4.6.3: Experimental and Instructions

Materials

• Equipment:
  • Nicolette iS50 FT-IR Spectrometer
  • Long Path Length Gas Cell with windows that are transparent to IR radiation (KBr windows)
• Reagents:
  • Hydrogen Chloride 99+%: Aldrich Chemical Company; Cat # 29,542-6; Cas # [7647-01-0]; FW = 36.46; safety information can be obtained from: Sigma Aldrich (Click).

HAZARD: This experiment uses concentrated HCl (strong acid)! 

The concentrated acid used in this experiment is a hazard. Go to the linked Safety Data Sheet and read the safety information related to these compounds:

• Hydrogen chloride is colorless, pungent, and corrosive .
• Contact will cause burns to the skin, severe burns to the eyes, and burns to the respiratory system if inhaled. 
• Use gloves, protective eyewear, and avoid inhalation throughout this experiment.

USE CARE: Gas Cells are FRAGILE and the KBr Windows are FRAGILE andare destroyed by water!

The cell is constructed of a glass body with KBr windows at each end. Ensure that the salt windows are kept free of moisture at all times. Please do not touch these windows.When not in use the IR gas cell must be stored in the desiccator.

The Group Notebook (ELN) Template for this experiment is here: FTIR Group Notebook Template.

This template is provided as a guide so that you know what data and information should be kept in a notebook. You may keep your personal notebook in any way you'd like, but you should translate your notes into this template for the group submission.

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Part IA: Computational prediction of vibrational frequency

1. First, Access Spartan from a Lab Computer (Click to expand)

1. In your group, go to a lab computer and open the FastX3 program.  Then double click on bohr.  Enter your Duke Net ID in the box, then your password.
   
2. In the new window, click the "+" sign at the top right corner. 
3. Then, in the next new window, double click xterm.
4. In the new xterm window, type "spartan" and press ENTER.
   
   The Spartan program should open in a few moments.  If it does not, or if it prompts you for a license, please inform the instructors.

 

3. Calculate the IR spectrum and bond length of HCl (Click to Expand)

With Spartan open....  

1. Build HCl: Choose Build from the toolbar, and then build HCl. To do this, choose Cl from the Model Kit at the right.  Then double click anywhere in the build screen (the turquoise part of the window).  Spartan will fill any unfilled valences with hydrogen, so there is no need to add H. If you'd like to see what your molecule looks like, click the icon that looks like sunglasses. 
   
2. Run Calculation: Next, chose the Calculations from the toolbar. In the calculations window, choose Equilibrium Geometry and any calculations method that allows you to predict an IR spectrum.  Be sure to record which calculation method you use.  Then press Submit.
   
3. View IR Spectrum and other Output: Wait until your calculation is finished.  You can view the Spectrum and other Output using the toolbar.  Hover your mouse over any icon in the toolbar to see what it is. 
4. View Bond Length: To view the calculated bond length, go to the top menu and click Geometry, then choose Measure Distance. Click the on the two atoms of the bond, and read the bond distance in the right bottom corner.
   
5. Record Data: Record the predicted IR spectrum in your group notebook.  Also record the predicted frequency of the vibration, the predicted bond length, and any other information you feel is worth recording.

 

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Part IB: Preparation of sample and collection of FTIR spectrum

SEE HAZARD and CAUTION STATEMENTS above.

Use one of the following methods of preparing a gas cell to collect (1) a spectrum of the background (2) a spectrum of HCl gas.   

Option 1: Preparation of a sample at low pressure using a vacuum line (Click to expand)

Vacuum-Line Start-Up

Most of the following will be completed by the TA before the lab period.

1. Check that all external valves on the working and main manifold are closed to the outside atmosphere.
2. Attach an empty N₂ trap to the main manifold. Hold this trap in place until a vacuum has formed.
3. Turn on the floor vacuum pump to start generating a vacuum.
4. Turn on all valves in the manifold that are connected to gages (marked with purple tape).
5. Turn on the diffusion pump.
   • Ensure that the floor pump is on BEFORE the diffusion pump. Otherwise, the diffusion pump will generate heat and the oil within the pump will oxidize, causing long-term damage to the diffusion pump.
6. Turn on Digivac electronic monitor. Use this to actively monitor the pressure at the end of the main manifold.
7. Check the analog pressure gauge on the diffusion pump to ensure the pressure is stable.
   • Throughout the experiment, the pressure should remain below 30 mtorr if the system is closed and stable.
8. Fill a 4L dewar with liquid N₂ from the shared Chemistry department supply.
9. Place a second dewar underneath the N₂ trap and fill with liquid N₂ from the 4L dewar such that the N₂ trap is submerged in liquid N₂
10. Check the level of N₂ as the experiment progresses. Refill if necessary to ensure the trap remains cold.
11. Check to ensure the pressure at the end of the main manifold is below 30 mtorr before proceeding.

Transfer HCl gas to the cell

These instructions involve using a high vacuum line to prepare a cell containing HCl.

