Identifying Hydrated Salts Using Simultaneous Thermogravimetric

Laboratory Experiment pubs.acs.org/jchemeduc

Identifying Hydrated Salts Using Simultaneous Thermogravimetric Analysis and Differential Scanning Calorimetry Jerry D. Harris* and Aaron W. Rusch Department of Chemistry, Northwest Nazarene University, Nampa, Idaho 83686, United States S Supporting Information *

ABSTRACT: An experiment for analytical chemistry is presented that utilizes simultaneous thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) to characterize colorless, hydrated salts with anhydrous melting points less than 1100 °C. The experiment could be used to supplement the lecture discussing gravimetric techniques. It is a straightforward experiment to introduce the instrumentation and analysis software to students, and it reinforces stoichiometric calculations. The students identify an unknown salt by determining the salt’s water content using mass loss data from the TGA and by measuring the anhydrous salt’s melting point using DSC data.

KEYWORDS: Second-Year Undergraduate, Analytical Chemistry, Laboratory Instruction, Hands-On Learning/Manipulatives, Thermal Analysis, Gravimetric Analysis, Quantitative Analysis, Instrumental Methods, Calorimetry/Thermochemistry

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However, many of these are colored and thermally decompose to the metal oxide when heated in air. Although a few of the above referenced experiments introduce thermal analysis in general chemistry laboratories, most are for upper-division advanced laboratories, where the students use the instrument to characterize synthetic products. Several of the experiments that introduce thermal analysis in general chemistry laboratories do it as a dry lab,5,9 where the students are given the TGA data as a handout. Gray, Smith, and Silva use a forensic science scenario to introduce undergraduate students to TGA and DSC analysis,11 where the students analyze unknown textile fibers and compare their data to that for known fibers to determine fiber identity. What is missing in the chemistry education literature is an introductory experiment that can be used to introduce the students to thermal analysis techniques and instrumentation while the topic is covered in lecture. The experiment described below seeks to satisfy that need by introducing simultaneous TGA/DSC to students, using colorless hydrated salts, which have well-defined melting points.

hermogravimetric analysis (TGA) is an instrumental technique that is introduced and discussed in many analytical chemistry courses. While a sample is heated, TGA instruments measure the sample mass as a function of temperature. Newer TGA instruments often combine differential scanning calorimetry (DSC) capability to measure heat flow of the sample simultaneous with the TGA data. Although these TGA/DSC instruments are not generally as sensitive as standalone DSC instruments, they are adequate for characterizing most routine samples and for teaching the concept of measuring heat flow into and out of samples during reactions or heating. Additional background information on DSC and TGA can be found in chapters one and three of Principles and Applications of Thermal Analysis.1 Thermogravimetric analysis and TGA/DSC instruments have been used to study a variety of substances in the teaching laboratory including, but not limited to, air-sensitive compounds,2 polymers,3−5 inorganic coordination polymers,6 carbon allotropes,7 primary standards,8 coal,9 layered inorganic materials,10 textile fibers,11 nicotine,12 xerogels,13 and hydrated metal oxalates.14 They have also been used to study the kinetics of decomposition in physical chemistry experiments. 15 Chittenden et al. reported the analysis of hydrated salts by TGA/DSC in upper-division laboratories to introduce students to the technique and to predict the final decomposition product.16 The salts that were used by Chittenden all had welldocumented thermal properties and included CuSO4·5H2O, FeSO4·7H2O, CaC2O4·2H2O, and several metal carbonates. These salts are typical examples used in analytical chemistry textbooks to demonstrate the capabilities of TGA instruments. © 2012 American Chemical Society and Division of Chemical Education, Inc.

