26 Chapter 2 substance undergoes several reactions,as does calcium oxalate as shown below in Figure 6(A),then these reactions are separate and unconnected. This neans tha it ea ch tion must betreated separate Each will hav h will generate a set of values,each running from 0 to 1.Thus the whole decomposition of calcium oxalate will generate three sets of a values. A TYPICAL THERMOGRAVIMETRIC EXPERIMENT An experiment is described to follow the mass losses when calcium oxalate m onohydrate is h eated rom room temperature to 1000C.The object of the experiment is to find out how many steps of weight loss are seen and the temperatures at which the processes occur and if any reactions overlap.The weight losses are to be calculated and com- pared with%losses from probable reaction mechanisms. Some of the instructions may not be a applicable to all types of equip ment wer models are e likely use,but analy ts ma y be faced with operating older models of thermobalance.Tips will be given on the best procedure. Preliminary Steps ingswitched on the apparatus,it should be allowed to about half an hou and the uring this period the zero mass reading of the bala is likely to drift.The measuring thermocouple system and furnace ther mocouples may require reference junctions.If the instrument uses a"cold junction"reference,at this point the thermos flask should be filled with fresh ice and the sheathed thermocouples inserted.This allows them to cool to 0C. Crucible Treatment The balance should be arrested and in the standby mode of the control system.Before removing the crucible from the balance,ensure that the crucible cannot be dropped into the furnace or anywhere else where it will be difficult to retrieve.This is obviously important where the furnace lies below the bala anism.There ld be a lever or knob on the balance to low the furnace away from the sample(ins me cases furnace is above the balance and lifts upwards or slides sideways).The crucible should be removed from the cradle or holder on the balance
26 Chapter 2 substance undergoes several reactions, as does calcium oxalate as shown below in Figure 6(A), then these reactions are separate and unconnected. This means that each reaction must be treated separately. Each will have a maximum mass loss for that mechanism alone and each will generate a set of a values, each running from 0 to 1. Thus the whole decomposition of calcium oxalate will generate three sets of a values. A TYPICAL THERMOGRAVIMETRIC EXPERIMENT An experiment is described to follow the mass losses when calcium oxalate monohydrate is heated from room temperature to 1000°C. The object of the experiment is to find out how many steps of weight loss are seen and the temperatures at which the processes occur and if any reactions overlap. The % weight losses are to be calculated and compared with % losses from probable reaction mechanisms. Some of the instructions may not be applicable to all types of equipment. Newer models are likely to be simpler to use, but analysts may be faced with operating older models of thermobalance. Tips will be given on the best procedure. Preliminary Steps Having switched on the apparatus, it should be allowed to equilibrate for about half an hour, so that the electronic system warms up and the conditions settle. During this period the zero mass reading of the balance is likely to drift. The measuring thermocouple system and furnace thermocouples may require reference junctions. If the instrument uses a “cold junction” reference, at this point the thermos flask should be filled with fresh ice and the sheathed thermocouples inserted. This allows them to cool to 0°C. Crucible Treatment The balance should be arrested and in the standby mode of the control system. Before removing the crucible from the balance, ensure that the crucible cannot be dropped into the furnace or anywhere else where it will be difficult to retrieve. This is obviously important where the furnace lies below the balance mechanism. There should be a lever or knob on the balance case to lower the furnace away from the sample (in some cases the furnace is above the balance and lifts upwards or slides sideways). The crucible should be removed from the cradle or holder on the balance
