In the Laboratory
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A Closer Look at Acid–Base Olfactory Titrations Kerry Neppel, Maria T. Oliver-Hoyo,* Connie Queen, and Nicole Reed Department of Chemistry, North Carolina State University, Raleigh, NC 27695; *
[email protected] requirements for potential olfactory indicators and tested protocols for using garlic, onions, and vanillin as acid–base olfactory indicators. In addition, as different types of onions exhibit different colored solutions when extracted in strong base, a comparative testing of the intrinsic visual colorimetric versus olfactory determination of different types of onions used as indicators in acid–base titrations is included.
Numerous chemistry experiments depend heavily on color-change detection. Routine acid–base titrations are a perfect example. Statistics point out that 7% of the male population and 4% of the female population suffer from color blindness (1). This means that 2.8 million people are affected by color deficiencies in the United States alone. In addition, 1.1 million people in the United States have severe visual impairment or blindness. Students in this latter category are often discouraged from actively performing laboratory experiments, restricting their role in a lab environment to one of passive recipients of information. An alternative feasible to all students is to create new experiments directed to appeal to senses other than vision. Alternatives to subtle color changes in chemistry experiments should be a welcome option to educators. We have exploited the sense of smell in end-point determinations of acid–base reactions. Its sensitivity makes it a viable option as a quantitative detection tool. Olfactory titrations using raw onions (2) and eugenol (3) as acid–base indicators have been reported. The similarities between the mechanisms of aroma release for onions and garlic led us to consider garlic as a viable option for olfactory end-point detection in acid–base titrations. In addition, previous reports on vanillin excluded this compound as an effective olfactory indicator owing to its solubility in water (3). We considered that fact an asset rather than an obstacle in using vanillin for olfactory end-point determinations and developed a successful experimental procedure using vanillin in acid–base titrations. An in-depth investigation on olfactory titrations is presented in this article to include O
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Background and Practical Considerations The first requirement for the effective use of an olfactory indicator is the ability of the experimenter to safely control aroma release. Potential olfactory indicators must be tested for pungency strength, characteristics of the aroma, and toxicity. Pungent, lingering aromas that numb the senses, overpowering repulsive aromas, or toxic volatiles are not feasible options (3). Plants offer potential sources of benevolent olfactory indicators. Strong penetrating aromas characterize plants in the genus Allium. This family includes onions, garlic, leeks, shallots, and chives. The strong aroma in this genus does not flourish unless the plant tissue is damaged and enzymes convert the aroma precursors to odorous volatiles (4). An odorless substrate is attacked by the enzyme producing a series of sulfur-containing compounds responsible for the characteristic aromas. Theoretically, to control olfactory experiments using plants, the release of aroma compounds could be accomplished enzymatically as long as some of the targeted volatiles are obtained through nonenzymatic reactions. The mechanism of odor release in garlic is depicted in Scheme I (4, 5). Disulfide monoxides (thiosulfinates), espe-
allinase
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Scheme I. Mechanism of odor release in garlic.
