6 Chapter 1 THE REASONS FOR USING THERMAL AND cALORIMETRIC METHODS Novice analysts may enquire why yet another technique is needed when gas chromatography,molecular and atomic spectrometry and elec trochemical analysis plus many other powerful analytical tools are avail- able.The answer might best be given by considering two practical examples. First,h can you analys ea mixture of pr ocessed minerals such as cement?Although X-ray diffraction might tell you the different minerals present and atomic absorption spectrometry could measure the elements quantitatively,this does not help to analyse how the cement would behave in practice.For this we need to compare the behaviour under conditions of mechanical and thermal stress and the thermoanalyt techniqu of TG,DTA and TMA are tan t tools for Second,the preparation of new chemica Is lor nev pharma ceutical products,synthetic materials and foods could add to the hazards which workers and customers face.Thermal instability and explosive behaviour can be extremely destructive and costly events.Reaction calorimetry and similar techniques can help to predict the likely behavic our of chemicals when and storage ed.7.8 ph ysiologica behaviour may vary with the nature and interconversion of these forms is often studied by thermal and calorimetric methods. Many analytical techniques require samples in a particular form.For example,gas-liquid chromatography and mass spectrometry need vol- atile UV-VIS sP ctrometry usu uses solut ons.Th fore,in analysing we destroy the structure of the matrix containing the sample.This has two disadvantages:(i)the behaviour of the sample in its original matrix may be different and(ii)it is time-consuming to alter the form.It is possible to use thermal methods to study the sample"as avoids laborious change the ther mal and molecular history of the gives in orma on to the analyst about the real sample and how it would behave in the situation or process where it is actually used. THE NEED FOR PROPER PRACTICE nou1uads rdues snbruu r u bands in an infrared spec toobtain the spectrum.whethertodise.Nujol mul or a solution and whether it is obtained by a dispersive or a Fourier
6 Chapter 1 THE REASONS FOR USING THERMAL AND CALORIMETRIC METHODS Novice analysts may enquire why yet another technique is needed when gas chromatography, molecular and atomic spectrometry and electrochemical analysis plus many other powerful analytical tools are available. The answer might best be given by considering two practical examples. First, how can you analyse a mixture of processed minerals such as a cement? Although X-ray diffraction might tell you the different minerals present and atomic absorption spectrometry could measure the elements quantitatively, this does not help to analyse how the cement would behave in practice. For this we need to compare the behaviour under conditions of mechanical and thermal stress and the thermoanalytical techniques of TG, DTA and TMA are important tools for doing thi~.~?~ Second, the preparation of new chemicals for new pharmaceutical products, synthetic materials and foods could add to the hazards which workers and customers face. Thermal instability and explosive behaviour can be extremely destructive and costly events. Reaction calorimetry and similar techniques can help to predict the likely behaviour of chemicals when reactions, transport and storage are concerned. 798 Physiological behaviour may vary with the nature and form of a drug, and the nature and interconversion of these forms is often studied by thermal and calorimetric methods. Many analytical techniques require samples in a particular form. For example, gas-liquid chromatography and mass spectrometry need volatile samples and UV-VIS spectrometry usually uses solutions. Therefore, in analysing we destroy the structure of the matrix containing the sample. This has two disadvantages: (i) the behaviour of the sample in its original matrix may be different and (ii) it is time-consuming to alter the form. It is possible to use thermal methods to study the sample “as received”. This avoids laborious preparation, does not change the thermal and molecular history of the sample and gives information to the analyst about the real sample and how it would behave in the situation or process where it is actually used. THE NEED FOR PROPER PRACTICE Some analytical techniques are sample specific. The “group frequency” bands in an infrared spectrum are largely independent of the method used to obtain the spectrum, whether it is run as a solid KBr disc, a Nujol mull or a solution and whether it is obtained by a dispersive or a Fourier
