Communication Cite This: J. Chem. Educ. XXXX, XXX, XXX−XXX
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Modified Siwoloboff−Wiegand Procedure for the Determination of Boiling Points on a Microscale Timothy L. Troyer,*,† Kristen R. Mounsey,† William J. King,‡ Laura M. Givens,‡ Jessica A. Hutton,‡ Melissa Hood Benges,‡ Kindra N. Whitlatch,‡ and Jacob D. Wagoner‡ †
Department of Chemistry, Huntington University, Huntington, Indiana 46750, United States Department of Chemistry, West Virginia Wesleyan College, Buckhannon, West Virginia 26201, United States
‡
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S Supporting Information *
ABSTRACT: Developing simpler, more accessible, and more affordable methods for the determination of the boiling point of microscale amounts of liquid may lead to increased utilization in research and teaching laboratories as an analytical technique. Our efforts toward that goal have revealed that digital hot plates can be used as the heat source. One option is to simply use a test tube and a digital thermometer, requiring 300 μL of liquid that accurately determines boiling points that range from 35 to 150 °C. An alternative method, requiring as little as 30 μL of liquid, utilizes a modified aluminum block and a temperature control probe coupled to the digital hot plate that results in the accurate determination of boiling points that range from 35 to 205 °C. Both modifications to the Siwoloboff−Wiegand method provide consistent, reliable, and accurate boiling points for a variety of liquids with a wide range of boiling points. KEYWORDS: General Public, Laboratory Instruction, Safety/Hazards, Hands-On Learning/Manipulatives, Instrumental Methods, Laboratory Equipment/Apparatus, Liquids, Microscale Lab, Molecular Properties/Structure, Physical Properties
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Mayo et al. introduced the creation of a microcapillary having one closed end that can be inverted into a standard capillary used for melting points instead of a much larger test tube.3 This procedure has the advantage of requiring only 3−4 μL and can be heated and observed in a standard melting point apparatus. A drawback is that the procedure can involve some basic glassblowing skill in creating the micro capillary, or bell, of the correct diameter and length.4 Another drawback is that the introduction of the liquid and microcapillary into the bottom of the standard capillary may require a microliter syringe and a centrifuge. Furthermore, the bell of the microcapillary can tend to rise with the bubbles formed.5 We desired to make modest improvements on the Siwoloboff−Wiegand method and the Mayo modification for determining boiling point. Our criteria were for the new procedure to require less than 0.5 mL of liquid, need no modification of glassware, and employ a simple yet clean heating source with the temperature determined electronically. Sand baths or mineral oil/glycerin in a bath or Thiele tube did not meet our criteria for simplicity and ease of post-analysis cleanup. Moreover, digital hot plates with a temperature probe
nlike melting points, boiling points do not see widespread use in the laboratory setting as an analytical technique for characterization of organic molecules. Rarely, if ever, are boiling points reported for new compounds. The underuse of boiling point is in part due to the cumbersome process required to obtain an accurate boiling point and also due to the impracticality of the amount of material required. Ideally, a boiling point is determined during a distillation; however, distillation, and thereby the determination of the boiling point, requires a significant amount of sample and is not amenable to the microscale. Boiling points have also been determined using a self-contained reflux apparatus.1 This procedure still requires, at a minimum, 2−3 mL of liquid sample. The Siwoloboff−Wiegand procedure has become the procedure of choice for the undergraduate laboratory.2 This procedure relies upon the vapor pressure of a liquid that has been drawn in to a capillary having one closed end. When a standard melting point capillary is used, the capillary is merely placed inverted into a small test tube containing a small amount of liquid. The simplicity of this Siwoloboff−Wiegand method is a clear advantage in that common laboratory equipment is utilized. The heat source for this method is typically a sand bath or mineral oil or glycerin in either a bath or Thiele tube which provide a controlled and consistent source of heat, but can require a lengthy cleanup especially in the undergraduate laboratory. © XXXX American Chemical Society and Division of Chemical Education, Inc.
Received: January 5, 2018 Revised: June 15, 2018
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DOI: 10.1021/acs.jchemed.6b00839 J. Chem. Educ. XXXX, XXX, XXX−XXX
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the standard deviation and absolute error increased when less than 300 μL of liquid was used (Table 1). It was also found that the liquid could evaporate before a boiling point could be determined when less than 300 μL of liquid was examined.
to control the temperature seemed like the most sensible heat source. Herein we report two options for utilizing a hot plate and a digital thermometer to accurately determine the boiling point of less than 300 μL of liquid with cheap and easily accessible glassware.