Thermogravimetry and Dericative Thermogravimetry 27 usnga par with pointed is veryom o beginners to drop the crucible into the furnace and often the sample powder as well.For this reason,place a plate over the furnace entrance at all times when the crucible is being removed or replaced.A piece of sheet aluminium is suitable and can be used also when heating the furnace for mistake is made and the isdropp into the furna ce,u der no circumstances s should oper “fshl the furnace us ng the tweezers to retrieve the crucible.This is because the tweezers could damage a thin wire thermocouple mounted so that it is just below the sample,when the furnace is raised.Instead the furnace must be disassembled and inverted to retrieve the crucible and/or tip out powder that has been spilt The crucibl t be eaned.Many rucibles are made of platinum mert o aci and heating up to 176Cutiaumin another metal is being used,the instructions given may have to be modified so as not to destroy or oxidise the crucible.To clean,place the crucible in a small beaker and soak it in moderately concentrated nitric acid.This should remove most inorganic substanc es,because all metal nitrates are sc roughly v with water.N ow p on a gauze and triangle and place a hot Bunsen burner under it.In theory the crucible should be heated to at least the temperature to be used in the run.In practice,glowing white-hot is sufficient.Allow the crucible to cool and then transfer it empty to the cradle or holder on the balance.While the crucible was being cleaned your fingers could handle it,but after ly be touc hed by the tw eezers,the of which ha v mber!fingers put fingerprints which consist of grease,onto the crucible.These will evaporate off and be recorded as a mass loss during the run. Zero Setting After the empty crucible is on the balance,move the furnace back into position around the crucible and start the flow of the purge gas that is used to remove product gasses.The flow rate is usually about 10cm min.Do not use a fast flow because it may influence the balance. causing ittos and so giv a fluctuati g rec ange the balance control to the released position and set a suitable mass range typically 10mg if 10mg samples are used.Carry out the procedure in the computer system to store the zero mass reading
Thermoyravimetry and Derivative Thermoyruvimetry 27 using a pair of curved tweezers with pointed ends. It is very common for beginners to drop the crucible into the furnace and often the sample powder as well. For this reason, place a plate over the furnace entrance at all times when the crucible is being removed or replaced. A piece of sheet aluminium is suitable and can be used also when heating the furnace for isothermal experiments. If a mistake is made and the crucible is dropped into the furnace, under no circumstances should the operator “fish” inside the furnace using the tweezers to retrieve the crucible. This is because the tweezers could damage a thin wire thermocouple mounted so that it is just below the sample, when the furnace is raised. Instead the furnace must be disassembled and inverted to retrieve the crucible and/or tip out powder that has been spilt. The crucible must be cleaned. Many crucibles are made of platinum, which is inert to acid and heating up to 1769 “C; but if alumina, ceramic or another metal is being used, the instructions given may have to be modified so as not to destroy or oxidise the crucible. To clean, place the crucible in a small beaker and soak it in moderately concentrated nitric acid. This should remove most inorganic substances, because all metal nitrates are soluble. Wash thoroughly with water. Now place the crucible on a gauze and triangle and place a hot Bunsen burner under it. In theory the crucible should be heated to at least the temperature to be used in the run. In practice, glowing white-hot is sufficient. Allow the crucible to cool and then transfer it empty to the cradle or holder on the balance. While the crucible was being cleaned your fingers could handle it, but after heating it should only be touched by the tweezers, the tips of which have also been flamed. Remember! fingers put fingerprints, which consist of grease, onto the crucible. These will evaporate off and be recorded as a mass loss during the run. Zero Setting After the empty crucible is on the balance, move the furnace back into position around the crucible and start the flow of the purge gas that is used to remove product gasses. The flow rate is usually about 10cm3 min-’. Do not use a fast flow because it may influence the balance, causing it to sway and so give a fluctuating mass record. Change the balance control to the released position and set a suitable mass range, typically 10mg if 1Omg samples are used. Carry out the procedure in the computer system to store the zero mass reading
