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7.4.3: Experimental

  • Page ID
    424827
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    General Steps to prepare the materials and calorimeter assembly

    1. Prepare room-temperature water: Fill a 2-liter volumetric flask to the TD ("to deliver") mark with deionized water at least 20 minutes before each run, so that the contents are equilibrated at room temperature prior to use. There are two flasks per station. Fill both flasks at the start of the laboratory period, and after the start of each run, so that the water for subsequent runs comes to thermal equilibrium while the prior run is being performed.
      *Volumetric flasks are "Class A" glassware, and the errors associated with their measurements can be found either on the flask itself or in a table of tolerances (like this one, click).
    2. Prepare sample pellet (See Figure \(\PageIndex{1}\)A-C).
      • Weigh approximately 0.85 g (between \(0.8000-0.9000 \) g) of solid sample.
      • Press the sample into a pellet using the pellet press.
      • Carefully re-weigh the pellet and be sure it is between \(0.8000-0.9000 \) g .
    3. Bomb head: Place the bomb head on its stand and put the ignition cup in place (See Figure \(\PageIndex{1}\)D).
    4. Place the weighed sample pellet in the ignition cup (See Figure \(\PageIndex{1}\)D/E).
    5. Place fuse wire: Cut about 9-10 cm of nickel chromium fuse wire (\(\Delta_{T_1} = -6.876 \; \mathrm{ kJ/g}\)\)) and weigh it carefully. Attach the ends of the wire to the two electrodes and loop it in such a way that it rests upon the surface of the pellet. Take care that it does not come into contact with the combustion cup. Assemble the bomb carefully; the pellet and fuse wire are in fragile contact (See Figure \(\PageIndex{1}\)E).
      Note

      Handle the fuse wire with care. If you allow the wire to bend too sharply or to develop kinks, it will burn off at the weak point upon ignition, and combustion of the pellet will not occur. Consequently no temperature rise will be observed and you will have to disassemble the bomb and prepare a new pellet and ignition wire.

    6. Your TA will supervise the assembly of the bomb and the flushing and filling of the bomb with pure oxygen (25 atm) (See Figure \(\PageIndex{1}\)F-H).
      Caution
      • YOU MUST WEAR A FACE SHIELD AT ALL TIMES WHEN THE BOMB IS PRESSURIZED.
      • DO NOT EXCEED 30 atm! If you do, open the valve to release the oxygen slowly over a one minute period, dismantle the bomb and begin again.
      Prior to removing the hose from the bomb, bleed the oxygen from the connecting line so that it may be detached from the bomb safely.
    7. The bomb is placed in the inner bucket (See Figure \(\PageIndex{1}\)I).
    8. Add 2 liters of distilled water, at room temperature. (Use a 2 L volumetric flask to measure the water and see step (1) above.)
    9. Assemble the apparatus and make sure that the thermistor is immersed in the water to the same depth as the stirrer propeller (See Figure \(\PageIndex{1}\)I-K).
    10. Power on the Parr Calorimetric Thermometer Unit using the on/off switch at the rear left of the instrument.
      • After a short "Self Test" routine the following screen will appear.
        CAL1
      • Press Operating Controls. Look over the settings; Method of Operation should be Combustion; Reporting Units should be cal/g; and, Spiking Correction should be off.
    clipboard_e1830717878b77ae3e7797e7483836c63.png
    Figure \(\PageIndex{1}\): Visual overview of some of the major steps in a bomb calorimetry run. (CC-BY-SA; Kathryn Haas)

    Standardization Procedure

    The calorimeter is calibrated using benzoic acid (\(\Delta_{T_1} = -26.436 \; \mathrm{ kJ/g}\)). To standardize the calorimeter (measure its heat capacity), begin by following steps 1-10 above using benzoic acid as the sample. Continue with the procedure outlined below and consult your TA as necessary.

    1. On the Parr Calorimetric Thermometer Unit screen, press Escape or Main Menu to go back to the Main Menu.
    2. Press Diagnostics.
      • Check the settings; the Data Logger should be off. If it is on, press the Data Logger button to turn it off.
      • Press the Delete Data Log File button to clear the data log.
      • Check to be sure that the Data Log Format is set to Data Format (csv).
      • Then, turn the Data Logger back on.
        CAL9
    3. Press the Start button on the right side of the screen.
      • The stirrer to the bomb calorimeter will turn on automatically.
      • You will be prompted to enter a Sample ID. Use the format: initialsMMDDYY-N, where: initials are your initials; MMDDYY is the date in month, day and year format; and, N is the number of the run (e.g., 1 for the first benzoic acid run).
        Note

        Note: When naming files,

        • First calibration run with Benzoic Acid: N=1
        • Second calibration run with Benzoic Acid: N=2
        • First run with carbohydrate: N=3
        • Second run with carbohydrate: N=4
      • Press Enter on the calorimeter screen.
      • You will be prompted for which bomb number to use: use 1. Press Enter.
      • Enter the sample pellet mass (in grams). Press Enter when you are done.
        BOMB2A4
        Note

        At this point your data collection has started and you are logging temperature and time data.

        There will be a seven-minute pre-run measurement of the starting temperature, followed by ignition, and then about seven minutes post-run measurement of the final temperature. Moments before ignition, the instrument will beep loudly. Stand back during ignition (when you hear the beeping) in case of failure of the bomb head.

