In the Laboratory
A Graduate Course in Modern Analytical Methods: Investigating the Structure, Magnetic Properties, and Thermal Behavior of CuSO4ⴢ5H2O Christian Näther,* Inke Jeß, Sabine Herzog, Christoph Teske, Karsten Bluhm, Herbert Pausch, and Wolfgang Bensch Institut für Anorganische Chemie, Christian-Albrechts-Universität zu Kiel, Olshausenstr. 40, D-24098 Kiel, Germany; *
[email protected] Modern analytical methods such as single crystal or powder diffraction, spectroscopy, thermal analysis, or microscopy play an important role in the practice of contemporary chemistry, physics, and materials science. Results of experiments often cannot be interpreted unambiguously and important questions cannot be answered by applying only one analytical technique. This is true for science at universities and also in industrial research. Therefore, it is necessary that chemical education guarantees that students come in contact with many different methods. However, because the time is limited and the theory of most of the methods is complicated, it is impossible for the students to master the complete theoretical background for several analytical methods. Practical courses must concentrate on those aspects that are really necessary in practice. Initially students should learn the problemoriented selection of a method. Strictly speaking the students should be able to select one or more of several analytical methods to solve a specific analytical problem. Depending on the information needed and on the properties of the material investigated, students should select the appropriate techniques. Such a material can be a gas, a solution, or a solid and a solid may be crystalline or amorphous. For a solid it is fundamental to know whether information about surface or bulk properties is required. Such information can include the qualitative or quantitative chemical composition of a compound, two or three dimensional structural information, or exact distances between atoms. A prerequisite is the knowledge of whether the sample is a pure compound or a mixture of two or more compounds. It is obvious that answering each of these questions requires a specific analytical method. To address these problems fundamental knowledge such as the advantages and limitations of the different methods must be known, as well as which information is accessible and which is not. We decided to develop and incorporate an obligatory twoweek course for the first-year graduate chemical education curriculum. The course consists of lectures, practical exercises, and discussions (see Table 1) and is held every semester. Students work about 45 h in the laboratory (including computer work); about 15 h are reserved for lectures and discussions. The students work on a specific solid state analytical problem that can be solved only by using a combination of several analytical methods. While good chemistry education requires students to be familiar with equipment currently used in research, students are often not provided with opportunities to do so. The equipment is expensive and sensitive to mishandling. Thus, such equipment is rarely used in the undergraduate or graduate chemical education, to students’ detriment.
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The Experiment In this course students investigate the thermal decomposition of CuSO4ⴢ5H2O (1), which involves these steps: CuSO4ⴢ5H2O → CuSO4ⴢ3H2O → CuSO4ⴢH2O → CuSO4. The experiments are carried out on pure CuSO4ⴢ5H2O as well as on a mixture of CuSO4ⴢ5H2O and Al2O3. We selected these compounds for several reasons. •
The compounds were readily available and not too expensive.
•
Handling the compounds did not require special equipment, that is, manipulations were possible under ambient conditions.
•
Several analytical methods were applicable to solve a single chemical problem.
The copper sulfate pentahydrate (CuSO4ⴢ5H2O) compound fulfils these requirements perfectly. All students are familiar with this simple compound. Nonetheless, the results of the investigations (such as the thermal behavior) may be surprising for the students. Single crystals of CuSO4ⴢ5H2O can be easily grown. In addition, the paramagnetic properties allow characterization using a simple Faraday balance. Alumina is used in the mixture because it is thermally very stable and it is diamagnetic. Both compounds are investigated during the course and the participants can additionally determine the composition of mixed samples. During the course each thermal decomposition intermediate of CuSO4ⴢ5H2O must be identified and students have to estimate the ratio of CuSO4ⴢ5H2O and Al2O3 in a mixture. For the investigations students use optical and electron microscopy, energy dispersive X-ray spectroscopy (EDX), differential thermal analysis, thermogravimetry, differential scanning calorimetry, as well as temperature and time-resolved X-ray powder diffraction. To gain insight of what happens on an atomic level the single-crystal structure of CuSO4ⴢ5H2O is determined. At the end of the course the magnetic behavior of CuSO4ⴢ5H2O is investigated. Depending on the number of participants, the students are divided into two groups with a maximum of eight students per group. If necessary one group starts making differential thermal analysis (DTA) measurements while another group makes differential scanning calorimetry (DSC) measurements at the same time, and vice versa. The lectures, discussions, and evaluations using computers are performed by all students. Each group performs all experiments. The
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practical work is divided in a way that, for instance, one student prepares the sample, whereas another participant sets up and starts the measurements. Every member of each group must evaluate the experimental data on a computer. After the course is finished each participant writes a report that includes the theoretical basics of one selected method. In the report the advantages and limitations of the technique should be emphasized. The results of the different experiments are presented by each group as a typical scientific publication, including introduction, results, experimental part, and conclusions. Finally, a short oral examination with each participant is carried out.
