REPORT pubs.acs.org/jchemeduc
Exploiting Mass Measurements in Different Environments: Density, Magnetic Susceptibility, and Thermogravimetry Nathan Bower*,† and Anne Yu‡ † ‡
Department of Chemistry, Colorado College, Colorado Springs, Colorado 80903-3294, United States Department of Chemistry, Pomona College, Claremont, California 91711-6338, United States ABSTRACT: The measurement of mass in different surroundings allows one to measure the density of materials and their interaction with magnetic fields or their temperature dependence. Although most curricula cover the use of analytical scales, many leave out mass measurements in different environments. We describe some of the current instrumentation and applications in this area, including a novel and inexpensive method for measuring magnetic susceptibility. KEYWORDS: First-Year Undergraduate/General, Second-Year Undergraduate, Upper-Division Undergraduate, High School/Introductory Chemistry, Analytical Chemistry, Instrumental Methods, Laboratory Equipment/Apparatus, Magnetic Properties, Physical Properties, Thermal Analysis FEATURE: Instrumentation Topics for the Teaching Laboratory
T
hough the measurement of mass is one of our oldest analytical methods,1 we often forget that mass is a fundamental unit included in the ideal gas, gravitational, and Planck constants, Avogadro’s number, the Bohr magneton, and the charge of an electron.2 Mass measurements are used in the search for extra-solar planets and, via mass spectrometry, for determining the age, origin, and identity of materials. Even common laboratory balances are underappreciated by teachers and students. Magnetic susceptibility, thermogravimetric, and density measurements can expand awareness of mass measurements, and this article highlights these methods.
Liquids and gases are readily determined by filling known volumes and weighing them. With buoyancy corrections, good values can be obtained and no special apparatus beyond flasks or pipettes are needed. For determining the specific gravity of solids (and liquids), Ohaus, Mettler-Toledo and A&D have kits that fit inside their balances that use Archimedes’ principle. These range from $200 to $1000, and they eliminate the wind currents that can plague hanging samples below a balance. Their kits include a thermometer for temperature corrections and a known density “sinker” used to determine the density of liquid media.
’ DENSITY MEASUREMENTS During the 20th century, improvements in how force exerted by a mass is measured using various springs, load cells, and strain gauges have allowed transducers to replace relatively slow measurements by mass comparison with balances.3 Piezoelectric crystals, although nonlinear in their output, are commonly employed in analytical scales because of their ruggedness and ease of use. Masses as low as 1 part in 108 or better can be resolved, and this precision is now a requirement for certified laboratories.4 Buoyancy differences in various media are at the heart of density measurements made using Archimedes’ principle. At least three or four significant figures are attainable using this method, but temperature compensation and an enclosure for a stable measurement are necessary for the best work. Precious metals, gems, and glasses with forensic interest are often measured by this nondestructive method.5 Even microscopic particles can be measured by suspending them in a medium with equivalent density and then measuring the medium’s density.
’ THERMOGRAVIMETRIC ANALYSIS Mass loss with heating is commonly employed to measure adsorbed and/or bound water. Similarly, volatile organic matter or other components are lost at fairly discrete temperatures, allowing identification based on the temperature and mass lost. A profile of temperature versus change in mass gives a thermogram used to identify the changes. The profiles can be used to determine thermal stability and the kinetics of various reactions. These parameters are especially useful in the paint and polymer industries, but many different materials, such as food, cement, plastic, wood, paper, rock, and ceramic have been characterized by thermogravimetric analysis (TG or TGA) in order to determine parameters such as the safe shelf life for food6 or to help detect forgeries.7 Three main configurations are used by TG instrument manufacturers. In some, the sample pan hangs vertically inside of a furnace,
Copyright r 2011 American Chemical Society and Division of Chemical Education, Inc.
