-aqueous
system containing excess of sulfur
dioxide, with a primary alcohol as
the solvent and a base as the
buffering agent
A modern KF titrator
Karl Fischer titration is a widely used analytical technique
for quantitative analysis of total water content in a material
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Karl Fischer Reaction and Reagents
CH3OH + SO2+ RN [RNH]+SO3CH3
-
[RNH]+SO3CH3- + H2O + I2 + 2RN [RNH]
+SO4CH3 + 2[RNH]+I-
0.2 M I2, 0.6M SO2, 2.0 M pyridine in methanol/ethanol
Pyridine Free (e.g. imidazole)
Endpoint detection: bipotentiometric detection of by adedicated pair of Pt electrodes
Detector sees a constant current during the titration, sudden
drop when endpoint is reached (I- disappears, and only I2 is
around when the reaction finishes)
Reaction:
Reagents:
ester
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Volumetric Karl Fischer Titration
Volumetric KFT (recommended for larger samples > 50 mg)
One component Titrating agent: one-component reagent (I2, SO2, base)
Analyte of known mass added
Two component (reagents are separated) Titrating agent (I2 and methanol)
Solvent containing all other reagents used as working medium in
titration cell
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Coulometric of Karl Fischer Titration
Coulometric KFT (recommended for smaller samples < 50
mg)
Iodine is generated electrochemically via dedicated Pt
electrodes
Q = 1 C = 1A x 1s where 1 mg H2O = 10.72 C
Two methods:
Conventional (Fritted cell): frit separates the anode from the
cathode
Fritless Cell: innovative cell design (through a combination offactors but not a frit), impossible for Iodine to reach cathode
and get reduced
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Common Problems with Karl Fischer Titrations
Titration solvents: stoichiometry of the KF reaction must be
complete and rapid
solvents must dissolve samples or water may remain trapped
solvents must not cause interferences
pH
Optimum pH is 4-7
Below pH 3, KF reaction proceeds slowly
Above pH 8, non-stoichiometric side reactions are significant
Other errors:
Atmospheric moisture is generally the largest cause of error in
routine analysis
When operated properly, KFT can yield reproducible water
titration values with 2-5% w/w precision
E.g. sodium tartrate hydrate (15.66% water theory) usually yields
KFT values in the 15.0-16.4% w/w range
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Aldehydes and Ketones
Form acetals and ketals respectively with normalmethanol-containing reagents
Water formed in this reaction will then be titrated to give
erroneously high water results
With aldehydes a second side reaction can take place,consuming water, which can lead to sample water
content being underestimated
Replacing methanol with another solvent can solve the
difficulties (commercial reagents are widely available)
Common Problems with Karl Fischer Titrations
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Oven Karl Fischer
Some substances only release their water at high
temperatures or undergo side reactions
The moisture in these substances can be driven off in an
oven at 100C to 300C.
The moisture is then transferred to the titration cell using an
inert gas
Uses:
Insoluble materials (plastics, inorganics)
Compounds that are oxidized by iodine
Results in anomalously high iodine consumption leadingto an erroneously high water contents
Includes: bicarbonates, carbonates, hydroxides,
peroxides, thiosulphates, sulphates, nitrites, metal
oxides, boric acid, and iron(III) salts.
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Thermal Analysis Thermal analysis: determining a specific physical property of a
substance as a function of temperature
In modern practice:
The physical property and temperature are measured and
recorded simultaneously
The temperature is controlled in a pre-defined manner
Classification:
Methods which measure absolute properties (e.g. mass, as
in TGA)
Methods which measure the difference in some propertybetween the sample and a reference (e.g. DTA)
Methods which measure the rate at which a property is
changing
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Thermal Gravimetric Analysis (TGA)
Concept: Sample is loaded onto an accurate balance
and it is heated at a controlled rate, while its mass ismonitored and recorded. The results show the
temperatures at which the mass of the sample changes.
