Demonstration Cite This: J. Chem. Educ. XXXX, XXX, XXX−XXX
pubs.acs.org/jchemeduc
Demonstrations of Magnetism and Oxidation by Combustion of Iron Supplement Tablets Max J. Palmer, Keri A. Martinez, Mayuresh G. Gadgil, and Dean J. Campbell* Mund-Lagowski Department of Chemistry and Biochemisty, Bradley University, Peoria, Illinois 61625, United States S Supporting Information *
ABSTRACT: Iron supplement tablets containing iron(II) sulfate can be used in chemistry demonstrations as a convenient, household source of small quantities of iron(II) ions. When the tablet is burned in air, oxygen converts the iron(II) in the tablet ultimately to iron(III) oxide in the hematite phase. Heating pure iron(II) sulfate heptahydrate also produces hematite. However, for the tablet, and for mixtures of iron(II) sulfate heptahydrate and starch, other iron oxide species such as magnetite and maghemite are produced before complete conversion to hematite. The presence of the starch, and cellulose in the tablets, appears to slow the oxidation of the iron(II). Magnetite and maghemite can be captured, observed by X-ray diffraction, and are much more strongly attracted to a magnet than hematite. The changes in color and magnetic susceptibility during iron supplement tablet combustion enable it to be used as an interesting demonstration of oxidation. KEYWORDS: General Public, First-Year Undergraduate/General, Demonstrations, Analogies/Transfer, Oxidation/Reduction, Oxidation State, Carbohydrates, Magnetic Properties
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BACKGROUND
combustion products were assessed by appearance, magnetic susceptibility, and X-ray diffraction.
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Iron supplement tablets containing iron(II) sulfate can be used as a household source of iron(II) ions for chemistry education purposes.1 These tablets are relatively inexpensive, are easily accessible, and contain consistent quantities of iron(II) ions in a solid, relatively stable form. Iron(II) compounds, including those in the tablets, can be readily oxidized to iron(III) compounds by a variety of chemical species, including oxygen, hydrogen peroxide, and sodium hypochlorite.2 Concepts associated with oxidation, transition metal colors, and magnetism can be demonstrated when the tablets are burned in air to produce various iron oxide compounds. Table 1 describes the iron compounds that are most relevant to this demonstration and some of their optical and magnetic properties. A white tablet containing iron(II) sulfate that is not noticably attracted to a magnet can be burned for a minute or two, depending on temperature, to produce dark-colored material containing magnetite and/or maghemite. These oxides can be readily attracted to a magnet. Continued combustion produces hematite, a red material that is weakly attracted to a magnet. By contrast, when pure iron(II) sulfate heptahydrate is burned, it does not produce material that is easily attracted to a magnet but instead converts to hematite. However, combinations of iron(II) sulfate heptahydrate and starch burn to produce products similar to those produced by the iron supplement tablets. It appears that the carbohydrates that are mixed with the iron(II) sulfate heptahydrate, whether they are cellulose or starch, slow the formation of hematite and allow other iron oxides to temporarily dominate the product composition. The © XXXX American Chemical Society and Division of Chemical Education, Inc.
EXPERIMENTS AND DEMONSTRATION
Combustion Tests
The samples that were the focus of the combustion studies follow: • Pure iron(II) sulfate heptahydrate • Iron supplement tablets, e.g., from Nature Made Nutritional Products, Mission Hills, CA; each 400 mg tablet contained 65 mg iron (equivalent to 325 mg iron(II) sulfate heptahydrate) and other substances such as cellulose gel • Iron(II) sulfate heptahydrate and starch, e.g., from J. T. Baker Chemical Company, Phillipsburg, NJ; this combination was designed as a simulation of the iron supplement tablets in which the starch simulated the cellulose gel in the tablets The samples were combusted in two general ways: • Samples were placed on a metal spoon supported by a clamp and heated from 0 to 180 s with a propane torch (flame temperature up to 2,000 °C).7 The samples were subjected to direct heat (from above) or indirect heat (from below). Received: June 29, 2017 Revised: November 9, 2017
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DOI: 10.1021/acs.jchemed.7b00475 J. Chem. Educ. XXXX, XXX, XXX−XXX
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Table 1. Properties of Iron Compounds That Are Relevant to the Iron Supplement Tablet Combustion Demonstrationsa
a
Names
Formula
Color
Magnetism
Magnetic Susceptibility (10−6 cgs)
Iron(II) sulfate heptahydrate, melanterite Iron(II, III) oxide, magnetite Iron(III) oxide, maghemite Iron(III) oxide, hematite
FeSO4·7H2O Fe3O4 γ-Fe2O3 α-Fe2O3
Pale blue-green Black Orange Red
Paramagnetic Ferrimagnetic Ferrimagnetic Canted antiferromagnetic
40 16,000−88,000 32,000−40,000 8−600
See refs 2−6.