Day One On the first day a brief lecture introduces the aim and organization of the course. The lecture also explains and stresses the importance of modern analytical methods in chemistry and related sciences and the necessity of using a combination of different methods to solve analytical problems. Students are then given the compound samples that will be investigated during the course. The exercises start with optical microscopy, and obviously the students see that one sample contains only beautiful blue single crystals of CuSO4ⴢ5H2O, whereas the second sample is a mixture of blue crystals and a fine white powder. The microscope is equipped with a video camera that is connected to a monitor so the whole group can follow the investigation. Afterwards a short lecture introduces the basics of electron microscopy and EDX with special attention focused on the practical aspects of the determination of the qualitative and quantitative composition of a material applying EDX. The environmental scanning electron microscope (ESEM; we used a Philips XL30 ESEM) allows investigations in a high vacuum environment and also experiments applying pressures up to 20 mbar within the sample chamber (ESEM mode). The composition of the phase-pure sample is readily determined using standard software (EDAX International’s EDX Control Software, Version 3.3). Because Al2O3 is an insulator the investigation of the mixture must be performed in the ESEM mode.
Day Two The second day starts with a lecture introducing thermoanalytical methods, with the main focus on DTA (using a DTA 404 EP device from Netzsch Instruments). Both samples are investigated with different heating rates. Using heating rates of 20 °C/min, the students find that the DTA curve of pure CuSO4ⴢ5H2O exhibits two endothermic signals at about 100 and 250 °C, of which the first peak seems to be split into two peaks. A second run with a lower heating rate of 2 °C/min shows that the first peak is better resolved, consisting of two independent thermal events. Students investigate the reversibility of the reaction by repeated heating and cooling between room temperature and 200 °C for one sample and between room temperature and 350 °C for another sample. Afterwards they isolate the intermediate products formed at about 200 and 350 °C for further characterization using X-ray powder diffraction and EDX. Day Three On the third day thermogravimetry (TG) and DSC are introduced. It is pointed out that these methods can supplement the results obtained from the DTA measurement. With TG a change in mass of the sample is monitored and DSC allows the measurement of the thermal energy of a physical or chemical reaction. The students investigate the samples on an STA-429 balance and a DSC-204 calorimeter (both from Netzsch), allowing repeated heating and cooling (Figures 1 and 2). Heating and cooling of the samples should demonstrate whether a thermal event is reversible or irreversible within the time scale of the experiment. In a lecture it is explained in detail how to prepare the samples and how to use the instruments. The measurements using slow heating rates of 2 °C/min are performed overnight. Day Four The fourth day starts with the evaluation of the experimental results using conventional software (Netzsch’s DSC204 Software, Version 3.3). The theoretical background for
o
-28.8 % -14.6 % -7.3 %
-14.3 % atmosphere: air (100ml/min.) theoretical values: 1. 14.4% 2. 14.4% 3. 7.2%
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Heat Absorption Rate
⌬m (arbitrary units)
o Tp = 123 C Tp = 143 C
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Tp = 94 C
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Tp = 98 C Tp = 94 oC
o heating rate: 20 C/min. heating rate: 2 oC/min.
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heating rate: 20 oC/min. heating rate: 2 oC/min.
o Tp = 113 C
Tp = 243 oC o
Tp = 216 C 288 J/g
o Tp= 71 C
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Temperature / oC Figure 1. TG curves for CuSO4ⴢ5H2O showing the dependence of the resolution on different heating rates (first weight loss using a heating rate of 20 °C/min cannot be evaluated due to poor resolution).