    4. In the meantime (during pre-run measurement)....press the Main Menu button on the right side of the screen to return to the Main Menu.
    5. Then press Calorimeter Operation. You will be taken to a screen that shows the progress of the current run. You will see your sample run information along with the temperature of the water (listed as the Bucket Temperature). At the bottom of the screen you will see a timer that will monitor the progress of the run.
    6. Press the Temperature Graph button on the calorimeter.
      • A real-time image of the temperature inside the calorimeter (the Bucket Temperature) will appear.
      • Press Setup and you will see the following screen.
      • Set the Bucket Min Value about 0.5 degrees below the pre-period baseline temperature and Bucket Max Value about 2.5 degrees above the pre-period baseline temperature.
      • Press Escape when you are done.
        tempgraph1
    7. Go to the computer associated with the calorimeter.
      • Click the Parr Calorimeter shortcut found on the main Windows desktop screen. This will take you to a Web page associated with the calorimeter.
      • Click on the LCD Image tab; a screen shot of what is currently on the calorimeter display will appear. This is not a “living” image, but one that is captured at the moment you clicked the LCD Image button. To update the image press the F5 key on the computer keyboard.BUTTONS
    8. About seven minutes after starting the run you will hear a beep and the calorimeter will ignite the sample.
      • Stand back when you hear this beeping.
      • After ignition, you can watch the temperature rise inside the system by monitoring the Temperature Graph either on the calorimeter display or by updating the LCD Image on the computer screen.
      • Watch the display closely for the first minute or two after ignition to be sure that the sample is burning. If there is an Ignition Failure (something that happens occasionally) you will get an error message on the calorimeter screen and the run will stop. If that happens, ask your TA to supervise as you carefully remove the bomb from the calorimeter; release the oxygen pressure by opening the outlet value, open the bomb; remove the old sample and wire; and, prepare another run.
        twtris4
    9. Near the end of the run you will hear another beep from the calorimeter telling you the run has completed.
    10. Upon completion, press F5 on the computer screen and capture the Temperature Graph on the computer screen at that moment.
      • Move the mouse toward the upper left of the computer screen and a small menu bar SAVE2 will appear. Click on the save button and save a copy of this image to c:\data\ initialsMMDDYYN.jpg (where N is the run number).
      • When the report for the run appears on the Parr Calorimeter screen press F5 on the computer keyboard to bring an image of the report onto the computer screen. Save this image to c:\data\ initialsMMDDYYNrpt.jpg (where N is the run number).
    11. Press Escape on the Parr Calorimeter screen and the report will disappear, leaving the thermogram.
    12. Press Escape again and you will return to the Calorimeter Operation Screen.
    13. Press Main Menu on the button bar and then press the Diagnostics button to navigate back to the Diagnostics Screen.
    14. Press the Data Logger button and when you get to the Data Logger screen press the new Data Logger Button to turn the Data Logger off.
      TWBZ4RP
    15. Return to the computer screen. Click on the Home button at the bottom of the browser window. Then Click on the Data Log tab.
      BUTTONS
      You will get a new window containing the logged data from the run. Each row contains the date, the time, the bucket temperature, the jacket temperature and two columns that are mostly zeros.
      • Use the mouse to highlight the entire data log (it will be long and you will need to scroll down the window as you highlight).
        CAL5
      • Select Copy from the Edit menu.
      • Open the Windows Notepad (Start →Accessories → Notepad) and Paste the data log into the Notepad window. Delete any text that may appear below the last row of data.
      • Select Save As from the File menu and save the Notepad file in c:\data\initialsMMDDYYN.txt.
        Imprtant: always save the data log!

        In this course, you must create and save the Notepad (.txt) files for every run in case you need to revisit the data later. Alwasy save the data log for each run using the instructions above.

    16. Ask your TA to supervise as you carefully dismantle the bomb.
      • Open the oxygen outlet value very slowly to relieve the oxygen pressure.
      • Open the bomb and inspect for incomplete combustion indicated by traces of soot.
      • The inside should be clean but covered with a dew-like layer of water.
      • If there is evidence of incomplete combustion (sample remaining after combustion), the run must be re-done.
      • If you find unburned wire it should be removed and weighed; Subtract the mass of unburt wire from the original mass of fuse wire to determine the actual mass of wire burned.
    17. Carefully clean and dry the apparatus.
    18. Repeat with another benzoic acid sample.

    Determination of the Average Heat of Combustion of a Carbohydrate

    1. Repeat the above experiment twice, substituting the sugar for the benzoic acid. You should clean any benzoic acid off of the pellet press with a bottle brush before making a pellet with sugar.
    2. Carefully clean and dry the apparatus after each use.

    Shut down and cleanup

    After you have collected two replicate datasets for the calibration and two replicate datasets for carbohydrate, please clean the instrument and workstations and prepare for the next person to use the instrument:

    • Clean and dry all components of the bomb and calorimeter. The bomb head should be on its stand with valve open, the inside of the bomb should be wiped with a paper towel to remove water.
    • Power off the instrument using the switch in the back.
    • Turn the oxygen tank completely off (ask your TA to supervise).
    • Wipe the bench area and sink where you were working.
    • Use the brushes (looks like a small paint brush) to gently remove any residue or powder from the inside and outside of the balance, then wipe the bench around the balance.

    Before you leave, go through all the steps of data analysis with logger pro and discuss the correction for non-adiabaticity with your TA. Use the rest of lab time to make progress on the data and error analysis so that your TA is available to answer any questions that come up.


    This page titled 7.4.3: Experimental is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by Kathryn Haas.

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