• Samples were heated in an evaporating dish in a muffle furnace from 0 to 300 s. The samples were heated at 500, 700, and 900 °C. Crushing the tablets into powder enabled them to burn more quickly than intact tablets. The tablets containing the iron(II) sulfate started out white. As they burned they turned dark brown or black and then eventually red, Figure 1. The red color corresponds well to hematite, but the dark colors tend to correspond to other iron oxides such as magnetite and maghemite, as discussed below.
Figure 2. Magnetic susceptibility of iron supplement tablets as they are burned in a muffle furnace at 500, 700, and 900 °C. The error bars represent one standard deviation for each averaged data point. Each point on the graph is based on triplicate measurements.
oxides magnetite and/or maghemite. The curve maxima shift to shorter times as the furnace temperature increases, which indicates that the conversion of iron(II) sulfate to hematite is faster at higher temperatures. Heating the iron supplement tablets on a spoon over a propane torch produces similar curves. By contrast, when pure iron(II) sulfate heptahydrate is burned, low magnetic susceptibility material is produced. However, combinations of iron(II) sulfate heptahydrate and starch burn to produce products with magnetic susceptibilities similar to those produced by the iron supplement tablets. The Supporting Information contains more detailed information, similar to that shown in Figure 2, describing magnetic susceptibility measurements for a variety of samples and combustion methods.
Figure 1. Left to right: General white−black−red progression of colors of an iron supplement tablet as it is burned.
Magnetic Measurements
The relative magnetic susceptibilities of the reactants and products in these combustion trials were measured by placing the samples over a strong magnet on stacked foam cups on an electronic balance.8 This stacked cup method allowed measurements of large magnetic susceptibilities and fast analysis of many samples. Specifically, two stacked foam cups were placed on the balance pan with a strong neodymium−iron−boron magnet on top. The top of the magnet was just below the top draft door of the balance. The sample was placed in a vial on top of the draft door. As the magnet was attracted to the sample, its mass appeared to decrease, and a negative mass change was produced on the balance. The absolute value of the change in the magnet’s mass was divided by the sample’s real mass to provide a relative magnetic susceptibility measurement. The Supporting Information contains a correlation between measurements made using the stacked cup method and a susceptibility balance. Figure 2 shows the magnetic susceptibility of iron supplement tablets placed in a muffle furnace at varying temperatures and times. Each curve exhibits a rise and fall in magnetic susceptibility with increasing time. Generally speaking, the white tablet material containing iron(II) sulfate corresponds to the low susceptibilities at short heating times, and the red tablet material containing hematite corresponds to the low susceptibilities at the long heating times. The dark tablet material generally corresponds more with the high magnetic susceptibilities, though it should be stressed that uneven heating of the tablet material can produce discrepancies between tablet color and magnetic susceptibility. As discussed further below, the materials with high magnetic susceptibility appear to contain significant amounts of the ferrimagnetic iron
X-ray Diffraction Measurements
Powder X-ray diffraction, performed on a Rigaku Smartlab diffractometer using Cu Kα radiation, was used to study the combustion products. Figure 3 shows some representative diffraction patterns from these studies. Elemental silicon was added to the samples as an internal standard, which aided in distinguishing patterns associated with magnetite and maghemite. The crushed, unburned iron supplement tablets produced patterns consistent with the presence of iron(II) sulfate in the monohydrate rather than the heptahydrate form. When the combusted tablets were allowed to cool to room temperature in the air, the resulting samples that had the highest magnetic susceptibility contained maghemite as the dominant iron oxide. However, if the hot, burnt tablets were quickly cooled in liquid nitrogen, the resulting samples that had the highest magnetic susceptibility contained magnetite as the dominant iron oxide. It is hypothesized that the cellulose in the tablet provided a somewhat reducing environment to help stabilize the iron(II) ions in the magnetite during combustion. Similar diffraction patterns were observed for mixtures of iron(II) sulfate and starch, with magnetite best observed for liquid nitrogen cooled B
DOI: 10.1021/acs.jchemed.7b00475 J. Chem. Educ. XXXX, XXX, XXX−XXX
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Figure 4. Burned iron supplement tablet that can be lifted by a magnet.