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EXPERIMENTAL OVERVIEW The goal of this experiment was to give analytical chemistry students experience using simultaneous TGA/DSC instrumentation to determine the identity of unknown hydrated salts. Salts were selected based on the following criteria: (i) the salts needed to be colorless so that salt color would not help with identification; (ii) the salts needed to have anhydrous melting Published: December 12, 2012 235

dx.doi.org/10.1021/ed300222y | J. Chem. Educ. 2013, 90, 235−238

Journal of Chemical Education

Laboratory Experiment

points that were less than 1100 °C, which is the maximum temperature for many TGA instruments; (iii) the salts all needed to be stable; (iv) the salts needed to be commercially available; and (v) the salts needed to be as nontoxic and environmentally friendly as possible. A survey of the “Physical Constants of Inorganic Compounds” section of the Handbook of Chemistry and Physics narrowed the list of candidate salts to the 16 given in Table 1.17 Melting points listed in the table

instrument in either small groups (2−3 students) or one-onone with the laboratory instructor. The time required for training, sample preparation, characterization, data analysis, and instrument cool down was about 40 min. It should be possible to analyze four to five samples on a particular instrument during a given 3-h laboratory period. Students were also provided with a list of possible unknowns and instructed to find known melting points and to determine the expected mass loss associated with dehydration for the possible unknowns, for comparison to data for their unknown. The students were expected to find melting point data and calculate theoretical water loss either outside of lab or while other students were using the TGA.

Table 1. Melting Points and Water Content of Colorless, Hydrated Salts Used for Characterization by TGA/DSC Compound Calcium chloride dihydrate Calcium chloride hexahydrate Calcium iodide tetrahydrate Lithium chloride monohydrate Potassium carbonate sesquihydrate Potassium fluoride dihydrate Sodium carbonate monohydrate Sodium iodide dihydrate Sodium metasilicate nonahydrate Sodium molybdate dihydrate Sodium pyrophosphate decahydrate Sodium sulfate decahydrate Sodium tungstate dihydrate Strontium bromide hexahydrate Strontium chloride hexahydrate Zinc phosphate tetrahydrate

Anhydrous Melting Temp/°C

Water (%)

CaCl2·2H2O

772

24.51

CaCl2·6H2O

772

49.34

CaI2·4H2O

779

19.69

LiCl·H2O

605

29.83

K2CO3·1.5H2O

891

16.35

KF·2H2O

858

38.28

Na2CO3·H2O

851

14.52

NaI·2H2O

661

19.38

1088

57.05

Na2MoO4·2H2O

687

14.89

Na4P2O7·10H2O

988

40.39

Na2SO4·10H2O

884

55.91

Na2WO4·2H2O

698

10.92

SrBr2·6H2O

643

30.40

SrCl2·6H2O

874

40.54

1060

15.70

Formula

Na2SiO3·9H2O

Zn3(PO4)2·4H2O

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EXPERIMENTAL DETAILS Hydrated salts were either obtained from commercial sources and used without further purification, freshly prepared by precipitation from aqueous solution, or dried in vacuum. Zinc phosphate was prepared as described by Boonchom et al.19 Salts were characterized using a Mettler Toledo TGA/DSC 1 thermal analyzer. The salts were heated in dry air at a ramp rate of 50 °C/min from 30 to 1100 °C, and typical sample size ranged from 5 to 15 mg. Sample sizes were kept small to avoid excessive expansion during the dehydration of the salts in the TGA furnace.

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HAZARDS None of the salts are considered extremely hazardous. Good laboratory practices should be sufficient to minimize the risk of ingestion, inhalation or contact. Crucibles used in the experiment reach 1100 °C and portions of the TGA can become hot. Care should be taken around the instrument when heating.

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RESULTS AND DISCUSSION This experiment reinforces many important chemical concepts that the students have learned or are learning in lecture. Most importantly, the experiment gives the students experience using thermal analysis instrumentation and the thermal analysis software. A dehydration experiment is often used in general chemistry laboratory courses to characterize such compounds as MgSO4·7H2O, or KAl(SO4)2·7H2O,20,21 but not on the scale or precision available with modern instrumentation. This experiment also provides the students with experience determining melting points using more sophisticated instrumentation than they use in organic chemistry laboratories and determining melting points at temperatures much higher than are routinely observed during organic experiments. It also reinforces stoichiometric calculations as the students calculate percent water for each of the possible hydrated unknowns. Additionally, the laboratory allows the students to experimentally observe endothermic heat flow for the dehydration and melting processes in the DSC trace. If desired, an extension of the laboratory would be to use the DSC data to determine the enthalpy of fusion and the enthalpy of dehydration for each unknown. This would provide the students with further experience with the analysis software. In preparing this experiment, it was observed that several of the excessively hydrated salts are prone to partially dehydrate with prolonged time on the stockroom shelves, so all samples should be analyzed prior to assigning them as unknowns. Dissolving and precipitating the salts from an aqueous solution