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Table 1. Effect of Volume on Boiling Point of Ethyl Acetatea
HAZARDS AND SAFETY PRECAUTIONS Specific hazards are dependent on the specific compound being used and should be determined prior to determination of boiling point. The flammability and flash points will also vary and should also be determined prior to any measurement of boiling point since both methods have an inherent ignition risk. Both methods have a risk of superheating and bumping. Observation should be done at a distance to avoid exposure to heated liquids. Proper personal protection is essential, and an additional clear shield can be added between the observer and the apparatus. Additionally, the temperature should be raised slowly to avoid superheating. The hot plate and aluminum block can cause burns, so contact with both must be avoided. There is no need to remove the aluminum block. The vials and capillaries should be removed using forceps. All cords should be secured to avoid contact with hot surfaces.
Entry
Volume Used (mL)
Average Observed Boiling Point (°C)b
Std Dev
Absolute Error (°C)c
1 2 3 4 5 6
0.1 0.2 0.3 0.5 0.8 1
75.8 75.4 76.2 76.1 74.3 75.4
0.8 2.5 2.0 1.1 1.3 0.7
−1.2 −1.7 −0.8 −0.3 −1.3 −1.1
a Measured using digital thermometer in a test tube. bAverage of three trials. Not corrected for variations in atmospheric pressure. c Determined from manufacturer provided boiling points. Purity not verified.
Accurate and reliable boiling points for a variety of liquids were determined using this method (Table 2). The boiling
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Table 2. Observed Boiling Points of Various Liquidsa
RESULTS AND DISCUSSION Our initial desire was for the temperature probe to be accurate enough for the determination of the boiling point without the need for an additional thermometer. Unfortunately, using only a temperature control probe on a digital hot plate for the Siwoloboff−Wiegand method in a test tube did not produce consistent nor accurate boiling points. This may have been in part due to needing a sufficient portion of the temperature probe immersed in the liquid for accuracy. In addition, the temperature throughout the system may not be uniform leading to errant temperature depending on the location of the temperature probe in the liquid. Subsequently, it was established that there was no difference between boiling points obtained by a digital thermometer relative to a standard glass thermometer. The boiling point was determined using the Siwoloboff−Wiegand method in a small test tube containing 500 μL of ethyl acetate and heated on a digital hot plate with only the bottom of the test tube in contact with the hot plate (see method 1 in Supporting Information). The inverted capillary was secured to a digital thermometer6 with the capillary about a millimeter above the bottom of the thermometer and then placed in the liquid. The capillary was the standard capillary used for melting point and was used without modification. The boiling point for ethyl acetate determined using a digital thermometer had an average of 76.1 °C measured over three trials, with a standard deviation of 1.10 and a ±1.12% error from the manufacturer reported boiling point.7 Boiling points were not corrected for atmospheric pressure. Digital thermometers have the advantage of a smaller outside diameter which allows them to fit in test tubes or vials with smaller inside diameters while leaving room for a capillary tube. Therefore, digital thermometers would allow for the measurement of boiling points of smaller amounts of liquid in addition to avoiding potential exposure to mercury or broken glass. With this information in hand, we then set about to determine the minimum amount of liquid required for an accurate determination of boiling point using this simple modification of the Siwoloboff−Wiegand method. In general,
Compound Acetone Dichloromethane Ethanol Methanol Isobutyraldehyde Acetic anhydride Isopropyl acetate 2-Butanol 2-Butanone Pentyl acetate 4-Methyl-2pentanone Toluene
Average Observed Boiling Point (°C)b
Std Dev
Absolute Error (°C)c
56.4 39.3 77.9 65.7 61.9 135.2 86.4 97.5 72.9 141.0 113.8
0.4 0.4 0.4 0.4 0.2 0.7 0.6 0.7 0.3 2.3 1.1
0.9 0.3 −0.1 0.7 −1.1 −3.8 −1.6 −0.5 −7.1 −4.5 −3.7
109.0
0.4
−1.5
Measured using digital thermometer in a test tube with 300 μL of liquid. bAverage of three trials. Not corrected for variations in atmospheric pressure. cDetermined from manufacturer provided boiling points. Purity not verified. Liquids not purified. a