Figure \(\PageIndex{1}\): Vacuum line set-up: working manifold and main manifold

1. Remove gas cell from the desiccator and attach to the middlemost section of the working manifold shown below in Figure \(\PageIndex{1}\).
2. Evacuate the back of the gas cell valvue and introduce a vacuum to the interior of the cell.
3. Remove cell from manifold, bring it to the FTIR instrument, and take a background spectrum (see "Obtaining an FTIR Spectrum using the Nicolet iS50" below).
4. Replace the gas cell onto the working manifold and open it to vacuum.
5. Connect a flask containing a few drops of concentrated HCl to the working manifold. Introduce a vacuum to the flask for only a few seconds to remove the air in the head space.
6. Isolate the Working Manifold.
7. Use the mercury pressure gauge on the working manifold to monitor the pressure due to evaporation of HCl into the working manifold.
8. When there is sufficient pressure of HCl in the manifold (and the cell), isolate the cell and the flask by closing all vales on the working manifold.
9. Record the FTIR spectrum using "Obtaining an FTIR Spectrum using the Nicolet iS50" below.
10. Once all data has been collected, return the gas cell to the working manifold and safely evacuate all remaining HCl from the cell and the working manifold.
11. Close and remove the evacuated gas cell and return it to the desiccator.

Vacuum Line Shutdown

1. Turn off the diffusion pump. Wait 5 mins for the pump to cool.
   • The pump should be cool before any oxygen is introduced to the system to prevent oil in the pump from oxidizing.
2. Close all valves with purple tape.
3. Turn off Digivac monitor.
4. Turn off the floor pump.
5. Pick an external valve and open it to release the vacuum and open both the working manifold and the main manifold to the air.
   • Do this quickly after step 4. The system should not be left under vacuum without an active vacuum pump.
6. Remove N₂ trap and transfer it to the fume hood to safely evaporate excess HCl gas.
   1. Do this quickly after step 5. Leaving the N₂ trap cold after the vacuum has been released may cause liquid O₂ to condense, which is highly volatile.

Option 2: Preparation of sample at atmospheric pressure (Click to expand)

Prepare the cell with background gas  

1. Put on clean, dry gloves before handling any components of the gas cell.
   Avoid touching the Potassium Bromide (KBr) windows with bare hands, and never expose them to water, as they are made of salt and will degrade upon contact with moisture.
2. Gather all required materials, including:
   • One Pyrex gas cell body
   • Two Potassium Bromide (KBr) windows
   • Two blue end caps
   • Four O-rings
   • Two septa
   • pH paper
   • Concentrated hydrochloric acid
   • Glass pipettes
   • Wipes
3. Insert a small piece of pH paper into one of the inlets of the gas cell.
   Fold the pH paper as needed so that it fits easily inside the inlet.
4. Cap the inlet containing the pH paper with a septum.
   The septum does not need to fold down or form a tight seal; it simply needs to securely plug the opening.
5. Prepare the end caps by placing one O-ring into the groove inside each end cap.
   Use your thumb to press the O-ring evenly into the groove.
6. Place one Potassium Bromide window into each end cap.
   If a window has one side that appears etched or foggy, orient that side so it faces inward toward the cell body.
   Keep the windows dry at all times.
7. Place a second O-ring on top of each Potassium Bromide window.
8. Gently screw the body onto the end caps without distrubing the windows and o-rings.
   Do not overtighten. The Potassium Bromide windows are fragile and can crack or crush if excessive force is applied.
   Tighten only until slight resistance is felt.
9. Collect a background spectrum on the sealed gas cell assembly using the FTIR instrument.

Introduce HCl into the cell

10. Carefully introduce hydrochloric acid into the cell.
    Using a glass pipette, add one drop of concentrated hydrochloric acid through the inlet.
    Ensure that the liquid does not contact the Potassium Bromide windows.
    If any acid is spilled, clean it immediately and change your gloves before proceeding.
11. Wipe the outside of the inlet to remove any residual acid, then cap the inlet with a septum.
12. Gently move and rotate the cell so that the liquid drop moves away from the Potassium Bromide windows and toward the center of the cell.
    Rotate the cell slowly to spread the liquid around the inner circumference, increasing surface area and promoting evaporation.
13. Monitor the pH paper.
    The cell is ready when the pH paper indicates the presence of acid (e.g., a color change).
14. Collect the FTIR spectrum of gaseous hydrochloric acid.

Disassemble the cell and clean up

15. Disassemble the cell carefully after data collection.
    Remove at least one septum before unscrewing the end caps.
    Unscrew the end caps slowly, ensuring that the acid does not contact the Potassium Bromide windows.
16. Set the end caps aside carefully and keep the windows dry.
17. Remove the pH paper using a clean glass pipette and tap it out of the cell body.
18. Change your gloves before handling the Potassium Bromide windows! Transfer the windows to their designated storage location inside a desiccator.
19. Flush all remaining cell components thoroughly with water and dry them with a towel.
20. Wipe bench spaces that may have acid residue with a wet towel, followed by a dry towel.
21. Dispose of all acid-contaminated waste in the appropriate waste container.

  

Data Analysis for HCl FTIR spectrum for Part I 

After collecting your first spectrum, please email your results to yourself, your group, and your instructors. Then assemble your group and begin group discussion and analysis of the data. The files linked at the top of this page will guide you to analyze the spectrum, derive mathematical models to fit the data, and calculate molecular parameters from the fits. (links copied here for convenience: Shared Files for Duke Students). Your instructors will be on hand to help guide you if you get stuck.

Part II: How does instrument resolution effect the certainty of derived parameters?

In Part II, you will collect the same spectrum of HCl at different instrument resolutions to determine how instrument resolution affects the error in your results. If you are finished with part I early, you may be able to collect this data in the first lab period. Otherwise, data collection for Part II will happen in the second week of this module.