Connections to Geology on Earth and Mars
Iron oxide minerals are common on Earth, and include the compounds produced in these demonstrations. Hematite and magnetite are major sources of the elemental iron produced worldwide.9 Iron sulfide minerals such as pyrite and marcasite can oxidize to form iron(II) sulfates such as the heptahydrate (i.e., melanterite). Many iron(II) compounds can be oxidized to iron(III) in a wide variety of environments. Examples include the combustion of coal containing iron(II) minerals10 and the production of reddish iron(III) hydroxides associated with water draining from some mining areas.11 Tablets containing iron(II) sulfate can also be oxidized by aqueous species, such as dissolved oxygen, hydrogen peroxide, or sodium hypochlorite.2 In contact with these species, the tablet disintegrates to eventually produce a reddish solid. Aqueous reactions involving the iron supplement tablets are the subject of related studies by these authors.1 Looking beyond Earth, the iconic red color of Mars is due to iron(III) oxides. 12 Martian dust contains a magnetic component that has been attributed to both magnetite and maghemite.6 A variety of sulfate minerals have also been studied on Mars.13 The short laboratory activity described in the Supporting Information connects the products of the tablet combustion to Martian minerals.
Figure 3. Representative X-ray diffraction patterns produced by iron supplement tablet combustion. In all patterns, Si indicates peaks associated with silicon added for use as an internal standard, and * represents peaks associated with the main compound of interest for that sample. (A) Iron(II) sulfate monohydrate within a crushed, unheated iron supplement tablet. Some low-intensity peaks were left unmarked for clarity. (B) Maghemite observed after heating a tablet at 700 °C for 100 s and cooling in the air. (C) Magnetite observed after heating a tablet at 700 °C for 100 s and cooling in liquid nitrogen. (D) Hematite observed after heating a tablet at 700 °C for 300 s. Diffraction data collected with a step size of 0.01° and speed of 1°/ min.
samples and maghemite observed for more slowly cooled samples. Longer combustion times for the iron supplement tablets and the iron(II) sulfate and starch mixtures produced hematite. X-ray diffraction also indicated that combustion of iron(II) sulfate alone produced hematite.
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CONCLUSIONS
Classroom Demonstration
Magnetic Sludge
As noted in the Safety section, combustion of iron supplement tablets containing iron(II) sulfate can produce chemical species in the air such as sulfur dioxide; thus, an air handling system, such as a fume hood, is recommended. The iron supplement tablets can be checked with a magnet by an instructor or student to show that there is no magnetic attraction. Then the tablet can be burned in a spoon or an evaporating dish using a Bunsen burner or a propane torch for 1−2 min, producing dark iron oxides that are attracted to the magnet, as shown in Figure 4. Repeating the experiment with a longer heating time (e.g., 10 min) produces iron oxides that are less magnetic and more red in color. Crushing the cooled tablets is often necessary to best reveal the colors of the oxides. A short laboratory activity based on tablet combustion and attraction to a magnet is described in the Supporting Information.