self-
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thiosulfonate (disulfide dioxide)
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In the Laboratory
cially the aliphatic members, readily disproportionate to thiosulfonates and disulfides. These compounds are responsible for the characteristic odor of vegetables in the genus Allium. Conditions that favor such a reaction include temperatures above 50 ⬚C, acidic medium, low pressure, and exposure to light. It is the pH dependence that is manipulated in olfactory acid–base titrations. The raw garlic odor is detected under neutral or acidic conditions but not under basic conditions. In garlic, the precursor allicin (diallyl thiosulfinate) is responsible for the aroma as it decomposes and rearranges to produce the disulfide and thiosulfonate. Small differences in structure can change dramatically the aroma exhibited by a compound. In the case of onions, a similar mechanism takes place with just a few differences (4, 6). The unstable sulfenic acid is 1-propene (instead of 2-propenesulfenic acid), which undergoes self-condensation leading into the onion-aroma precursor and a lachrymator, thiopropanal S-oxide, which is absent in the garlic mechanism. Similarities between garlic’s mechanism and the mechanism of odor release in onions strongly suggested to us garlic’s potential as an olfactory indicator. The second requirement deals with olfactory sensitivity. An ideal olfactory indicator would be one whose odor can be “arrested” at the beginning of the titration and eventually released at the equivalence point. The starting solution must be aroma-free so that the onset of the odor can be easily detected for an accurate determination of the end point of the titration. In the case of garlic and onions, the aroma is quenched in strong basic solutions and evolves in neutral and acidic solutions. An extensive study on the optimal conditions for effective use of garlic and onions as olfactory indicators was conducted. Our findings are the basis for the experimental protocols included in the Supplemental Material.W The successful implementation of olfactory titrations with the general student population in our labs required a closer look at aroma control, aroma fatigue, and safety issues. Control of the aroma in the laboratory is achieved by placing as many experimental setups as possible in available hoods and running one experiment per lab bench in open space. Numbing of senses is another practical issue to consider. Flair and Setzer have previously reported that after 10 titrations with eugenol, they expected numbing of the senses, however, they observed no significant evidence of that (3). Our experimental setup consists of a mini fan placed behind the titration setup (Figure 1). This permits slow air circulation from the back of the titrating setup toward the experimenter. The incorporation of the mini fan in the experimental setup proved to be a crucial addition for obtaining reproducible results as well as controlling aroma fatigue. As the end point of the titration approaches, the aroma bursts from the solution only to disappear in a couple of seconds. Without the mini fan, students must approach the flask at close range, constantly being exposed to the disappearing smell prior to the lingering smell. The mini fan not only provides a safe distance from the equipment, but it disperses the immediate and transient aroma reducing exposure and decreasing the chance of numbing the sense of smell. Once the end point is reached, the lingering smell bursts out of solution in a sharp and distinguishable manner. Between titration runs, some students choose to “purge” their senses by smelling 608
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Figure 1. Olfactory lab setup.
coffee crystals. Students who did this reported that this step allowed them to start “clean” every trial run. The effectiveness of the protocols we developed was measured by comparing calculated molarities of analyte solutions to standardized values. Visual indicators could complement olfactory indicators. In this scenario, visually impaired students can make determinations by smell while at the same time students with no visual impairment make the determination by changes in color or smell. Phenolphthalein was previously reported as such a complement in onion titrations (2). However, the difficulties in observing the completion of the phenolphthalein color change when titrating a strong base with a strong acid can be easily demonstrated (7). We noticed that the odorless filtrates exhibited different colors depending on the type of onions used. A colorless filtrate results from white onions, bright-yellow from yellow onions, and brownish-yellow from red onions. We found that the intrinsic color from yellow onions disappears with acid addition making it as useful as commercial indicators in determining acid–base end points colorimetrically. In the case of the yellow-colored filtrate from yellow onions, the color disappears right after the onset of the onion aroma. Red onions undergo two color changes, although these changes tend to run into each other. The first change is very close to the olfactory end point, however, students had difficulties stopping the titration at the first color change (brownish-yellow to beige-yellow) before the second change (beige-yellow to beige with a hue of pink) was obtained. Therefore, red onions were not as efficient as the yellow onions for colorimetric determinations. A comparison between commercial indicators and the natural indicators found in yellow onions shows that their intrinsic pigments can be used for visual end-point determinations as effectively as commercial indicators. Experimental Overview When garlic or onion are used as olfactory indicators, the chopped vegetables are soaked in solutions of sodium hydroxide to quench the characteristic smells. The vegetable兾NaOH solution is decanted into a clean beaker and a portion of the analyte is pipetted and titrated with hydrochloric acid. The concentrations that work best for each vegetable are given in the student experimental procedure (see the Supplemental MaterialW). In the case of vanillin, drops of a saturated vanillin solution in ethanol are added to the quenching basic solution.
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In the Laboratory 14
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Figure 2. Aroma end points using red onions and corresponding pH values superimposed on a potentiometric titration curve for students A and B.