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Temperature / C Figure 2. DSC curves for CuSO4ⴢ5H2O showing the dependence of the resolution on different heating rates (single crystals; atmosphere: air).
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In the Laboratory Table 1. Course Syllabus for Modern Analytical Methods, Christian-Albrechts-Universität* heDay
Event
heOne
L P
Topic
L P
Introduction: Aim of the course Investigations using optical microscopy, electron microscopy and energy dispersive X-ray spectroscopy (EDX) Optical microscopy of CuSO4ⴢ5H2O and Al2O3 Introduction to electron microscopy and energy dispersive X-ray spectroscopy Determination of the composition using energy dispersive X-ray spectroscopy
L P
Differential thermal analysis (DTA) Introduction to differential thermal analysis Investigations of the thermal behaviour of CuSO4ⴢ5H2O
L P
Differential scanning calorimetry (DSC) and thermogravimetry (TG) Introduction to differential scanning calorimetry and thermogravimetry Investigations of the thermal behaviour of CuSO4ⴢ5H2O
heTwo
heThree
heFour L
P D P heFive L P D heSix L P D heSeven L P D heEight
Differential scanning calorimetry (DSC) and thermogravimetry (TG) Introduction how to determine characteristic temperatures in DTA and DSC curves, accurate determination of thethe weight loss in TG measurements, application of conventional computer software; determination of the thecomposition of the phase mixture Determination of characteristic temperatures and the weight loss Interpretation of the results; critical evaluation of the results Determination of the composition of the intermediate products obtained from the TG measurements after heating to 200兾° C and 350兾° C by EDX Single crystal structure determination of CuSO4ⴢ5H2O Introduction to X-ray structural analysis Selection of a single crystal using optical microscopy; crystal mounting; searching for reflections; starting the he he he measurement Development of a measuring strategy Single crystal structure determination of CuSO4ⴢ5H2O Introduction to structure solution and refinement Integration of intensities; absorption correction, structure solution and refinement, drawing of figures and the thewriting a report with the computer Evaluation of the quality of the structure model; critical discussion of the results X-ray powder diffraction Introduction to X-ray powder diffraction Preparation of the samples; starting the measurements; calculation of the powder pattern of CuSO4ⴢ5H2O thefrom single crystal data Development of a measuring strategy
D
X-ray powder diffraction Introduction to the computer software Identification of the compounds in the phase mixture as well of the intermediate products using computers and theconventional software Evaluation of the results
L P D
Magnetic investigations Fundamentals of magnetism Preparation of the samples; starting the measurement Development of a measuring strategy
L P D
Magnetic investigations Introduction to the analysis of the data Analysis of the data Final discussion: criticism and suggestions
L P
heNine
heTen
*NOTE: Components of this two-week graduate-level course include lectures (L), practical exercises (P), and discussions (D).
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the determination of the experimental weight loss, the extrapolated onset temperature, the peak temperature, and the enthalpy are introduced. It is explained that the extrapolated onset temperatures are much more suitable than peak temperatures because they are nearly independent of the heating rates. However, for CuSO4ⴢ5H2O the decomposition temperatures depend on several parameters like the preparation of the samples or the atmosphere and therefore are not exactly comparable. From the TG curve the students learn that all thermal events are accompanied with a loss in mass, ∆m (Figure 1). From the ∆m values the students calculate that decomposition of CuSO4ⴢ5H2O leads first to CuSO4ⴢ3H2O as intermediate, than to CuSO4ⴢH2O, and finally CuSO4 is formed. However, they have to prove these results later with other methods. Additionally, Al2O3 is inert in the temperature range investigated. The ratio between CuSO4ⴢ5H2O and Al2O3 (approximately 60:40) can be calculated, as well as the sum of the enthalpies of all reactions involved (Figure 2). With these exercises the part of thermal analysis is finished. On the same day the students investigate the intermediate products obtained after heating to 200 °C and 350 °C using EDX. The composition of the samples determined agreed well with those calculated for CuSO 4ⴢH 2O and CuSO4 using SW-STA-531.01, a software program for the analysis of DTA and TG data from Netzsch Instruments.