The recent popularity of magnetic putty toys14 led to an exploration of the use of making similar magnetic composite materials with the iron oxides produced by heating the tablets. These research efforts included mixing the iron oxides with borax and white glue combinations, also referred to as slime or gluep,15 but the results were inconsistent and high concentrations of the iron oxides appeared to be necessary to produce magnetically produced deformation of the material. However, a simple alternative was to crush a burned iron supplement tablet into a few milliliters of a relatively viscous liquid, such as liquid soap, corn syrup, or white glue. The resulting sludge could be magnetically deformed by the field of a strong magnet. For example, four burned tablets crushed and mixed into 7−9 mL of old corn syrup and 1 mL of water responded well to a strong magnet, as shown in Figure 5. C
DOI: 10.1021/acs.jchemed.7b00475 J. Chem. Educ. XXXX, XXX, XXX−XXX
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procedure being mentioned only in the Safety portion of this paper.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available on the ACS Publications website at DOI: 10.1021/acs.jchemed.7b00475. Information on magnetic susceptibility measurements of the iron supplement tablets and related iron(II) sulfate systems, and a more detailed X-ray diffraction (XRD) pattern associated with magnetite and maghemite measurements (PDF, DOCX) Short laboratory activity based on tablet combustion and attraction to a magnet, called Making Magnetic Mars Minerals, used for bonus points in a General Chemistry I Lab (PDF, DOCX)
Figure 5. Burnt iron supplement tablets mixed with corn syrup and water responding to a strong magnet.
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If the magnetic iron oxides are mixed with a liquid that can turn into a solid, the oxide particles can be aligned by an external magnetic field to make a composite magnet, reminiscent of a flexible sheet refrigerator magnet.16 For example, burned tablets crushed and mixed into white glue produced a dark slurry that could be hardened in a drying oven. When the slurry was hardened in a plastic weighing boat over a strong neodymium−iron−boron magnet, the composite solid was able to produce its own magnetic field after the strong magnet was removed, with strengths sometimes exceeding 0.02 mT as detected by a Vernier LabQuest2 module and Magnetic Field Sensor.17
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Dean J. Campbell: 0000-0002-2216-4642 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors are grateful to Bradley University for funding through the Sherry, Special Emphasis, and Building Excellent Scientists for Tomorrow Programs. We also thank the Illinois Space Grant Consortium and the Illinois Heartland Section of the American Chemical Society for support. We are also grateful for contributions from Emily Brewer, Shreya Bellur, and Vishwaarth Vijayakumar. We would also like to thank the participants in various outreach events held by the Bradley University Chemistry Club Demo Crew and the students in the Summer, 2017, General Chemistry I Lab for observing and participating in various iterations of these demonstrations.
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SAFETY The combustion of the iron(II) sulfate heptahydrate, whether contained within a tablet or mixed with starch, requires heating to several hundred degrees Celsius. Precautions must be made to protect people and equipment from thermal damage. Performing the demonstration over a heat resistant surface is important, and supporting the equipment with clamps and stands, rather than human hands, is highly recommended. Iron oxides are the desired combustion products of iron(II) sulfate, but the combustion also produces byproducts such as sulfur dioxide and possible nanoscale magnetite particles. Sulfur oxides can be harmful if inhaled and can damage the surfaces of materials.10 Magnetite nanoparticles are capable of moving from combustion sources into the human brain, where they potentially cause adverse neurological health effects.18 Adequate ventilation during this demonstration is highly recommended. An alternative that was explored was to burn the iron supplement tablet within a borosilicate glass test tube to produce the magnetic iron oxides. To contain the combustion vapors, the end of the test tube was connected via a rubber stopper and tubing to a vacuum flask containing a couple of centimeters of a sodium bicarbonate solution. This flask in turn was connected via tubing to the bottom interior of a large graduated cylinder full of a sodium bicarbonate solution. The solution was used to absorb acidic vapors, and the vacuum flask was used to prevent any solution from backing up the tubing into the test tube as it cooled and the gases within contracted. A photograph of the assembly is in the Supporting Information. The smell of sulfur oxides was not noticed when this system was kept closed, though those vapors could still be released when the system was opened. The risk of melting/cracking the borosilicate test tube is sufficiently significant to warrant this
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REFERENCES
(1) Campbell, D. J.; Staiger, F. A.; Peterson, J. P. Demonstrations Using (Mostly) Household Items. Presented at ChemEd 2015, Kenesaw, GA, July 28−August 1, 2015. (2) Young, J. A. Iron(II) Sulfate Heptahydrate. J. Chem. Educ. 2003, 80, 141. (3) CRC Handbook of Chemistry and Physics, 70th ed.; CRC Press: Boca Raton, FL, 1989. (4) Berger, P.; Adelman, N. B.; Beckman, K. J.; Campbell, D. J.; Ellis, A. B.; Lisensky, G. C. Preparation and Properties of an Aqueous Ferrofluid. J. Chem. Educ. 1999, 76, 943−948. (5) Hunt, C. P.; Moskowitz, B. M.; Banerje, S. K. Magnetic Properties of Rocks and Minerals. In Rock Physics & Phase Relations: A Handbook of Physical Constants; Ahrens, T. J., Ed.; American Geophysical Union: Washington, DC, 1995; DOI: 10.1029/ RF003p0189. http://wellog.com/RF003p0189.pdf (accessed Nov 2017). (6) Bertelsen, P.; Goetz, W.; Madsen, M. B.; Kinch, K. M.; Hviid, S. F.; Knudsen, J. M.; Gunlaugsson, H. P.; Merrison, J.; Nørnberg, P.; Squyres, S. W.; Bell, J. F., III; Herkenhoff, K. E.; Gorevan, S.; Yen, A. S.; Myrick, T.; Klingelhöfer, G.; Rieder, R.; Gellert, R. Magnetic Properties Experiments on the Mars Exploration Rover Spirit at Gusev Crater. Science 2004, 305, 827−829.