were obtained from either the Handbook of Chemistry and Physics or from Mathematica.18 Several other salts were initially included, but were removed from the list for a variety of reasons including lack of commercial availability, similar melting point and water percent as others on the list, the need to be stored under an inert atmosphere, or the inability to obtain reasonable TGA and DSC data. A complete list of all of the potential compounds is provided in the Supporting Information, along with the rationale for why each was not included. This laboratory was repeated and modified three times during three sequential school years and was preformed late in the semester to correspond with the topic of thermal analysis covered in lecture. The number of students in the laboratory ranged from 5 to 10 depending on the year, and all were either chemistry majors or chemistry minors. The students were given a 3-h lab period to complete the experiment, but several opted to come in outside of the lab period for more individual instruction, which was possible, given the limited number of students in the laboratory. Each student was provided an unknown and was given hands-on instruction on how to use the TGA/DSC instrument. Students were trained to use the 236




Figure 1. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) curves for the heating of 28.6290 mg of Na2WO4·2H2O to 1100 °C in dry air at a rate of 50 °C/min.

yielded fully hydrated salts. In contrast, other salts are very hygroscopic, such as LiCl, and may actually contain more waters of hydration than listed, and the monohydrate was obtained by drying the salt in vacuum overnight. It is necessary to measure both melting point and percent water loss to accurately identify one of the unknowns, as several salts have similar water content (i.e., Na2CO3·2H2O = 14.52%, Na2MoO4·2H2O = 14.89%), but different melting points (i.e., Na2CO3·2H2O = 851 °C, Na2MoO4·2H2O = 687 °C). Fast heating rates were used to shorten the required time for the experiment and to obtain well-defined melting point peaks. Slower heating rates could be used to observe water lose in multiple steps. An example of data obtained from the experiment is provided in Figure 1 for Na2WO4·2H2O; both the observed percent water loss and melting point for Na2WO4·2H2O compare well to the published values. In the figure, endothermic peaks point down and exothermic peaks point up. The reproducibility of the data for each salt in the experiment was excellent. Analysis of 40 samples of the 16 salts listed in Table 1 yielded an average standard deviation for water loss of ±1.52% water and an average standard deviation for the melting points of ±2.45 °C. The measured data also correlates well to the theoretical values. On average, the measured water percents were within 1.5% of the calculated values, and the measured melting points were within 8 °C of the published values. Sodium metasilicate nonahydrate and sodium pyrophosphate decahydrate were outliers, with melting points that differed on average by 24 °C from the literature values. The students were reasonably successful at determining their unknown. Approximately 95% of the students were able to deduce their unknown, using both the melting point and water loss data. Thermal data for all of the salts listed in Table 1 are provided in the Supporting Information. A student handout is also provided in the Supporting Information.

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CONCLUDING REMARKS

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ASSOCIATED CONTENT

The emphasis of this experiment is to give analytical students first-hand experience using a TGA/DSC instrument with compounds that do not decompose to oxides below 1100 °C or require chemical synthesis, therefore allowing the students to concentrate on learning the technique and instrument software. Exposing students to the capabilities of TGA/DSC instrumentation in a hands-on experience not only reinforces the material covered in an analytical chemistry lecture, but it also provides a foundation on which they can build in upper-division courses, such as inorganic chemistry and thermodynamics, where thermal characterization techniques are also often discussed. When done using an unknown in a discovery-based laboratory, the students find the experiment to be a fun and meaningful way to learn about these techniques. The unknown challenges them to achieve a goal, and the students feel rewarded for their effort when they correctly determine the salt.

S Supporting Information *

Student handout; instructor notes; thermal data for the salts listed in Table 1. This material is available via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. 237



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ACKNOWLEDGMENTS The authors gratefully acknowledge financial support from the National Science Foundation through grant DMR-0840265 for the purchase of the TGA/DSC-MS system.

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REFERENCES

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Identifying Hydrated Salts Using Simultaneous Thermogravimetric

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