Inexpensive Method for Creating Robust Barium Sulfate Plates for Use

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Inexpensive Method for Creating Robust Barium Sulfate Plates for Use in a UV−Vis Integrating Sphere Eric V. Campbell* and Joel S. Miller* Department of Chemistry, University of Utah, Salt Lake City, Utah 84112-0850, United States

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S Supporting Information *

ABSTRACT: Expensive, highly reflective barium sulfate plates are typically used to obtain solid-state UV−vis electronic spectra. Through normal laboratory use these plates become dirty, chip, and break, thus requiring replacement. An alternative to purchasing new plates is to make your own by combining a mechanical stabilizer, such as gum arabic, with barium sulfate, and using a common laboratory hydraulic press, quickly forming a plate for a fraction of the cost of purchasing new plates.

KEYWORDS: Upper-Division Undergraduate, Graduate Education/Research, Analytical Chemistry, Hands-On Learning/Manipulatives, Solid State Chemistry, UV−Vis Spectroscopy

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gram.8−10 Gum arabic has been used with barium sulfate to improve its coating ability as a contrast medium for X-ray examination of the human colon.11 However, to the best of our knowledge, gum arabic has not been used for binding nonabsorbing reflective materials to be used in UV−vis spectroscopy. By using gum arabic as a binding agent, robust plates suitable for UV−vis spectroscopy can be made.

INTRODUCTION Solid-state UV−vis electronic spectroscopy often employs an integrating sphere attachment to measure the reflectance and transmittance of solid samples.1 Placed behind the sample is a highly reflective plate, commonly made of barium sulfate (BaSO4) or aluminum oxide (Al2O3).2−4 As a result of normal use, these plates become dirty and chipped and can crumble if dropped. Typical replacements cost $100−200 per plate and usually are purchased in sets of two or four. Alternatively, a BaSO4 plate can be made quickly using a common laboratory hydraulic press and a die set. Unfortunately, these plates are brittle and after 3−5 uses become chipped or shattered and need to be replaced. A common undergraduate organic-chemistry laboratory experiment is the Friedel−Crafts Acylation of ferrocene to acetylferrocene and 1,1′-diacetylferrocene.5,6 Students typically purify the products via column chromatography followed by analysis using 1H NMR, infrared spectroscopy, melting-point, or UV−vis spectroscopy. Over the course of a year, because of the number of students, analysis by UV−vis spectroscopy can result in significant solvent waste. To move toward greener chemistry, solid-state UV−vis spectroscopy can help reduce unnecessary solvent waste.7 This still poses a problem because of the short lifetime of the plates due to frequent use. An alternative to commercially available plates can help reduce costs and improve reliability, eliminating the wait time for a purchased replacement to be delivered. Gum arabic (acacia gum) is commonly used as a thickener or stabilizer in the food industry as well as a binding agent for pigments and can be purchased for as little as $0.04 per © XXXX American Chemical Society and Division of Chemical Education, Inc.

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EXPERIMENTAL SECTION Ten grams (10.0 g) of barium sulfate (Acros Organics, extra pure) was added to 2.0 g of gum arabic powder (Jacquard) and finely ground using a mortar and pestle. The mixture was added to a die set with a 5 cm inner diameter, and using a Carver 4350-L hydraulic press, 4 tons of pressure was applied to form a 5 × 1 cm plate. The size of the plate was chosen to closely match commercially available plates; however, other sizes will work equally well if die sets are available and if they fit in the sample holder and cover the ports of the integrating sphere. Higher pressures may be used and may form a more durable plate; however, separating the die set at higher pressures may prove too difficult to be practical. The free-fall drop test was performed with testing conditions similar to those of American Society for Testing and Materials (ASTM) D5276-98.15 A yardstick was attached to a ring stand, and from the indicated height, three plates of each type were dropped, flat-side down, onto a benchtop three times. The Received: April 18, 2018 Revised: June 13, 2018

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DOI: 10.1021/acs.jchemed.8b00283 J. Chem. Educ. XXXX, XXX, XXX−XXX

Journal of Chemical Education

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Figure 1. UV−vis spectra of nonabsorbing reflective plates: (a,d) commercially available plate after several years of use, (b,e) plate made from pure barium sulfate, and (c,f) barium sulfate plate made with gum arabic. The UV−vis spectra of several plates of each type were studied, and only typical spectra are depicted. Spectra d−f show ferrocene (blue) and 1,1′-diacetylferrocene (red).