point measurements were shown to have low standard deviation and absolute error over a range of boiling points. Those liquids whose boiling points that showed a more significant error were hypothesized to be impure since none of the liquids were purified nor was their purity verified. Boiling points were also not corrected for atmospheric pressure. Acetic anhydride was freshly distilled, and the boiling point was found to be 138.3 °C which is an absolute error of −0.7 °C. 2Butanone was also freshly distilled, and the boiling point was found to be 78.4 °C which is an absolute error of −1.6 °C. The experimental details of this method (method 1) are described in the Supporting Information. It was also envisaged that smaller volumes could be used with a few modifications. Miniaturizing the vessel holding the liquid would prevent a thermometer from being in the vessel in addition to the capillary. Thus, a simple apparatus would need to be designed such that the temperature outside the vessel would be equal to that of the liquid. Whereas Thiele tubes are B
DOI: 10.1021/acs.jchemed.6b00839 J. Chem. Educ. XXXX, XXX, XXX−XXX
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Communication
significant difference between the temperature indicated on the hot plate using the temperature probe and the actual temperature measured by a digital thermometer (Table 4,
an excellent example of where this is generally true, they have the drawback of requiring mineral oil or glycerin as a heating liquid. We hypothesized that an aluminum block on top of the hot plate would provide for the uniform heating environment we desired and would have less chance of a mess. Furthermore, it was anticipated that the temperature control probe could accurately and consistently help to maintain the temperature of an aluminum block. Thus, the temperature setting of the hot plate would represent the temperature of the aluminum block and thereby the liquid inside the vessel and capillary, which would eliminate the need for a separate thermometer. Kinzer has reported the use of a modified aluminum block as a heat source for a variety of applications including the determination of boiling point.8 Kinzer’s boiling point was determined as part of a distillation in a microstill and not by the Siwoloboff− Wiegand method. Another distinction is that the aluminum block was heated by an internal heating element and does not utilize a thermocouple for direct temperature control. A quick initial test showed that the depth and diameter of the hole for the temperature control probe and vessel containing the liquid are critical for obtaining accurate boiling points. As mentioned previously, when the temperature probe was placed in the liquid, it was necessary to immerse enough of the probe in what was being heated in order to get an accurate temperature measurement. However, we also discovered that the temperature of the aluminum block was not consistent with the temperature setting when there was too much gap between the aluminum block and the probe. It was also discovered that accurate temperatures could not be obtained when the hole in the aluminum block for the temperature probe extended all the way through the block so as to allow the probe to be in direct contact with the top of the hot plate. To determine the optimum position and depth of the temperature probe in the aluminum block we compared the temperature displayed on the hot plate with the temperature control probe inserted in the aluminum block relative to a digital thermometer6 at various locations and depths for both probe and thermometer within an aluminum block (Figure 1 in Supporting Information). All holes were of a diameter that left very little gap between the aluminum and the control probe or thermometer. The data shown in Table 3 was determined at a specific depth which resulted in less precision and accuracy, yet the error and standard deviation were relatively constant regardless of the distance from the center of the block which suggests that the temperature was uniform throughout the block. What was found to be critical was the depth of the temperature control probe in the aluminum block. Shallow depths showed a
Table 4. Effect of Depth on Temperaturea Depth of Temperature Entry Control Probe (cm) 1 2 3 4 5 6
Distance from Center (cm)
Average Difference in Temperature (°C)b
Std Dev
1 2 3 4 5 6
0.00 0.45 1.00 1.35 1.62 2.25
2.3 5.6 4.7 4.2 2.8 3.9
0.4 3.0 3.9 1.7 0.9 1.0
Average Difference in Temperature (°C)b
Std Dev
0.64 1.11 1.43 0.64 1.11 1.43
12.3 5.4 3.6 7.2 0.7 0.2
3.1 0.6 2.1 2.1 0.4 0.5
a
Comparison between temperature control probe and digital thermometer at minimally different distances from center in aluminum block at 60, 80, and 100 °C. bAverage difference from three different temperatures.