Published: March 07, 2011 536
dx.doi.org/10.1021/ed200074q | J. Chem. Educ. 2011, 88, 536–539
Journal of Chemical Education
REPORT
Table 1. Comparative Features of Some Thermogravimetric Analyzers
Temperature Range
Entry-Level TGA
Multi-Option TGA
Ambient to 1000 °C; single pan,
Ambient to 1200 °C; Curie point calibration
TGA and DSC Combined Ambient to 1500 °C; dual pans, horizontal furnace
vertical furnace Heating Rates
0.1 °C/h to 99.9 °C/min
0.1-500 °C/min; modulated, hi-res heating
0.1-100 °C/min
Sample Size
20/200 mg; 1 μg sensitivity
100 mg; 0.1 μg sensitivity;25 position
200 mg; 0.1 μg sensitivity; DTA sensitivity 0.001 °C
List Price
$20,000; $23,000 for H option
$60,900 to $78,200 with options
$70,000
Some Options
TGA50H goes to 1500 °C
FTIR detector an option
$2,500 for specialized software
Example Vendors
http://www.shimadzu.com/ http://www.perkinelmer.com/
http://www.tainstruments.com/ http://www.thermoscientific.com/
http://www.tainstruments.com/ http://www.netzsch.com/
(accessed Feb 2011)
http://www.netzsch.com/
(accessed Feb 2011)
autosampler
(accessed Feb 2011)
and in others, the furnace rests on a ceramic rod on top of a balance. In the third type the pan is inserted horizontally. Some manufacturers use more than one of these configurations. (See Table 1 for links to a few of the many vendors available.) In each configuration, the temperature sensor is close to the sample for accuracy. Thermal inertia, especially with larger samples, can produce errors. The horizontal configuration is less affected by turbulence in gas flow and the longer balance arm typical in horizontal systems can provide greater sensitivity. It is used in the simultaneous TG-DSC (differential scanning calorimetry) instruments. However, sample size is more limited. The most common configuration (Figure 1) uses a single pan suspended in a furnace that is raised and lowered around the pan, providing a versatile arrangement that is easily loaded. In most instruments, the pan is held at a constant position by using a null-deflection balance, held in place by a circuit that senses deflection and produces a counterforce to restore its original position. Sample masses of about 10 mg are common with an instrument baseline drift of (0.025 mg. Cool-down times are 20-30 min, but low mass furnaces can reduce this by half. “High-resolution” mass change measurements can be achieved using dynamic feedback controls, for example, slowing the heating rate during periods of greater weight loss. Quantitative analysis of reactions is enhanced if the quantity of a reactive gas is controlled with known pulses injected into the reaction chamber. Kinetic analysis of decompositions, volatilizations, or other reactions can be measured in a single run with modulation of the temperature program. Otherwise, multiple runs at different heating rates are used in plots of log (rate) versus inverse temperature (K). Modulation also removes the need for a priori (or assumed) knowledge of the rate equation. Methods for heating samples include infrared sources as well as resistance heating, and high-end instruments allow purge gases to be introduced into an FTIR or MS for further characterization. Temperature calibration may be done using Curie point standards or substances with known weight loss at given temperatures. Although not considered a true standard, calcium oxalate is often used because of its loss of H2O, CO, and CO2 at known temperatures.
Figure 1. Vertical configuration thermogravimetric analysis balance with the furnace lowered. (Image used with permission of Shimadzu, Columbia, MD.).
’ MAGNETIC SUSCEPTIBILITY When mass measurements are conducted in a magnetic field, the force between a sample’s electrons and the magnetic field will add a component to the total mass measured, depending on whether the electrons are all paired or not. Diamagnetic substances with paired electrons will be repelled by the field, while paramagnetic, ferromagnetic, ferrimagnetic and antiferromagnetic substances will be attracted.
Figure 2. Evans magnetic susceptibility balance. (Image used with permission of Johnson Matthey Gas Purification Technology, West Chester, PA.).
