In the Laboratory edited by
Safety Tips
Timothy D. Champion Johnson C. Smith University Charlotte, NC 28216
Safe Preparation of HCl and DCl for IR Spectroscopy William R. Furlong and W. Tandy Grubbs* Department of Chemistry, Stetson University, DeLand, FL 32720; *
[email protected] Several schemes have been outlined in this Journal (1– 3) and elsewhere (4) for obtaining HCl and DCl gas samples for vibrational–rotational spectroscopy. Cost considerations lead many to choose the direct synthesis of these gases over purchasing the gases in cylinders; “lecture bottles” of HCl and DCl along with the requisite corrosion-free regulator can cost several hundred dollars, and this cost does not include the waste disposal fee for these cylinders.
A
oil bubbler
24/40 joint
3 L Florence flask
moisture trap (submerged in ice/salt bath)
B to pump
vacuum manifold to pressure gauge 24/40 joint 12/30 joint
infrared gas cell (NaCl windows)
3 L Florence flask from above
Figure 1. An alternate method for capturing the synthesized gas at atmospheric pressure and transferring it to the infrared sample cell. (A) Apparatus for synthesizing and capturing a mixture of HCl/DCl (carried out in a fume hood). (B) An illustration of the connection of the Florence flask to a vacuum line for subsequent expansion into the infrared gas cell.
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Journal of Chemical Education
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The hydrolysis of benzoyl chloride is one of the more popular methods for synthesizing these gases. The procedure, loosely based upon the method described in ref 4, involves heating benzoyl chloride with a 1:5 mixture of H2O:D2O inside of a closed vessel and freezing the resultant HCl兾DCl gas in a liquid nitrogen trap (a 1:5 ratio of H2O:D2O is selected to compensate for the kinetic isotope effect on the reaction rate and thereby obtain approximately equal quantities of HCl and DCl). The trap containing the frozen HCl兾DCl sample is then connected to a vacuum manifold line, where it is slowly heated and the gas is allowed to expand into the infrared gas cell. A potentially hazardous situation can arise as the frozen HCl兾DCl sample is heated. Namely, the vapor pressure of the sample will rapidly rise to a dangerous level if the sample is heated too quickly. For example, the vapor pressure of the frozen兾liquid sample in equilibrium with its vapor is roughly 0.1 bar at 77 K, and rises to 1 bar at 188 K, 4 bar at 220 K, and 12 bar at 250 K. Consequently, the vacuum line will develop a positive pressure of hazardous gas at temperatures above 188 K (provided unvaporized sample remains in the trap), which can in turn “pop” a glass joint loose and rapidly vent the sample into the face of the user. (This, in fact, happened the first time we attempted this procedure in our lab.) To circumvent this hazard, we have developed an alternate method (Figure 1) for capturing the synthesized gas at atmospheric pressure and transferring it to the infrared sample cell. Instead of freezing the sample in a liquid nitrogen trap, the gas is allowed to flow through a large Florence flask (Figure 1A). As the gas exits the Florence flask, it is passed through an oil bubbler. The generation of HCl兾DCl is allowed to proceed for approximately 15 minutes to adequately purge atmospheric gases from the Florence flask. The Florence flask is then sealed off and transferred to the vacuum manifold (Figure 1B), whereby the HCl兾DCl is allowed to expand into an evacuated infrared gas cell. Since the sample is at atmospheric pressure, there is no chance that a positive pressure will develop in the vacuum line. This procedure yields about 0.9 bar of HCl兾DCl gas for infrared analysis, which is more than adequate for observing the fundamental absorption peaks and even allows a weak observation of the overtone spectrum. Literature Cited 1. 2. 3. 4.
Lawrence, B. A.; Zanella, A. W. J. Chem. Educ. 1996, 73, 367. Ganapathisubramanian, N. J. Chem. Educ. 1993, 70, 1035. Buettner, G. R. J. Chem. Educ. 1985, 62, 524. Shoemaker, D. P.; Garland. C. W.; Nibler, J. W. Experiments in Physical Chemistry, 6th ed.; McGraw-Hill: New York, 1996; pp 401–404.
Vol. 82 No. 1 January 2005
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