In the Laboratory edited by
The Microscale Laboratory
R. David Crouch Dickinson College Carlisle, PA 17013-2896
A Safe and Efficient Technique for the Production of HCl/DCl Gas Steven G. Mayer,* Raymond R. Bard, and Kevin Cantrell Department of Chemistry, University of Portland, Portland, OR 97203; *
[email protected] We present a safe and efficient technique to generate HCl/ DCl gas for use in any experiment that requires a relatively low partial pressure of said gas. The most apparent use of this preparation would be the classic physical chemistry experiment that introduces students to ro-vibrational spectroscopy. Traditionally, this experiment has employed a common technique for producing HCl/DCl gas that is tedious and presents a significant risk to the safety of the students (1a). As this experiment has been a staple of the undergraduate physical chemistry laboratory
N2 gas
H2O
IR cell
thionyl chloride
indicator solution
Figure 1. The microscale apparatus used to produce and transfer HCl/DCl gas to an IR gas cell.
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curriculum, several articles have appeared in this Journal that present modified versions of the standard preparation technique (1b, 1c, 2–4). The most recent article presents a technique that mitigates the inherent risk to the students but is still time and equipment intensive. In contrast, the technique that we present employs the use of standard microscale glassware commonly used in the second-year organic chemistry laboratory and the reaction produces the HCl/DCl gas and delivers it to the IR gas cell in one step at atmospheric pressure. The reaction involves thionyl chloride and a mixture of water and deuterium oxide to produce HCl/DCl gas and SO2 gas as follows.
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Wavenumber / cmź1 Figure 2. The infrared absorption spectrum of the gas sample. Note the peaks centered at 2900 and 2100 cm−1 are due to HCl and DCl, respectively.
SOCl2(l) H2O(l)
2HCl(g) SO2(g)
(1)
It is worthwhile to note that the reaction produces ~ 2:1 mixture of H35Cl to H37Cl and likewise for DCl, corresponding to the relative abundances of the chlorine isotopes. The simple microscale apparatus employed in this technique is shown in Figure 1. Thionyl chloride is placed in the 5 mL microvial that is connected to a Claisen adapter with the side arm attached to a connecting adapter, which, in turn, is attached to the IR gas cell via Tygon tubing and to a N2 regulator. The top of the Claisen adapter is sealed with a septum. Water is injected through the septum with a microliter syringe so that the HCl/DCl gas is directly transferred to the IR cell. The outlet valve of the gas cell is connected to a glass sparge by a piece of Tygon tubing that is, in turn, placed in a 500 mL beaker of water with thymol blue to monitor the progress of the reaction. Approximately three drops of thymol blue is placed into a beaker of water. The microscale apparatus is constructed in a fume hood. One milliliter of thionyl chloride is admitted to the reaction vessel and the entire apparatus is purged with N2. The N2 line is then closed with a pinch clamp to prevent HCl/DCl gas from migrating back to the regulator. Approximately 2 mL of deuterium oxide and 1 mL of deionized water are poured into a small vial. One milliliter of the D2O/H2O mixture is drawn into a microliter syringe. The syringe is then inserted through the septum and the water is added to the reaction vessel dropwise until the reaction begins to produce gas as is immediately evident from the bubbles escaping from the outlet tube in the beaker of water. The reaction is allowed to proceed until the thymol blue changes color from yellow to red. Once it has been neutralized, it can be disposed of appropriately. The infrared absorption spectrum of the gas sample shown in Figure 2 was collected using a resolution of 1 cm‒1. The peak centered at 2900 cm‒1 is due to HCl and the peak centered at 2100 cm‒1 is due to DCl. The remaining four peaks, centered at 2500, 2300, 1356, and 1150 cm‒1 are due to SO2 (5). Since the
© Division of Chemical Education • www.JCE.DivCHED.org • Vol. 85 No. 6 June 2008 • Journal of Chemical Education
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In the Laboratory
Hazards
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Thionyl chloride is a lachrymator. HCl gas is corrosive and care must be taken to contain it within the optical cell and bubble the excess through water.
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Literature Cited
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Figure 3. The P-branch of the infrared absorption spectrum of HCl gas. Note that there are two sets of peaks. The higher intensity peaks are due to H35Cl and the red-shifted peaks of lower intensity are due to H37Cl.
1. (a) Shoemaker, D. P.; Garland, C. W.; Nibler, J. W. Experiments in Physical Chemistry, 7th ed.; McGraw-Hill: New York, 2003; pp 408–409. (b) Ganapathisubramanian, N. J. Chem. Educ. 1993, 70, 1035. (c) Pattacini, S. C. J. Chem. Educ. 1996, 70, 822. 2. Buettner, G. R. J. Chem. Educ. 1985, 62, 524. 3. Lawrence, B. A.; Zenella, A. W. J. Chem. Educ. 1996, 73, 367. 4. Furlong, William R.; Grubbs, W. Tandy. J. Chem. Educ. 2005, 82, 124. 5. Herzberg, G. Molecular Spectra and Molecular Structure; Krieger Publishing Company: Malabar, FL, 1991; Vol. 2, p 285
Supporting JCE Online Material
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SO2 peaks do not interfere with either of the peaks due to HCl and DCl, there is no need to separate the gases. Figure 3 is an expanded view of the P-branch of HCl clearly showing the most intense peaks due to H35Cl with a separate set of lower intensity peaks due to H37Cl. The same structure can be seen for the Rbranch of HCl and for DCl.
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Journal of Chemical Education • Vol. 85 No. 6 June 2008 • www.JCE.DivCHED.org • © Division of Chemical Education