Magnetic susceptibilities can be determined by many means, including measuring how the sample affects the frequency of an inductively coupled ac electric field, or how it shifts an indicator solvent peak in an NMR.8 At least three methods employ balances or scales in the measurement. In the traditional Gouy 537
dx.doi.org/10.1021/ed200074q |J. Chem. Educ. 2011, 88, 536–539
Journal of Chemical Education
REPORT
Table 2. Comparative Features of Some Magnetic Susceptibility Instruments MSB Mark I Evans Balance Range
1 10-7 to 1 10-2 SI units (Xv)
Extra Components
Bartington MS3
Analytical Balance and Magnet
1 10-6 to 26 SI vol. susceptibility (Xv)
1 10-6 to 1 10-1 SI units (Xv)
Extra sample tubes $40/each (2 are
Different probes allow liquid or solid samples,
1 L plastic beaker inverted over magnet mounted
included with purchase)
low and hi temps
above balance
Sample Size
250 mg; 50 mg with thin cell
0.2 or 1 mL for lab probe
1 mL typical; 10 mg minimum
List Price
$7250
$3000 þ lab probe ($3000)
$10 magnet þ ($3000 balance)
Some Options
1 10-9 SI possible with the Auto MSB
Portable with various probes for fieldwork;
Various magnets with 2000-5000 G work
and Notes
Mark II ($16,500)
vials $1/ea
Example Vendors
http://pureguard.net/ http://www.geneq.ca/
http://www.gmw.com/index.html http://www.ascscientific.com/mag03.html
In-house: using magnet from, e.g.: http://www.kjmagnetics.com/
(accessed Feb 2011)
(accessed Feb 2011)
(accessed Feb 2011)
9
method, a sample is suspended from a balance so that only part of it is in a magnetic field, creating a torque that is measured as a gain or a loss in mass. A Faraday balance works similarly, but the sample is located completely within a uniform magnetic field, allowing smaller sample sizes to be measured.10 Both Gouy and Faraday balances are usually user-built from components. In the Evans-type balance (Figure 2),11 a null-deflection scale is used by providing a counterforce through an electromagnet that keeps the sample in the same position.12 Evans scales from various vendors are typically used by chemists, while other approaches are popular in geology (Table 2). In teaching, they can be used following a synthesis,13 allowing students to compare their results for magnetic susceptibility to those obtained using Pascal’s constants.14 Finely ground samples and standards of 50-250 mg are packed into glass tubes and approximately 5-10 min of instrument time is sufficient for multiple measurements. In one lab period, 24-30 students can finish their measurements using one or two of these scales.
’ LOW-COST ALTERNATIVES A number of aftermarket companies sell density kits, but users should find it is easy enough to construct their own (Figure 3A). Tripod designs provide extra stability while hindering access. Air currents are a major source of noise, so it is worth assuring the apparatus fits completely inside the sample chamber. As students are just developing a conceptual understanding of density in their mid teens, experiments that make use of Archimedes’ principle are especially useful for making mental connections. In the undergraduate curriculum, comparisons of the errors encountered by various procedures for measuring densities can be quite useful for introducing significant figures, error propagation, and method development. Gouy balances in education are often constructed from recycled balances and permanent magnets.15 We note that good results are also possible using one side of a 1.9 cm 0.32 cm disk (>2000 G) or 1.9 cm cube (>5000 G) rare-earth magnet16 mounted on, but well above, the pan of a 0.1 mg analytical balance (Figure 3B). Because a single pole is used with an inhomogeneous field, a force is exerted similar to but different from that created in Gouy and Faraday balances, making this a third type of magnetic susceptibility scale. The field curvature of these magnets suggests use of equivalent volume samples and standards near 1 cm3 will produce the best results. An inverted 1 L beaker or plastic food container placed immediately above (but not touching) the magnet provides a platform for samples and standards in plastic or aluminum weighing boats. After the balance is tared with an empty boat, relative magnetic
Figure 3. In-house assembled density and magnetic susceptibility apparatuses.
susceptibilities of samples are measured. Calibration with standards, such as 1.0 g pieces of Bi, Cu, Al, Cr, and Mn,17 allows absolute magnetic susceptibilities to be calculated. Electron spin in transition metal complexes (e.g., see Garland, et al.18) are easily explored with this apparatus using 0.01 mol of the solid salts in plastic vials, and its single-sided magnetic probe allows for new applications, such as a nondestructive magnetic imaging scanner for documents and paintings.19 Although permanent magnets should not themselves be heated or cooled, their magnetic fields will penetrate glass or Styrofoam cups, permitting the measurement of various materials’ Curie points if they fall within the range achievable by heating or cooling solutions within the cups.