Days Five and Six On the next two days the students learn how to determine the single-crystal structure of CuSO4ⴢ5H2O (2). The single-crystal structure is used to ascertain what happened on an atomic level during thermal decomposition. In addition, the results of the structure determination are the basis to calculate the powder pattern that is needed for the X-ray powder investigations. On the first day of this two-day section the lecturer introduces the fundamentals of single-crystal structure determination, including the characteristics of a crystal, the concept of symmetry, and diffraction. Exercises for the students include selection of a single crystal using optical microscopy, mounting the crystal, and starting measurements using a conventional four-circle diffractometer (AED-II) and an imaging plate diffraction system (IPDS), both from STOE & CIE. The data collection is performed overnight. The next day the students prepare the data for structure solution, including determination of the crystal system, refinement of the lattice parameters, and integration of the intensities. (The position sensitive detector used for this measurement is from STOE & CIE; students consult the STOE & CIE IPDS software manual, version 2.89.) Another lecture explains absorption correction techniques, structure solution, and structure refinement strategies. With this knowledge the students determine the structure using conventional computers and software (STOE & CIE X-SHAPE, Version 1.03; Siemens’s SHELXTL-PC). Each step is described in detail in a manual and little additional help is needed. After successful structure determination and refinement, the results are discussed in detail. It is explained how to rank the results of a determination and how to judge the quality of the structural model obtained from the refinement.
Days Seven and Eight The basics of X-ray powder diffraction are introduced on the seventh day. Special attention is devoted to the identification of pure compounds or in mixtures of two or more unknown crystalline components comparing measured diffraction patterns with calculated patterns. The patterns must be calculated by the students with conventional computer software (STOE Win XPOW, Version 1.02) using the singlecrystal data. The practical exercises involve the investigation of the mixture as well as the characterization of the intermediate products isolated after the DTA measurement using a transmission diffractometer (STOE & CIE) and a D-5000 diffractometer (Siemens) with Bragg–Brentano geometry. Special attention is paid to the preparation of the samples and some important practical aspects for the improvement of the measured powder patterns are mentioned. The next day starts with the evaluation of the results of powder diffraction. First, the participants have to identify both components in the mixture by comparing the calculated pattern of CuSO4ⴢ5H2O with the pattern of the mixture. The remaining peaks that cannot be explained by CuSO 4 ⴢ5H 2O obviously belong to Al2O3, and are identified using a searchmatch routine combined with a powder diffraction database that is part of the software. One of the two powder patterns of the mixtures showed no reflections of Al2O3. Obviously the material was nano-crystalline with particle sizes being too small to satisfy the conditions for Bragg reflections. This result is of great interest to the students because they learn one of the limitations of the method. In the next step both intermediate products of thermal decomposition must be identified with the search-match routine. It is demonstrated that after heating to 200 °C the monohydrate CuSO4ⴢH2O forms and above 350 °C CuSO4 forms. Finally, the students investigate what happens in detail up to 200 °C because the two thermal events at about 100 °C could not be resolved. The adequate method to solve the problem is temperature-resolved X-ray powder diffraction (Figure 3). In this experiment the sample is heated stepwise to a desired temperature and at each temperature a complete powder pattern is collected with a position sensitive detector. The analysis of each pattern performed by the students clearly shows that the decomposition occurs via CuSO4ⴢ3H2O as an intermediate product.
Figure 3. Temperature and time-resolved X-ray powder patterns obtained during the thermal decomposition of CuSO4ⴢ5H2O.