Figure 4. Aroma and visual end points with corresponding pH values using vanillin: (A) olfactory end point, 12.20 mL at pH 8, (B) phenolphthalein end point, 12.25 mL at pH 7.7, (C) phenol red end point, 12.32 mL at pH 6, and (D) bromothymol blue end point, 12.30 mL at pH 5.9.
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Figure 3. Aroma and visual end points with corresponding pH values obtained using different types of onions: (A) olfactory end point, 16.22 mL at pH 6.8, (B) yellow onion, intrinsic visual end point, 16.28 mL at pH 6.6, (C) white onion, phenol red end point, 16.30 mL at pH 6.3, (D) white onion, bromothymol blue end point, 16.37 mL at pH 5.8, and (E) red onion, intrinsic visual end point, 16.40 mL at pH 5.4.
Common visual acid–base indicators can be added to the white onion兾NaOH solution and compared to the olfactory determinations. We recommend bromothymol blue or phenol red to determine the end point visually. In the case of a colored filtrate (from yellow onions), the onset of the onion aroma may be compared to the disappearance of the intrinsic color of the solution. Data collection can be done in different ways in the laboratory. Some groups may perform olfactory determinations recording the onset of aromas while others may perform visual end-point determinations recording color changes. Data collected from the class as a whole can be used to compare results of both types of titrations. An alternative way is to run blind trials where each partner takes turns to monitor the smell or the visual color change. The student perceiving the aroma is blindfolded (so that the changes in color do not influence his or her determination) while his or her partner records the titrant volume at the time of the onset of the aroma and the titrant volume at the time of the visual color change (Figure 2 for this type of data collection results). Eiwww.JCE.DivCHED.org
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Volume of 1M HCl / mL Figure 5. Aroma and visual end points with corresponding pH values using garlic:(A) olfactory end point, 19.11 mL at pH 8.5, (B) phenol red end point, 19.22 mL at pH 7.4, (C) phenolphthalein end point, 19.15 mL at pH 6.1, and (D) bromothymol blue end point, 19.40 mL at pH 5.2.
ther of these data-collection methods can be further compared with potentiometric titrations. Results obtained from potentiometric and end-point determinations can be compared and discussed as part of the laboratory exercise. Hazards Sodium hydroxide is a strong base and hydrochloric acid is a strong acid. Both are corrosive and will cause severe burns if in contact with skin or eyes. Ingestion or inhalation may cause severe burns of mouth, throat, and stomach. Proper eye and skin protection are necessary. Results Potentiometric curves for stock solutions are presented in Figures 2–5 with the data obtained from olfactory and visual titrations superimposed. These figures contain information regarding olfactory and colorimetric end points as well as the pH values that students simultaneously obtained at each olfactory or colorimetric determination. These data allowed students to compare sensorial and potentiometric val-
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In the Laboratory Table 1. Average Molarity Calculations Average Molarities/(mol L᎑1) End Point
Yellow Onion (n = 45)
Red Onion (n = 36)
White Onion (n = 65)
Garlic (n = 36)
Vanillin (n = 36)
Olfactory aroma
1.698 ± 0.019
1.685 ± 0.023
1.682 ± 0.013
1.906 ± 0.019
0.9841 ± 0.0015
Visual color: intrinsic or exogenous
1.680 ± 0.008 Intrinsic
1.621 ± 0.009 Intrinsic
1.617 ± 0.002 Exogenous
1.904 ± 0.003 Exogenous
1.004 ± 0.001 Exogenous
NOTE: The concentration of the standardized NaOH was 1.9599 M for the onion titrations and 0.9942 M for the vanillin titration.