’ CONCLUSIONS Mass measurements in different environments are useful for characterization of complex materials. Research into alternative energy sources and storage systems has been enhanced by the kinetic information that TGA offers, especially when coupled with magnetic susceptibility.20 The recent interest in magnetic nanoparticle separation and drug delivery and in vivo magnetic resonance imaging of neurons has also increased the importance of introducing students to magnetic measurements.21
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. 538
dx.doi.org/10.1021/ed200074q |J. Chem. Educ. 2011, 88, 536–539
Journal of Chemical Education
REPORT
’ ACKNOWLEDGMENT We thank Steve Burt for help constructing the in-house magnetic susceptibility and density kits, and Arron Wolk and the Instrumental Analysis students for beta testing them. ’ REFERENCES (1) Sanders, L. A Short History of Weighing; W. & T. Avery, Ltd.: Birmingham, England, 1947; revised 1960. http://www.averyweightronix.com/download.aspx?did=6249 (accessed Feb 2011). (2) Recent Advances in Metrology and Fundamental Constants, Vol. 146; Quinn, T., Leschiutta, S., Tavella, P., Eds.; IOS Press, Inc.: Amsterdam, Netherlands, 2001. (3) OMEGA. Technical Engineering Reference, Introduction to Load Cells. http://www.omega.com/prodinfo/loadcells.html (accessed Feb 2011). (4) Fraley, K.; Harris, G. Advanced Mass Calibration and Measurement Assurance Program for State Calibration Laboratories; NISTIR 5672; NIST, U.S. Dept. of Commerce: Washington, DC, 2005. (5) For example: (a) Smale, D. J. For. Sci. Soc. 1973, 13, 5–15. (b) Cobb, P. G. W. J. For. Sci. Soc. 1968, 8, 29–31. (6) Ferrasse, J.-H.; Lecomte, D. Chem. Eng. Sci. 2004, 59 1365–1376. (7) Marcolli, C.; Wiedemann, H. G. J. Therm. Anal. Calorim. 2001, 64, 987–1000. (8) For example: (a) Evans, D. F. J. Chem. Soc. 1959, 2003–2005. (b) Ostfeld, D.; Cohen, I. A. J. Chem. Educ. 1972, 49, 829. (9) Gouy, L.-G. C. R. Acad. Sci. 1889, 109, 935–937. (10) Lindoy, L. F.; Katovic, V.; Busch, D. H. J. Chem. Educ. 1972, 49, 117–120. (11) Evans, D. F. J. Phys. E: Sci. Instrum. 1974, 7, 247. (12) Woolcock, J.; Zafar, A. J. Chem. Educ. 1992, 69, A176–A179. (13) King, H. C. A. J. Chem. Educ. 1971, 48, 482–484. (14) Bain, G. A.; Berry, J. F. J. Chem. Educ. 2008, 85, 532–536. (15) For example:(a) Saunderson, A. Phys. Educ. 1968, 3, 272–273. (b) Viswanadham, P. J. Chem. Educ. 1978, 55, 54. (c) Watton, E. C.; Quinlan, M. J. Aust. Sci. Teachers J. 1975, 21, 107–110. (16) Magnet BCCC (N42) or DC2 (N52). K & J Magnetics, Inc. http://www.kjmagnetics.com/ (accessed Feb 2011). (17) Handbook of Chemistry and Physics, 84th ed.; Lide, D. R., Ed.; CRC Press: Boca Raton, FL, 2003. (18) Garland, C. W.; Nibler, J. W.; Shoemaker, D. P. Experiments in Physical Chemistry, 7th ed.; McGraw-Hill: New York, 2003. (19) Bower, N. W.; Reuer, M. K.; Burt, S. E. Development of a Novel Magnetic Imaging System Useful in Art Conservation and Authentication. ACS Division of Analytical Chemistry Poster CO-1312, Pittcon 2011, Atlanta, GA, March 13-17, 2011. (20) G€ok, A.; Sarı, B.; Talu, M. Synth. Met. 2004, 142, 41–48. (21) Gijs, M. A. M. Microfluid. Nanofluid. 2004, 1, 22–40.
539
dx.doi.org/10.1021/ed200074q |J. Chem. Educ. 2011, 88, 536–539