Selected applications: determining the presence and quantity of hydrated
water
determining oxygen content
studying decomposition
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TG Instrumentation
Components: Sensitive
analytical balance
Furnace
Purge gassystem
Computer
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Applications of TGA
H20
Ca(C00)2
CO
CaC03
CO2
Ca0
200 400 600 800 1000
Sample Temperature (C)
Sam
pleWeight
Decomposition of calcium oxalate Composition
Moisture Content Solvent Content
Additives
Polymer Content
Filler Content
Dehydration
Decarboxylation
Oxidation
Decomposition
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Typical TGA of a Pharmaceutical
1.080%(0.06419mg)
9.615%(0.5717mg)
18.90%(1.124mg)
0.0
0.2
0.4
0.6
0.8
1.0
1.2
Deriv.
Weight(%/C)
20
40
60
80
100
Weight(%)
0 50 100 150 200 250 300 350
Temperature (C)
Sample: SB332235Size: 5.9460 mgMethod: Standard MethodComment: CL42969-112A1
TGAFile: Y:...\TGA\SB332235\CL42969-112A1.001Operator: J BrumRun Date: 18-Feb-05 14:45Instrument: TGA Q500 V6.3 Build 189
Universal V3.8B TA Instruments
Blue line shows derivative
Green line shows mass changes
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Differential Thermal Analysis (DTA)
Concept: sample and a reference material are heated at
a constant rate while their temperatures are carefullymonitored. Whenever the sample undergoes a phase
transition (including decomposition) the temperature of
the sample and reference material will differ.
At a phase transition, a material absorbs heat withoutits temperature changing
Useful for determining the presence and temperatures at
which phase transitions occur, and whether or not aphase transition is exothermic or endothermic.
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DTA Instrumentation
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General Principles of DTA
H (+) endothermic reaction - temp of sample lags behind temp of reference
H (-) exothermic reaction - temp of sample exceeds that of reference
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General Principles of DTA
Glass transitions
Crystallization
Melting
OxidationDecomposition
T = Ts
- Tr
Endothermic Rxns:fusion, vaporization, sublimation, ab/desorption
dehydration, reduction, decomposition
Exothermic Rxns:
Adsorption, Crystallization
oxidation, polymerization and catalytic reactions
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Applications of DTA
Jacobson (1969) - studied effects of stearic acid and sodium
oxacillin monohydrate
simple inorganic
species
Phase transitions
determine melting,
boiling,decomposition
polymorphism
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Differential Scanning Calorimetry (DSC)
Analogous to DTA, but the heat input to sample and
reference is varied in order to maintain both at a constanttemperature.
Key distinction:
In DSC, differences in energy are measured In DTA, differences in temperature are measured
DSC is far easier to use routinely on a quantitative basis,
and has become the most widely used method for thermalanalysis
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DSC Instrumentation
There are two common DSC methods
Power compensated DSC: temperature of sample andreference are kept equal while both temperatures are
increased linearly
Heat flux DSC: the difference in heat flow into the
sample/reference is measured while the sampletemperature is changed at a constant rate
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Heat Flow in DSC
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DSC Step by Step
MeltingGlass transition Recrystallization
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Applications of DSC
DSC is usually carried
out in linear increasing-
temperature scan mode
(but can do isothermal
experiments)
In linear scan mode,
DSC provides meltingpoint data for
crystalline organic
compounds and Tg for
polymers
Easily used for detection of bound crystalline water
molecules or solvents, and measures the enthalpy of
phase changes and decomposition
DSC trace of polyethyleneterphthalate (PET)
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Applications of DSC
DSC is useful in studies
o polymorphism inorganic molecular
crystalline compounds
(e.g. pharmaceuticals,
explosives, food
products)
Example data from two
enantiotropic
polymorphs
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DSC of a Pharmaceutical Hydrate
84.39C
56.35C34.97J/g
153.30C
134.06C116.0J/g
-1.5
-1.0
-0.5
0.0
0.5
HeatFlow(W/g)
0 50 100 150 200 250 300
Temperature (C)
Sample: SB332235Size: 3.0160 mgMethod: STANDARD DSC METHODComment: CL42969-112A1
DSCFile: Y:...\DSC\SB332235\CL42969-112A1.002Operator: J BrumRun Date: 24-Feb-05 09:53Instrument: DSC Q1000 V9.0 Build 275
Exo Up Universal V3.8B TA Instruments
Loss of water
Melt Decomposition