Introduction 7 transform instrument.Similarly the titration of an acid with a base should give the same result whether the end-point is detected by an indicator or electrochemically. This is not always so in the case of thermal methods.The results obtained de epend upon the conditions used to pre pare the sa mple,the instrumental parameters selec ed for the run and the chemical react involved.That is not to say that results are not reproducible provided similar conditions are selected.For example,it is possible to compare samples of a polymer to see if their behaviour is "good"or "bad"accord- ing to their potential use,but the experimental parameters used for ng ea mple must be the s The useful acronym (sample-crucible-rate of heating atmosphere-mass)will enable the analyst to obtain good,reproducible results for most thermal methods provided that the following details are recorded for each run: The sample:A proper chemical description must be given together with e source pre-tre ents.The hist can al y of the sample impur rities and dilution with inert materia affec sults The crucible:The material and shape of the crucible or sample holder is important.Deep crucibles may restrict gas flow more than flat,wide ones. and platinum crucibles catalyse some reactions more than alumina ones. The type of holder or clamping used for thermomechanical methods is mportant.The make and type of instrur ment used should also be recorded The rate of heating:This has most important effects.A very slow heating rate will allow the reactions to come closer to equilibrium and there will be less thermal lag in the apparatus.Conversely,high heating rates will give a faster experiment,deviate more from equilibrium and cause grea ater therma The pa eters sof special heating pro grammes,such as modulated temperature or sample control,must b noted. The atmosphere:Both the transfer of heat,the supply and removal of gaseous reactants and the nature of the reactions which occur,or are prevented,depend on the chemical nature of the atmosphere and its flow. Oxidations will ell i n oxygen les ssoin air and not at all in argo Product removal by a fairly rapid gas flow may prevent reverse reaction occurring. The mass of the sample:A large mass of sample will require more energy,and heat transfer will be determined by sample mass and dimen- sions.These include the volume.packing.and particle size of the sample Fin epowders eact ranidly more slowly.Large samples may allow the detection of sm effects.Comparison of runs should preferably be
Introduction 7 transform instrument. Similarly the titration of an acid with a base should give the same result whether the end-point is detected by an indicator or electrochemically. This is not always so in the case of thermal methods. The results obtained depend upon the conditions used to prepare the sample, the instrumental parameters selected for the run and the chemical reactions involved. That is not to say that results are not reproducible provided similar conditions are selected. For example, it is possible to compare samples of a polymer to see if their behaviour is “good” or “bad” according to their potential use, but the experimental parameters used for running each sample must be the same. The useful acronym “SCRAM” (sample-crucible-rate of heatingatmosphere-mass) will enable the analyst to obtain good, reproducible results for most thermal methods provided that the following details are recorded for each run:9 The sample: A proper chemical description must be given together with the source and pre-treatments. The history of the sample, impurities and dilution with inert material can all affect results. The crucible: The material and shape of the crucible or sample holder is important. Deep crucibles may restrict gas flow more than flat, wide ones, and platinum crucibles catalyse some reactions more than alumina ones. The type of holder or clamping used for thermomechanical methods is equally important. The make and type of instrument used should also be recorded. The rate of heating: This has most important effects. A very slow heating rate will allow the reactions to come closer to equilibrium and there will be less thermal lag in the apparatus. Conversely, high heating rates will give a faster experiment, deviate more from equilibrium and cause greater thermal lag. The parameters of special heating programmes, such as modulated temperature