entry 1). When the temperature probe is more significantly immersed in the aluminum block the difference became negligible (Table 4, entry 6). It is notable that the depth of the digital thermometer was not as critical (Table 4, entry 5), which suggests that the depth of the vessel containing the liquid for boiling point determination can be placed at a shallower depth allowing for an easier visualization of bubbles emitting from the capillary so long as the temperature probe is sufficiently deep. With this knowledge in hand, a new aluminum block was made with a hole for the temperature control probe at a depth of 1.43 cm and three holes to situate vials at a depth of 0.953 cm (Figure 2 in Supporting Information). Even though it was shown to be unnecessary, all holes were kept near the center of the block. The boiling points were determined for several liquids across a broad range of temperatures. Gratifyingly, accurate and consistent boiling points were obtained using only 30 μL of liquid in a GC autosampler vial low volume insert (Table 5). The procedure was essentially the same as before, utilizing the same standard, unmodified capillary having one closed end used for melting point, but with the temperature control probe inserted into the aluminum block instead of the vial. Boiling points that had absolute errors of greater than 2 °C using this method were also thought to be due to impure samples. Once again, upon freshly distilling heptane, the boiling point was found to be 97.4 °C which is an absolute error of −1.1 °C. Freshly distilled toluene was found to have a boiling point of 110.6 °C which is an absolute error of 0.6 °C. Freshly distilled acetic anhydride was found to have a boiling point of 137.7 °C which is an absolute error of −1.3 °C. Experimental details for this method (method 2) are available in the Supporting Information. It should be noted that the diameters of the holes are dependent on the specific brand of hot plate with temperature control probe and microvial chosen. It is suggested that this method can be successfully performed with any brand of hot plate and microvial. The procedure was then employed in a chemistry course with nonmajors. Owing to the robust nature of the procedure, novice students were also able to obtain accurate and consistent boiling points (Table 6).
Table 3. Effect of Position on Temperaturea Entry
0.64 1.11 1.11 1.43 1.43 1.43
Depth of Digital Thermometer (cm)
a
Comparison between temperature control probe and digital thermometer equidistant from center of aluminum block, both at a depth of 1.11 cm. Difference measured at 60, 80, and 100 °C. b Average difference from three different temperatures. C
DOI: 10.1021/acs.jchemed.6b00839 J. Chem. Educ. XXXX, XXX, XXX−XXX
Journal of Chemical Education
Communication
Table 5. Observed Boiling Points of Various Liquidsa Compound
Average Observed Boiling Point (°C)b
Std Dev
Absolute Error (°C)c
Dichloromethane Acetone Chloroform Methanol Ethanol Ethyl acetate 2-Propanol Heptane 2-Butanol Dioxane Toluene Acetic anhydride 2-Heptanone Diethyl malonate Benzyl alcohol Cyclohexane Cyclohexanone
41.0 55.6 58.1 65.3 80.2 79.3 82.4 93.6 96.1 102.9 112.0 140.3 150.0 191.0 206.3 83.6 155.7
0.3 1.0 2.1 0.6 1.9 1.7 0.5 2.9 1.0 4.5 3.5 5.9 5.6 3.0 2.1 1.4 0.6
2.0 0.1 −2.4 0.3 2.2 2.3 0.4 −4.9 −1.9 1.9 −1.5 1.3 −1.5 −7.5 1.0 2.9 0.7
temperature that will be safely assumed to be very similar to the temperature of the liquid held in a small glass vessel in a separate location in the aluminum block. Because the temperature of the aluminum block is relatively uniform the aluminum block can have several positions to hold the vessels containing liquid, thereby allowing for the determination of the boiling points for several different liquids more rapidly. Despite the somewhat higher cost of a digital hot plate with a heat probe, it is not prohibitive. The aluminum block and autosampler vial inserts (or some equivalent container) are rather inexpensive while all other components are generally commonly stocked items. Fabricating the aluminum block is easier using a band saw and a drill press but can be accomplished with a hacksaw and a drill and therefore does not require any particular skills. Decreasing the amount of material needed to determine a boiling point not only saves resources but limits exposure to potentially hazardous materials. Safety is further increased by eliminating the need for a mercury thermometer. For these reasons, we suggest that these options are preferable for adoption in the undergraduate laboratory. It should be note that Mayo’s method offers the same advantages while using even less liquid. The choice of method for determining the boiling point of a liquid will undoubtedly be a matter of personal preference. The improvement in safety and versatility is worth the investment in digital hot plates with temperature control probes.
a
Measured using temperature probe controlled digital hot plate and a modified aluminum block. A 30 μL portion of liquid held in an autosampler vial low volume insert. bAverage of three trials. Not corrected for variations in atmospheric pressure. cDetermined from manufacturer provided boiling points. Purity not verified. Liquids not purified.