Chapter 2 Adding the Sample Arrest the balance again and lower the furnace.The calcium oxalate which is a white with the crucible on the balance using a micro spatula and a steady hand! A plate over the furnace entrance is essential or,alternatively,the crucible may be removed and the sample added away from the balance. The exact mass used is not usually critical,so guess work may be used,the exact mass being found when the crucible is back on the balance.If the mass reading is critical,then a separate ndard】 ratory balance ma be used in the normal way to weigh out the sample.Replace the crucibl on the balance and raise the furnace.Switch the balance to the released position and leave for the gas flow to be established and the sample to stop swaying. Starting the Run If the furnace temperature has a dial contro,turn it back toroom temperature,ie.20C.On other systems set the computer control to a start temperature.Set the rate of heating to the required value.This is typically 10C min1,but might be 1,20 or 50C min-for certain experiments.There should be some means of setting the maximum tem- aps on the dial that indicates the e control panel.Set this to 1000 e experimen described.The control system may give alternative control programmes such as controlled heating followed controlled cooling.In the present experiment,set the control to heat to the maximum temperature and switch off.If the furnace has a cooling jacket,turn on the water flow and adjust the flow rate;it only needs a slow flow.Next switch on the data logging system.When th ing is steady,turn on the temperatur control to heat.The run should now be automatic. Ending the Run When the maximum temperature is reached the furnace control should switch oft.If cooling water was used.leave it running until the furnace cools to near room temperature,then turn it off.Turn off the furnace contr ter supply and gas purge.The compute colectin thedata shoud show the mass loss on the seree sample temperature.The plot may be printed and the data may be stored
28 Chapter 2 Adding the Sample Arrest the balance again and lower the furnace. The calcium oxalate, which is a white powder, may now be added. This may be done with the crucible on the balance using a micro spatula and a steady hand! A plate over the furnace entrance is essential or, alternatively, the crucible may be removed and the sample added away from the balance. The exact mass used is not usually critical, so guess work may be used, the exact mass being found when the crucible is back on the balance. If the mass reading is critical, then a separate standard laboratory balance may be used in the normal way to weigh out the sample. Replace the crucible on the balance and raise the furnace. Switch the balance to the released position and leave for the gas flow to be established and the sample to stop swaying. Starting the Run If the furnace temperature has a dial control, turn it back to room temperature, i.e. 20°C. On other systems set the computer control to a start temperature. Set the rate of heating to the required value. This is typically 10°C min-l, but might be 1, 20 or 50°C min-l for certain experiments. There should be some means of setting the maximum temperature reached, perhaps on the dial that indicates rising temperature or otherwise on the control panel. Set this to 1000°C for the experiment described. The control system may give alternative control programmes such as controlled heating followed controlled cooling. In the present experiment, set the control to heat to the maximum temperature and switch off. If the furnace has a cooling jacket, turn on the water flow and adjust the flow rate; it only needs a slow flow. Next switch on the data logging system. When the mass reading is steady, turn on the temperature control to heat. The run should now be automatic. Ending the Run When the maximum temperature is reached the furnace control should switch off. If cooling water was used, leave it running until the furnace cools to near room temperature, then turn it off. Turn off the furnace control, balance control, water supply and gas purge. The computer collecting the data should show the mass loss on the screen against sample temperature. The plot may be printed and the data may be stored
Thermogravimetry and Derivative Thermogravimetry 29 Example Experiment Details of the experimental conditions for the typical experiment above. Apparatus used-any suitable type of thermobalance. Sample-calcium oxalate mono hydrate powder,Hopkin Williams Standard Grade Sample was used untreated Scan conditions-20C to 1000C at a linear rate of 10C min-1 Purge gas-nitrogen,"white spot",passed through drying tube;flow 20cm3 min-1. Crucible-plati Sample mass-8. agnara5caoeha5am Furnace cooling water-5cm3min. If there is no built-in differentiating procedure,applying the Sav- itzky-Golay+method,using a 15-point smoothing/differentiation rou- tine,based on a quadratic fit,produces the DTG result. Results Figure 6(A)shows the mass loss as the temperature is taken up to 1000C ndmay be seen to consist ofthree stages or steps Fir point of maximum weight loss. should be noted that ath e TG curve is relatively smooth the DTG curve has many fluctuations.This