Topic 10 Classical and Thermal Methods

7/29/2019 Topic 10 Classical and Thermal Methods

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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

Topic 10 Classical and Thermal Methods

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Transcript of Topic 10 Classical and Thermal Methods