ues. Each figure represents data collected during a particular laboratory session. A blindfold data collection is presented in Figure 2 where each partner took turns for the olfactory determinations using red onions. At the olfactory end points, the pH values were also obtained. Each student conducted three runs represented in this typical graph. Student A reported greater variability for the three runs than student B, however, the difference between the two most dissimilar runs (run 1A and run 2A) was less than 0.18 mL. Colorimetric titrations were performed using exogenous indicators with white onions and the intrinsic colors exhibited by yellow and red onions. Consistently, the aroma detection preceded all color changes. Figure 3 shows the results of these titrations. End-point determinations using vanillin as an olfactory indicator and vanillin solutions with commercial visual indicators are shown in Figure 4. Olfactory and visual end-point determinations for garlic are presented in Figure 5. Vanillin and garlic gave the most accurate olfactory results when molarity calculations were compared to the standardized values (Table 1). Molarity calculations based on olfactory and visual endpoint determinations were compared for a number of olfactory indicators. Values in Table 1 are the averages for all titrations using a particular olfactory indicator conducted by students during the past two years (n = number of runs). In the case of white onions, garlic, and vanillin, commercial visual indicators were added to the solutions for comparison purposes since the filtrates were colorless. Values for all types of exogenous indicators (phenolphthalein, bromothymol blue, and phenol red) used with a specific substance were lumped together so that a general comparison between intrinsic and exogenous can be observed. Conclusions Acid–base titrations are common volumetric analysis experiments that include potentiometric and colorimetric methods. This experiment contributes yet another viable way of determining end points and concentrations of unknown basic solutions. Students can be exposed to the three methods to compare and discuss results. The preparation times, cost, and safety issues are comparable to traditional titration experiments. Results show that onions, garlic, and vanillin are effective olfactory indicators in acid–base titrations, giving consistent results when tested by a diverse student population. However, garlic and vanillin gave the most accurate results when calculated NaOH molarities were compared to the standardized values. Intrinsic pigments in yellow and red onions were tested as visual indicators and compared to commercial indicators added to white onion solutions. In the case 610
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of red onions, the results are not as accurate and straightforward as in the case of yellow onions as a result of two color changes often difficult to separate and great dependence on the portion of the onion chopped. Visual impairment is a particular disadvantage in the chemistry laboratory since visualization of chemical phenomena is a primary goal. Instructional materials providing instructors with the appropriate kind of experiments that can be used by visually impaired students will certainly be welcomed by any instructor who has the opportunity to teach this community of people. At institutions where the enrollment of handicapped people is low, the justification of specialized equipment can be very difficult to obtain. Besides the cost factor, these experimental alterations also offer the advantage of practically the same time of preparation as laboratories commonly performed as well as being cost effective. All students can perform these olfactory titrations and the level of instruction can be expanded from a phenomenological point to the complex aroma chemistry behind the scenes. W
Supplemental Material
Experimental protocols for the students using different types of olfactory indicators and notes for the instructors are available in this issue of JCE Online. Acknowledgments The authors would like to thank Monique Williams, Adrian Sutton, and Jennie Warren, undergraduates at North Carolina State University, whose contributions were invaluable for the development of these protocols. Also, we would like to express our sincere appreciation to the chemical engineering students who willingly and enthusiastically helped us conduct the blind experiments. Partial funding from the REU NSF program was invaluable to the progress of this work. Literature Cited 1. (a) Statistics on Eye Disease. http://whyfiles.org/003eye/ statistics.html (accessed Jan 2005). (b) Color Blindness. http:// www.hhmi.org/senses/b130.html (accessed Jan 2005). (c) Hopskins Q&A: Colorblindness. http://www.medhelp.org/news/ HC/2000Feb1.htm (accessed Jan 2005). 2. Wood, J. T.; Eddy, R. J. Chem. Educ. 1996, 73, 257–258. 3. Flair, M.; Setzer, W. N. J. Chem. Educ. 1990, 67, 795–796. 4. Block, E. Angew. Chem., Int. Ed. Engl. 1992, 31, 1135–1178. 5. Lawson, L. D.; Hughes, B. Planta Med. 1992, 58, 345–350. 6. Block, Eric. Phosphorous, Sulfur, and Silicon 1991, 58, 3–15. 7. Huefner, R.; Richmond, T. G. Chem. Educ. 2000, 5, 181–182.
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