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RESULTS AND DISCUSSION UV−vis spectra were measured from 240 to 2600 nm with a scan rate of 300 nm/min, and the spectra were baseline corrected. The absorbance scan was then taken under the same parameters. Figure 1a shows the UV−vis spectrum of a commercially available plate after several years of use. An unexpected positive absorbance is seen from 350 to 500 nm, and an unexpected negative absorbance is seen from 550 to 1000 nm. Figure 1b shows data from a plate of barium sulfate; this plate has a flat absorbance averaging at the absorbance limit of the instrument with the integrating-sphere attachment (±0.002 absorbance units), and these plates easily crumble. Figure 1c shows the absorbance of plates made with gum arabic and barium sulfate. The absorbance scan resembles the absorbance of pure barium sulfate, indicating that gum arabic does not absorb in the scanned region. The sharp increase in

temperature and humidity were both ambient at the time of testing. Solid-state UV−vis spectra were obtained at room temperature on a Hitachi U4100 spectrometer equipped with a 60 mm integrating sphere with BaSO4 plates used as a nonabsorbing reflectance reference.

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HAZARDS

Barium sulfate is considered nontoxic, but if handling large quantities, a mask is recommended to avoid inhaling fine particulates.12 The U.S. Food and Drug Administration (USDA) identifies gum arabic as ‘generally recognized as safe’ and no specific precautions need to be taken in its use.13,14 Hydraulic presses pose a crushing hazard. Proper training and eye protection must be worn before operation. B

DOI: 10.1021/acs.jchemed.8b00283 J. Chem. Educ. XXXX, XXX, XXX−XXX

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noise seen in Figure 1a−f from 875 to 925 nm is due to switching detectors, and the increase in noise from 2200 to 2600 nm is due to the integrating sphere as reported by Hitachi.16 The solid-state UV−vis spectra of ferrocene and 1,1diacetylferrocene17,18 is shown in Figure 1d−f. After years of use, the commercially available plates have greater noise in the 1500−2600 nm region and have a sloping absorbance from 600 to 1000 nm (Figure 1d). This slope is not observed in the barium sulfate plates made with and without gum arabic (Figure 1e,f). The free-fall drop tests were performed to determine if adding gum arabic to a barium sulfate plate would increase its durability under typical laboratory conditions. If the plates were dropped from the corresponding height, seen in Table 1,

Pure BaSO4 (Pass or Fail)

BaSO4 with Gum Arabic (Pass or Fail)

1 2 3 6 12 18 24

pass pass pass fail fail fail fail

pass pass pass pass pass pass pass

*E-mail: [email protected] (E.V.C.). *E-mail: [email protected] (J.S.M.). ORCID

Eric V. Campbell: 0000-0002-7466-1747 Joel S. Miller: 0000-0001-5743-8327 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank Shelby Harris for verifying our methodology and independently reproducing our results. The authors also thank Travis J. Moore for photography and photo editing.

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REFERENCES

(1) Workman, J.; Springsteen, A. Reflectance Spectroscopy: An Overview of Classification and Techniques. In Applied Spectroscopy; Academic Press: San Diego, 1998; pp 193−224. (2) Stenzel, O.; Wilbrandt, S.; Du, S.; Franke, C.; Kaiser, N.; Tünnermann, A.; Mende, M.; Ehlers, H.; Held, M. Optical properties of UV-transparent aluminum oxide/aluminum fluoride mixture films, prepared by plasma-ion assisted evaporation and ion beam sputtering. Opt. Mater. Express 2014, 4, 1696−1707. (3) Chantrapornchai, W.; Clydesdale, F.; McClements, D. J. Theoretical and Experimental Study of Spectral Reflectance and Color of Concentrated Oil-in-Water Emulsions. J. Colloid Interface Sci. 1999, 218, 324−330. (4) French, R. H.; Müllejans, H.; Jones, D. J. Optical Properties of Aluminum Oxide: Determined by Vacuum Ultraviolet and Electron Energy-Loss Spectroscopies. J. Am. Ceram. Soc. 1998, 81, 2549−2557. (5) Hwa, R.; Weizman, H. Revisiting the Separation of Ferrocene and Acetylferrocene by Adsorption Chromatography: Adding a Third Component. J. Chem. Educ. 2007, 84, 1497−1498. (6) Gilbert, J. C.; Monti, S. A. The Separation of Ferrocene, Acetylferrocene, and Diacetylferrocene. A dry-column chromatography experiment. J. Chem. Educ. 1973, 50, 369−370. (7) Molina-Diaz, A.; Garcia-Reyes, J. F.; Gilbert-Lopez, B. Solidphase spectroscopy from point of view of green analytical chemistry. TrAC, Trends Anal. Chem. 2010, 29, 654−666. (8) Vasile, F. E.; Martinez, M. J.; Pizones Ruiz-Henestrosa, V. M.; Judis, M. A.; Mazzobre, M. F. Physiochemical, interfacial and emulsifying properties of a non-conventional exudate gum (Prosopis alba) in comparison with gum arabic. Food Hydrocolloids 2016, 56, 245−253. (9) Nakauma, M.; Funami, T.; Noda, S.; Ishihara, S.; Al-Assaf, S.; Nishinari, K.; Phillips, G. O. Comparison of sugar beet pectin, soybean soluble polysaccharide, and gum arabic as food emulsifiers. 1. Effect of concentration, pH, and salts on the emulsifying properties. Food Hydrocolloids 2008, 22, 1254−1267. (10) Moschakis, T.; Murray, B. S.; Biliaderis, C. G. Modifications in stability and structure of whey protein-coated o/w emulsions by interacting chitosan and gum arabic mixed dispersions. Food Hydrocolloids 2010, 24, 8−17. (11) Tonariya, T. Y.; Wada, S. Y.; Yamaguchi, H. K.; Yamazaki, S. T.; Sakai, F. I.; Yokoi, T. S.; Togashi, O. M.; Noguchi, O. Y. Barium Sulfate Contrast Medium for X-Ray Examination of the Large Intestine. U.S. Patent 5,160,724, Nov 3, 1992. (12) Barium Sulfate; MSDS No. 243353; Sigma-Aldrich: Saint Louis, MO, May 23, 2016. https://www.sigmaaldrich.com/catalog/product/ sigald/243353 (accessed Feb 2018). (13) Acacia; MSDS No. 260770; Sigma-Aldrich: Saint Louis, MO, Jan 03, 2016. https://www.sigmaaldrich.com/catalog/product/ aldrich/260770 (accessed Feb 2018).