or sample control, must be noted. The atmosphere: Both the transfer of heat, the supply and removal of gaseous reactants and the nature of the reactions which occur, or are prevented, depend on the chemical nature of the atmosphere and its flow. Oxidations will occur well in oxygen, less so in air and not at all in argon. Product removal by a fairly rapid gas flow may prevent reverse reactions occurring. The mass of the sample: A large mass of sample will require more energy, and heat transfer will be determined by sample mass and dimensions. These include the volume, packing, and particle size of the sample. Fine powders react rapidly, lumps more slowly. Large samples may allow the detection of small effects. Comparison of runs should preferably be
8 Chapter 1 made using similar sample masses,sizes and shapes. Specific techniques reauire the recording of other parameters.for mple the load on the sample in thermomechanical analysi method. too,require attention to the exact details of eac experiment.In the following chapters the principles and practice of thermal analysis and of calorimetry will be described and illustrated with some of the many examples of its use in industry,academic research and testing. FURTHER READING An extensive list of reference sources and specialist texts is given in Appendix 1B.Some general texts which introduce thermal analysis and calorimetry for analytical studies are listed here. General Analytical Chemistry Books with Chapters on Thermal and Calorimetric Methods G.D.Christian and J.E.O'Reilly,Instrumental Analysis,Allyn Bacon Inc.Boston.2nd edn.1986. F.W.Fifield and D.Kealey,Principles and Practice of Analytical Chemis- Kener.J-M.Mermet,M.Otto and H.M.Widmer (ed. ackw ,Oxford,5th ,200 Chemistry,Wiley-VCH,Weinheim Chichester,1998. I.M.Kolthoff,P.J.Elving and C.B.Murphy (ed.),Treatise on Analytical Chemistry,Part 1,Theory and Practice(2nd edn.)Vol.12,Section J,Wiley, New York 1983 D.A.Skoog and J.L.Leary Principles of Instrumental Analysis,Saun ders,New York,4th edn.,1992. C.L.Wilson,D.W.Wilson (ed.),Comprehensive Analytical Chemistry, Elsevier,Amsterdam,1981-1984,Vol XII,A-D. J.D.Winefordner (ed.),Treatise on Analytical Chemistry,Wiley,New York 1993 Part 1 vol 13 R.A.Meyers(ed.),Encyclopedia of Analytical Chemistry,Wiley,Chiches ter,2000 REFERENCES 1.Quantities,Units and Symbols,Royal Society,London,1971 2 I Mills T Cvitas K H nn,N Kallay and K. itsu,Quanti- ties,Units and Symbols in Physical Chemistry,IUPAC,Blackwell Oxford,1993
8 Chapter 1 made using similar sample masses, sizes and shapes. Specific techniques require the recording of other parameters, for example the load on the sample in thermomechanical analysis. Calorimetric methods, too, require attention to the exact details of each experiment. In the following chapters the principles and practice of thermal analysis and of calorimetry will be described and illustrated with some of the many examples of its use in industry, academic research and testing. FURTHER READING An extensive list of reference sources and specialist texts is given in Appendix 1B. Some general texts which introduce thermal analysis and calorimetry for analytical studies are listed here. General Analytical Chemistry Books with Chapters on Thermal and Calorimetric Methods G. D. Christian and J. E. O’Reilly, Instrumental Analysis, Allyn & Bacon Inc., Boston, 2nd edn., 1986. F. W. Fifield and D. Kealey, Principles and Practice of Analytical Chemistry, Blackwells, Oxford, 5th edn., 2000. R. Kellner, J-M. Mermet, M. Otto and H. M. Widmer (ed.), Analytical Chemistry, Wiley-VCH, Weinheim & Chichester, 1998. I. M. Kolthoff, P. J. Elving and C. B. Murphy (ed.), Treatise on Analytical Chemistry, Part 1, Theory and Practice (2nd edn.) Vol. 12, Section J, Wiley, New York, 1983. D. A. Skoog and J. L. Leary, Principles of Instrumental Analysis, Saunders, New York, 4th edn., 1992. C. L. Wilson, D. W. Wilson (ed.), Comprehensive Analytical Chemistry, Elsevier, Amsterdam, 198 1-1984, Vol XIT, A-D. J. D. Winefordner (ed.), Treatise on Analytical Chemistry, Wiley, New York, 1993, Part 1, Vol. 13. R. A. Meyers (ed.), Encyclopedia of Analytical Chemistry, Wiley, Chichester, 2000. REFERENCES 1. 2. Quantities, Units and Symbols, Royal Society, London, 1971. I. Mills, T. Cvitas, K. Homann, N. Kallay and K. Kuchitsu, Quantities, Units and Symbols in Physical Chemistry, IUPAC, Blackwell, Oxford, 1993