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(7) Bernzomatic. TX9 14.1 oz. Propane Hand Torch Cylinder. http://www.bernzomatic.com/Products/Fuel-Cylinders/Hand-TorchCylinders/TX9 (accessed Nov 2017). (8) Ellis, A. B.; Geselbracht, M. J.; Johnson, B. J.; Lisensky, G. C.; Robinson, W. R. Teaching General Chemistry: A Materials Science Companion; American Chemical Society: Washington, DC, 1993. (9) Shaw, M. Types of Iron Ore: Hematite vs. Magnetite. http:// investingnews.com/daily/resource-investing/base-metals-investing/ iron-investing/types-of-iron-ore-hematite-vs-magnetite/ (accessed Nov 2017). (10) Campbell, D. J.; Wright, E. A.; Dayisi, M. O.; Hoehn, M. R.; Kennedy, B. F.; Maxfield, B. M. Classroom Illustrations of Acidic Air Pollution Using Nylon Fabric. J. Chem. Educ. 2011, 88, 387−391. (11) vanLoon, G. W.; Duffy, S. J. Environmental Chemistry: A Global Perspective, 3rd ed.; Oxford University Press: New York, 2011. (12) Wolchover, N. Why Is Mars Red? https://www.space.com/ 16999-mars-red-planet.html (accessed Nov 2017). (13) Flahaut, J.; Massé, M.; Le Deit, L.; Thollot, P.; Bibring, J.-P.; Poulet, F.; Quantin, C.; Mangold, N.; Michalski, J.; Bishop, J. L. Sulfate-rich Deposits on Mars: A Review of Their Occurrences and Geochemical Implications. In Proceedings of the Eighth International Conference on Mars, Pasadena, CA, July 14−18, 2014. https://www. hou.usra.edu/meetings/8thmars2014/pdf/1196.pdf (accessed Nov 2017). (14) Simply Clever Toys. Magnetic Thinking Putty. http:// simplyclevertoys.com/magnetic-thinking-putty (accessed Nov 2017). (15) Fun with Chemistry: A Guidebook for K-12 Activities; Sarquis, M., Sarquis, J., Eds.; Institute for Chemical Education: Madison, WI, 1993; Vol. 2, pp 81−88. (16) Olson, J. A.; Calderon, C. E.; Doolan, P. W.; Mengelt, E. A.; Ellis, A. B.; Lisensky, G. C.; Campbell, D. J. Chemistry with Refrigerator Magnets: From Modeling of Nanoscale Characterization to Composite Fabrication. J. Chem. Educ. 1999, 76, 1205−1211. (17) Vernier Software & Technology, LLC. LabQuest 2. http://www. vernier.com/products/interfaces/labq2/ (accessed Nov 2017). (18) Maher, B. A.; Ahmed, I. A. M.; Karloukovski, V.; MacLaren, D. A.; Foulds, P. G.; Allsop, D.; Mann, D. M. A.; Torres-Jardón, R.; Calderon-Garciduenas, L. Magnetite pollution nanoparticles in the human brain. Proc. Natl. Acad. Sci. U. S. A. 2016, 113, 10797−10801.
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DOI: 10.1021/acs.jchemed.7b00475 J. Chem. Educ. XXXX, XXX, XXX−XXX
