THERMAL METHODS

THERMAL METHODS

1. 1. Measure some property P as a function of T P = f(T) 2. Measure a thermal property as a function of another property P (ch.T or ch. H)* = f(P )

   • choice of P  is very wide (mass, length, magnetism, etc.) and includes thermal properties (DT, DH)

2. DISTINCTION

   • Heat – measure of the total kinetic energy (KE) of the particles in a body – an EXTENSIVE property - depends on how many particles
   • Temperature – measure of the average kinetic energy (KE) of the particles in a body – an INTENSIVE property - independent of # of particles
     •    Example 1 mL H2O at 70 °C contains 1000 times more heat than 1 mL H2O at 70 °C   

3. Special features of thermal methods

   • Thermal ‘events’ - (processes or changes that involve a heat or temperature change) are ubiquitous (occur for all materials)
   • widely applicable
   • rarely compound specific
   • qualitative and quantitative

4. THERMOGRAVIMETRY (TG, TGA)

   • Principle - m = f(T)
   • Apparatus
     • accurate - change in mass proportional to the change in current
   • Key problems
     • Convective phenomena (affects mass measurement)
       • make sure equal heating of the compound
     • Measurement of sample T
       • make sure both the furnace and the sample are at the same temp
   • Features & Notes
     • T change usually linear & slow
     • Furnace (sample) thermally isolated from balance
     • Weighing uses ‘null’ principle
     • Furnace atmosphere controllable (N2, O2, vac.) any gases
     • Sample size ug - g (1 - 100 mg common)
   • Thermogram
     • - The m vs. T function shows steps each of these correspond to specific thermal ‘events’ – can use derivative of curves to highlight changes 
   • Applications
     • 1. Stability domains
       •    – useful for drying or decomposition of materials   – provides quantitative info about reactions & products e.g. CaC2O4×H2O   
     • 2. Qualitative identification
       • Not very useful (like UV-Vis)   although plastics and some polymers arean exception - analyse different polymers- difft polymers have diffet temp to break down   
     • 3. ‘Broadbrush’ analysis
   • Extensions
     • 1. Differential TG
       •    Take derivative (¶m/¶T) of thermogram    ¨  peaks   ¨  easier to identify overlapping ‘events’   
     • 2. Evolved gas analysis
       •   Analyse what is coming off sample rather than what is left behind

5. DIFFERENTIAL THERMAL ANALYSIS (DTA)

   • Principle
     • dT = f(T)
       • temperature difference (DT) between sample & inert reference; DT is NOT = 0 during thermal ‘event’ thermal event - prodcution/consumption of heat
     • DTA measures the difference in T between sample & an inert reference material as T is scanned
       • nxo- rise the change in temp endo- reduce in change in temp
   • Apparatus
     • Features & Notes
       • Change in T small - uses thermocouples -slow scan
       • Al2O3 or SiO2 - ref materials - x undergo thermal event
       • DTA curve shows peaks
       • Peaks +ve or -ve if process exo- or endo-thermic
   • Applications
     • identification of sediments investigation of flame retardants - fabrics (cotton & satin) - also used in building materials COMPARISON ANALYSIS - glucose polymers analysis - cornstarch, dextran, glycogen etc - amino acids - diffnt aa have diffnt pattern of DTA - a complex mixture of EDTA - different mixture w different metal cations - diffnt pattern in DTA
     • 1. Qualitative analysis
     • 2. Quantitative analysis
       • complex materials: clays, soils, minerals. 
   • typical graph - removal of water crystallisation - endo

6. DIFFERENTIAL SCANNING CALORIMETRY (DSC)

   • Principle (dQ = f(T))
     • – measuring heat (DQ) rather than DT creates a quantitative thermodynamic technique but keeps analytical advantages of DTA
   • Apparatus (works in ‘null’ mode)
     • null temperature
     • Features & Notes
       • Heat added to sample (endothermic process) or reference (exothermic) to maintain dT = 0
       • Electric power integrated to give energy
       • Area under curve proportionate to dH
   • Applications
     • 1. Characterization of phase behaviour
       • investigation of new materials eg, polymers, microemulsions, etc.
     • 2. Thermodynamic properties
       • determination of mp & dH
       • A = k X m X dH (Determine k by measuring A for known amount m of substance with known dH, eg, melting of indium or benzoic acid)
         • A - area under curve k - calorimeter constant m - mass/moles dH - enthalpy change
     • 3. Purity determination of solids
       • Based on freezing point depression (a colligative property)
         • depends only on number of particles (not their chemical character) - generic (detects all impurities)        ≳0.01 mol % - impurities (hv foreign material - req lower temp to change state - hence can compare the graphs)
       • - pure cmpds have precise melting pt. - DSC will detect the difference melting pt of a compound if its not pure (mixed w some other compounds) - and detect the amt of heat that actually occur
       • can calculate k value of the pure compound. - if the k value is known, then can calculate the amt of impurity.
     • 4. ‘Broadbrush’ analysis
     • Overview of phase behaviour of polymers as determined by DSC

7. USE OF THERMAL METHODS FOR ‘BROADBRUSH’ ANALYSIS

   • determination of broad classes of components
   • complex and composite materials

8. THERMOMETRIC TITRATIONS

   • Principle (T = f(V))
     • V = ie, number of moles of titrant rxn occur - heat generated end point - no rxn - no heat generated
   • Features
     • -Temperature is a function of volume added. -Do not depend only on dG
     • All chemical processes have heat effects
     • dH = dG + TdS
       • dH = may be large even if dG ~ 0 dG = most analytical methods req large dG dG - determine whether the rxn is spontaneous or not (-ve = spontaneous)
       • e.g. – titration of weak acids & bases
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   • Applications
   • Apparatus
     • Simple: thermometer, insulated vessel, burette
     • If Ctitrant >> Canalyte no thermostat required as V(titrant) << V(analyte)
     • dT small (<< 1 C) – use thermistor (detects dT ≳ 10–5 C)
       • C- temp
   • thermometric titration curve
     • Trace I:&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; 0.01 M boric acid titrated with 1 M NaOH Trace II:&nbsp;&nbsp;&nbsp;&nbsp; 0.01 M HCl titrated with 1 M NaOH Trace III:&nbsp;&nbsp; theoretical calculation of I. A-B: pre-titration equilibration B: start of titration C: end point of titration C-D: excess reagent line Note: temperature of titrant was 1.2 ºC less than the aliquot. The titrant try to pull the temp down, but the rxn push the temp up. after rxn finish, the temp goes down

9. DIRECT INJECTION ENTHALPIMETRY

   • Principle - a one point titration – add analyte to excess of reactive reagent
     • can do duplicates if you have excess - will tell the uncontrol variable
   • Applications . e.g. determination of nitrite
   • Apparatus - As for thermometric titrations but use syringe not burette

10. OTHER THERMAL METHODS

    • Example: thermomechanical analysis – measure effects of T on mechanical properties such as tensile strength, compressibility, etc. – widely used in the study of new materials

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