Introduction 9 3.R.C.Mackenzie,in Treatise on Analytical Chemistry,ed.I.M. Kolthoff,P.J.Elving and C.B.Murphy,Part 1,Theory and Practice (2nd edn.),Vol.12,Section J,Wiley,New York,1983,pp.1-16. 4.W.Hemminger and S.M.Sarge,Handbook of Thermal Analysis and Calorimetry,ed.M.E.Brown,Elsevier,Amsterdam,1998,Vol.1,Ch 5.Recommendations for the Testing of High Alumina Cement Concrete by Thermal Techniques,Thermal Methods Group,London,1975. 6.H.G.Wiedemann and M.Roessler,in Proc.7th ICTA,ed.B.Miller, Wiley.Chichester,1982.p.1318. 7.U.von Stockara and I.Marison,Thermochim.Acta,1991,193,215. 8.M.Angberg.C.Nystrom and S.Cantesson,Int.J.Pharm.,1990,61 67. 9.P.J.Haines,Thermal Methods of Analysis,Blackie,Glasgow,1995
Introduction 9 3. R. C. Mackenzie, in Treatise on Analytical Chemistry, ed. 1. M. Kolthoff, P. J. Elving and C. B. Murphy, Part 1, Theory and Practice (2nd edn.), Vol. 12, Section J, Wiley, New York, 1983, pp. 1-16. W. Hemminger and S. M. Sarge, Handbook of Thermal Analysis and Calorimetry, ed. M. E. Brown, Elsevier, Amsterdam, 1998, Vol. 1, Ch. 1. Recommendations for the Testing of High Alumina Cement Concrete by Thermal Techniques, Thermal Methods Group, London, 1975. H. G. Wiedemann and M. Roessler, in Proc. 7th ICTA, ed. B. Miller, Wiley, Chichester, 1982, p. 1318. U. von Stockara and I. Marison, Thermochim. Acta, 1991,193,215. M. Angberg, C. Nystrom and S. Cantesson, Int. J. Pharm., 1990,61, 67. P. J. Haines, Thermal Methods of Analysis, Blackie, Glasgow, 1995. 4. 5. 6. 7. 8. 9
Chapter 2 Thermogravimetry and Derivative Thermogravimetry G.R.Heal Formerly of Department of Chemistry and Applied Chemistry, University of Salford,UK INTRODUCTION AND DEFINITIONS Thermogravimetry(TG)is an experimental technique used in a complete evaluation and interpretation of results when it is known as Thermog ravimetric Analysis(TGA).The technique has been defined by ICTAC (the International Confederation for Thermal Analysis and Calorimetry) as a technique in which the mass change of a substance is measured as a function of temperature whilst the sub stance is subjected to a controlled temperature programm .The temperature programme st be tak en to include holding the sample at a constant temperature other than ambient, when the mass change is measured against time.Mass loss is only seen if a process occurs where a volatile component is lost.There are,of course, reactions that may take place with no mass loss.These may be detected by the allied techniques of Differential Thermal Analysis (DTA)and Differential Scanning Calorimetry(DSC)which are described in Ch e 3.Results are presented as a plot of mass,m,against temperature,T time,t.The mass loss then appears as a step.This is shown in Figure 1(A) The temperature range shown in this plot has been restricted to 400 to 600C to show the detail of the step.In a normal experiment the tempera- ture might be run from room temperature to 1000C or higher.It should be noted that the shape mass loss occurs around one temperature,where the line is steepest,some 10
Chapter 2 Thermogravimetry and Derivative Thermogravimetr y G. R. Heal Formerly of Department of Chemistry and Applied Chemistry, University of Salford, UK INTRODUCTION AND DEFINITIONS Thermogravimetry (TG) is an experimental technique used in a complete evaluation and interpretation of results when it is known as Thermogravimetric Analysis (TGA). The technique has been defined by ICTAC (the International Confederation for Thermal Analysis and Calorimetry) as a technique in which the mass change of a substance is measured as a function of temperature whilst the substance is subjected to a controlled temperature programme.' The temperature programme must be taken to include holding the sample at a constant temperature other than ambient, when the mass change is measured against time. Mass loss is only seen if a process occurs where a volatile component is lost. There are, of course, reactions that may take place with no mass loss. These may be detected by the allied techniques of Differential Thermal Analysis (DTA) and Differential Scanning Calorimetry (DSC) which are described in Chapter 3. Results are presented as a plot of mass, rn, against temperature, T or time, t. The mass loss then appears as a step. This is shown in Figure l(A). The temperature range shown in this plot has been restricted to 400 to 600°C to show the detail of the step. In a normal experiment the temperature might be run from room temperature to 1000°C or higher. It should be noted that the shape is sigmoid in nature, that is, although most mass loss occurs around one temperature, where the line is steepest, some 10