is caused by minor variations in the TG curve.It is a characteristic of the differentiation process that "noise"on an experimental curve is accen- tuated.The first decomposition is widely separated from the second stage Ho wever,the cond stage only just finish fore the third stage starts In less favourable decompositions two such steps may overlap with n horizontal part between the stages on the TG curve and no return to the baseline on the DTG plot.In some of the worst cases a DTG peak will appear as a small shoulder on the side of a larger peak.In these cases the experiment should be re-run using a much slower rate of heating to try to allow one reac tion to finish before th next sta ts The first stage,which takes place between 100 and 200C,is likely to be loss of water of crystallisation since the substance is known to be a salt hydrate.The decomposition of the anhydrous material takes place in two steps.Some oxalates are known to decompose through an intermediate carbonate.so this may account for the two steps.The final product of decomposition is usually the commonest oxide(most stable oxida tion state),but sometimes the metal is formed in an inert atmosphere.A
Thermogravimetry and Derivative Thermogravimetry 29 Example Experiment Details of the experimental conditions for the typical experiment above. Apparatus used - any suitable type of thermobalance. Sample - calcium oxalate monohydrate powder, Hopkin & Williams, Sample was used untreated. Scan conditions - 20°C to 1000°C at a linear rate of 10°C min-’. Purge gas - nitrogen, “white spot”, passed through drying tube; flow 20cm3 min-’. Furnace cooling water - 5 cm3 min-’. Crucible - platinum, cylindrical; diameter 0.5 cm, height 0.5 cm. Sample mass - 8.404 mg; tamped. Standard Grade. If there is no built-in differentiating procedure, applying the Savitzky-Golay4 method, using a 15-point smoothing/differentiation routine, based on a quadratic fit, produces the DTG result. Results Figure 6(A) shows the mass loss as the temperature is taken up to 1000°C and may be seen to consist of three stages or steps. Figure 6(B) is the DTG curve, that is the derivative form of Figure 6(A). This helps to pinpoint the point of maximum weight loss. It should be noted that although the TG curve is relatively smooth the DTG curve has many fluctuations. This is caused by minor variations in the TG curve. It is a characteristic of the differentiation process that “noise” on an experimental curve is accentuated. The first decomposition is widely separated from the second stage. However, the second stage only just finishes before the third stage starts. In less favourable decompositions two such steps may overlap with no horizontal part between the stages on the TG curve and no return to the baseline on the DTG plot. In some of the worst cases a DTG peak will appear as a small shoulder on the side of a larger peak. In these cases the experiment should be re-run using a much slower rate of heating to try to allow one reaction to finish before the next starts. The first stage, which takes place between 100 and 200°C, is likely to be loss of water of crystallisation since the substance is known to be a salt hydrate. The decomposition of the anhydrous material takes place in two steps. Some oxalates are known to decompose through an intermediate carbonate, so this may account for the two steps. The final product of decomposition is usually the commonest oxide (i.e. the most stable oxidation state), but sometimes the metal is formed in an inert atmosphere. A
30 Chapter 2 possible reaction scheme is given,together with the molecular masses of the compounds: Ist stage Ca(COO)2H2O=Ca(COO)2+H2O (2) 146.115 128.100 18.01 2nd stage Ca(COO)2=CacO3+CO 3) 128.100 100.08928.011 3rd stage CaCO3=Cao +CO2 4 100.08956.07944.010 9 THERMOGRAVIMETRY,CALCIUM OXALATE MONOHYDRATE MASS n TEMPERATURE IN DEGREE CELSIUS DERIVATIVE THERMOGRAVIMETRY.CALCIUM OXALATE MONOHYDRAT m/d 00 TEMPERATURE IN DEGREE CELSIUS Figure6 Resul -calcium oxalate monohydrate,(A)TG curve
30 Chapter 2 possible reaction scheme is given, together with the molecular masses of the compounds: 1st stage Ca(COO),.H,O = Ca(COO), + H20 146.1 15 128.100 18.015 2nd stage Ca(COO), = CaCO, + CO 128.100 100.089 28.01 1 3rd stage CaCO, = CaO + CO, 100.089 56.079 44.010 A THERMOGRAVI M ETRY, CALCl U M OXALATE MONOHY DRATE MASS m in mg 9.00 8.00 7.00 6.00 5.00 4.00 3-00 , 8 I I I 0.0 200.0 4oa a 6GQ D 800.0 1000.0 1200.C TEMPERATURE IN DEGREE CELSIUS DERIVATIVE THERMOGRAVIMETRY, CALCIUM OXALATE MONOHYDRATE 1.00 -1.00 dm/dT in 3.00 mg/DEG. -5 00 -7 00 0.0 200.0 4oa a’ BGQ D 500.0 1000.0 1200.( TEMPERATURE IN DEGREE CELSIUS Figure 6 Results of a typical TG experiment - calcium oxalate monohydrate, (A) TG curve, (B) D TG curve