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Day Nine On the last two days of the course magnetic measurements are conducted using a Faraday balance (B-SU 60, model Kiel from Bruker). All calculations are performed using self-programmed computer software. A linear fit to the reciprocal susceptibility data gives the Curie constant as well as the Weiss constant. The magnetic moment calculated by the participants supports the Cu(II) d9 electronic state. Day Ten After evaluation of the magnetic data is finished the remaining time is reserved for a rigorous discussion among the students and teachers. The aim is a critical evaluation of the content and organization of the course to improve the education and also to incorporate changes suggested by the students. Assessment The course has been held five times to date. The criticism and suggestions made by participants demonstrated that the course is very satisfying for both the students and teachers. The final discussions, the reports written by the students, and the outcomes of the final examination showed that most of the goals of the course were reached. We are sure that this highly successful result is due to the principles underlying this course. Very often modern analytical methods are introduced as isolated tools without demonstrating the necessity of applying different methods to obtain a comprehensive understanding of a complex analytical problem. We believe that the combination of these practical exercises with lectures and critical discussions is one of the most important reasons for the success of the course. Indeed, the students are confronted with a huge amount of new information, and it is obvious that they cannot learn every detail within just two weeks. It is extremely important that at every stage of the course the teachers explain in detail how to use the techniques they have learned through their own experience and training. Teaching students without explaining the practical importance of a method is not adequate. This was one of the main conclusions drawn by the students. For most participants it was an exciting new experience to learn how the combination of different methods can help to solve specific analytical problems, even investigating a very simple compound like CuSO4ⴢ5H2O. The discussions during the course, the results of the reports, and the students’ oral exams clearly show that the basic knowledge of the techniques was successfully imparted and most of the students evaluated the results obtained with the different methods critically. Very often young people tend to believe the numbers produced by a computer or apparatus without any critical evaluation. But it is our opinion that the critical assessment of data is much more important than the knowledge of the whole theoretical background of each method. In this context it was a great advantage that the whole group worked on the same problem from the beginning until the end of the course. This allowed students to help each other and to work as a team on the problem. The students learned to work and think like scientists in a scientific community. 324
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For some parts of the course a manual describing the most important details of a method was supplied to the students in advance, which saved time because the students were prepared. The manual also helped to solve problems without further assistance from the instructor. According to the suggestions made by the students for future course offerings, the course section concerned with solution and refinement of the structure (Day Six) should be one day longer, and the course section on thermal analytical methods (Days Three and Four) should be reduced by one day. In light of student feedback, we will incorporate a short section on thermomicroscopy, since this method is very useful for understanding the thermal decomposition reaction. One important point that should be stressed is the need for instruction in each method used during the course by teachers who are specialists and can effectively communicate the salient aspects of the method. The major advantages of the course design presented here include the high degree of versatility, and the ease of adapting the content according to the demands of the students and the teachers as well as to the equipment available. If, for example, a TG coupled with mass spectrometry or atomic absorption spectroscopy is available, these methods can be included very easily. On the other hand, if differential thermoanalysis is not available, this course section can be done using only differential scanning calorimetry. Conclusions The impact of modern analytical methods on research is far-reaching. Therefore, it is absolutely necessary in chemical education to emphasize the interdependency of multiple analytical methods to solve chemical problems. It is also important to emphasize the practical aspect of each method to students. To learn how analytical problems can be solved, students need to work on analytical problems. In order to recognize why different methods are required teachers must assign problems that can only be solved by a combination of different methods. This is an exemplary aspect of our course. After the students have performed all the experiments on CuSO4ⴢ5H2O using the methods presented above they developed a complete picture of what happens during this simple reaction. For the success of the course it is necessary to concentrate from the beginning only on those facts that are important for a general understanding of the methods and that are necessary to interpret the results of an experiment. Offering too much complicated theory is counterproductive. Finally, the students have to learn to critically assess the results of an experiment, because this is the basis for any further chemical discussion. In many discussions the participants articulated their great satisfaction with the content and organization of the course: we, too, are extremely satisfied with the results of the newly developed course. This is only a first attempt to adapt modern analytical methods using state-of-the-art equipment for use in the graduate chemical education. In cases where other compounds with different properties or reactivity could be investigated, other methods can be included depending on the equipment available. Even the equipment for the
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experiment presented here can be modified to that which is available, for example, the different phases that occur during thermal decomposition can also be identified by spectroscopic measurements. The most important prerequisite for the selection of the compounds for analysis is that a successful solution of the problem can only be achieved using the multiexperimental approach presented here. The educational goal of the course is to teach students how different methods should be applied to solve a single chemical problem. The methods that are needed depend only on the problem that needs to be solved.
Acknowledgment Financial support from the state of Schleswig–Holstein, Germany, is gratefully acknowledged. Literature Cited 1. Gmelins Handbuch der anorganischen Chemie, 8. Auflage, Kupfer Teil B, Syst.-Nr.: 60, Verlag Chemie: Weinheim/ Bergstr. 1958, 491–525. 2. Varghese, J. N., Maslen, E. N. Acta Crystallogr. 1985, B39, 184–190.
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