and did not crack or shatter, then it was considered to pass. If, during any of the three attempts, the plate cracked or shattered, then it was considered to have failed. If a plate failed, then a new plate was made and tested at the next height. The pure barium sulfate plates failed when dropped onto a flat benchtop from 6 in. or greater. The plates made with gum arabic withstood being dropped three times from 24 in., indicating that adding the gum arabic binding agent increased the mechanical integrity of the plates. To ensure reproducibility, an undergraduate student followed the above methodology to make new plates and performed a drop test, obtaining the same results.

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SUMMARY Herein, an inexpensive method to make plates for solid-state UV−vis spectroscopy has been demonstrated. Mixing a binding agent with barium sulfate strengthens its mechanical integrity, improving its longevity as a plate under normal laboratory conditions. UV−vis spectroscopy shows that gum arabic does not absorb in the 240−2600 nm region, and hence does not interfere with sample signals in this region. Other plates could be made by combining gum arabic as a binding agent with other reflecting materials, such as titanium oxide or aluminum oxide. Instructional laboratories that use UV−vis spectroscopy, such as Physical Chemistry, Instrumental Chemistry, and Inorganic Chemistry laboratories, can benefit from the ease of plate fabrication.

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AUTHOR INFORMATION

Corresponding Authors

Table 1. Drop Test of Barium Sulfate Plates Height (in.)

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ASSOCIATED CONTENT

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The Supporting Information is available on the ACS Publications website at DOI: 10.1021/acs.jchemed.8b00283. Parts list with cost estimates (PDF) C

DOI: 10.1021/acs.jchemed.8b00283 J. Chem. Educ. XXXX, XXX, XXX−XXX

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(14) 21 CFR 582.7330 − Gum Arabic. Code of Federal Regulations; Food and Drug Administration: Silver Spring, MD, 2010. (15) Standard Test Method for Drop Test of Loaded Containers by Free Fall; Standard ASTM D5276-98(2017); American Society for Testing and Materials International: West Conshohocken, PA, 2017. (16) Hitachi High-Tech. Hitachi UV-VIS-NIR Spectrophotometer Accessories Catalog. https://www.hitachi-hightech.com/products/ images/8091/UH4150-AccessoriesHTB-E101.pdf (accessed April 2018). (17) Taniguchi, M.; Lindsey, J. S. Database of Absorption and Fluorescence Spectra of > 300 Common Compounds for use in PhotochemCAD. Photochem. Photobiol. 2018, 94, 290−327. (18) Rabie, U. M. Intra- and intermolecular charge transfer: Twin themes and simultaneous competing transitions involving ferrocenes. Spectrochim. Acta, Part A 2009, 74, 746−752.

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DOI: 10.1021/acs.jchemed.8b00283 J. Chem. Educ. XXXX, XXX, XXX−XXX