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Table 6. Student Obtained Boiling Points of Various Liquidsa Compound
Average Observed Boiling Point (°C)b
Std Dev
Absolute Error (°C)c
Chloroform Methanol Acetone Hexanes Ethyl acetate 2-Propanol
62.3 65.4 58.7 68.3 75.7 80.8
4.01 3.2 5.0 2.6 1.8 1.7
1.8 0.4 2.2 −0.7 −1.3 −1.2
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available on the ACS Publications website at DOI: 10.1021/acs.jchemed.6b00839. Experimental details of method 1 (Tables 1 and 2) and method 2 (Tables 5 and 6) and the design of the aluminum block used to determine optimum probe depth (Tables 3 and 4) and the aluminum block used in method 2 (PDF, DOCX)
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a
Measured using temperature probe controlled digital hot plate and a modified aluminum block. A 30 μL portion of liquid held in an autosampler vial low volume insert. bAverage of data from 18 different students each performing one trial. Not corrected for variations in atmospheric pressure. cDetermined from manufacturer provided boiling points. Purity not verified. Liquids not purified.
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
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Timothy L. Troyer: 0000-0003-0250-2699
CONCLUSIONS Two modifications to the Siwoloboff−Wiegand method are now available as options for determining accurate boiling points on a microscale. Both options utilize a digital hot plate equipped with a temperature control probe. The data supports the conclusion that a digital hot plate can be used as a heat source in the Siwoloboff−Wiegand procedure thereby avoiding the potential difficulties associated with using mineral oil or glycerin. The temperature control probe can accurately present the temperature of the liquid for which a boiling point is being determined. However, the probe does need to be sufficiently immersed for the temperature to be accurate. The immersion limits the miniaturization of the method if a test tube is used to hold the liquid for which the boiling point is being determined. A test tube of sufficient diameter to allow the temperature probe and inverted capillary requires more liquid. An aluminum block can be used in which the temperature probe is held at the right depth within the block and will maintain a
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We are grateful to Ruth Nalliah and Ed Wovchko for their help with the preparation of this paper. We are also indebted to all the students in CH141 at Huntington University for providing their data.
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REFERENCES
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of Boiling Points of Very Small Quantities of Liquids. Trans. Faraday Soc. 1908, 4, 95. (d) Biltz, H. Dijodacetylen und Tetrajodäthylen. Ber. Dtsch. Chem. Ges. 1897, 30, 1200. (e) Smith, A.; Menzies, A. W. C. Studies in Vapor Pressure: I. A Method for Determining Under Constant Conditions the Boiling Points of Even Minute Quantities of Liquids and of Non-Fusing Solids. J. Am. Chem. Soc. 1910, 32, 897. (f) Wiegand, C. Bestimmung des Schmelzpunktes und des Siedepunktes im Schmelzpunktsröhrchen. Angew. Chem. 1955, 67, 77. (3) (a) Mayo, D. W.; Pike, R. M.; Butcher, S. S.; Meredith, M. L. Microscale Organic Laboratory, III: A Simple Procedure for Carrying Out Ultra-Micro Boiling Point Determinations. J. Chem. Educ. 1985, 62, 1114. (b) Chaco, M. C. Microboiling Point Determination at Atmospheric Pressure. J. Chem. Educ. 1967, 44, 474. (c) Bulbenko, G. F. A. Modified Micro-Boiling-Point Technique. J. Chem. Educ. 1968, 45, 43. (4) Rausch, G.; Tonnis, J. Improvements in the Ultra-Micro BoilingPoint Technique. J. Chem. Educ. 1992, 69, A264. (5) Brouwer, H. Ultramicro-Boiling-Point Determination − A Modification. J. Chem. Educ. 2000, 77, 1480. (6) Go!Temp Temperature Probe; Vernier Software & Technology: Beaverton, OR. (7) Ethyl acetate boiling point = 76.5−77.5 °C, Sigma Aldrich Inc., Milwaukee, WI. (8) Kinzer, D. A Microscale Laboratory Heating System. J. Chem. Educ. 1997, 74, 1333−1334.
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DOI: 10.1021/acs.jchemed.6b00839 J. Chem. Educ. XXXX, XXX, XXX−XXX
