Online Photographic Pre-lab Demonstration for Bomb Calorimetry

Note: the complete text of the experimental procedure from the "2004-2005 Physical Chemistry Lab Manual" is given here (in this type). If additional comments or directions are needed, they are added (in italics). So that each photo has its own block of text, extra numbers have been added as needed (i.e. 2a), 2b), etc.).  The text is above the picture illustrating it in each case. For simple or known operations, no photo is included.

1)                          Weigh out the proper amount of your sample. Remember no more than 1.1 g or 33000 J (or 8000 cal) per sample. The energy limit must be calculated for each new substance before ignition. Make sure the material is pure, with no foreign material in it.

2)                          Prepare a pellet of your sample using the pellet press. Weigh the pellet carefully - this is the amount of sample you will use.

2a) Carefully pour the weighed sample into the die. The die rests in a holder, which is placed with the

larger opening up, so the die can rest inside the holder. Note the orientation of the die and holder.

2b) Carefully place the die (with the sample in it) and the holder on to the pellet press. The holder

rests on an adjustable base which may screwed up or down to get the proper height.

2c) Firmly push down so as to make a solid pellet. Apply strong pressure for a few minutes for best results.

Note that the holder is the same size as the adjustable base.

2d) Once pressed, the pellet is formed at the bottom of the die, but must still be removed.

Turn the base upside down so the die will rest on top of it (note the narrower, shallower opening).

2e) With the holder in "upside down", carefully slide the die into the press. Pushing down gently on the press will

force the pellet out of the die and into the holder.

2e) Success! The pellet has been gently pushed out of the die and fallen into the holder.

2) (cont.)  Once the pellet has been pressed, try to avoid handling it with your fingers - use gloves or tweezers. Reweigh the pellet. If the pellet breaks, press it anew. If the bomb needs to be vented and refilled before ignition, reweigh the pellet. Place the pellet in an ignition cup.

3)                          Make sure the bomb is clean and dry before use. The bomb consists of three parts: 1) the body (like a vase); 2) the cap, which is inserted into the body and contains the electrodes, ignition cup holder, and gas valves; and 3) the ring, which screws onto the body sealing it and the cap. Also shown are a) and ignition cup and b) the ignition wire. Visually inspect the O-rings and gaskets and make sure there are no nicks, cuts or damaged areas on them. Replace as needed.

4)                          Put 1.0 mL of water (use the same volumetric pipet each time) in the bottom of the bomb before putting the ignition cup with the pellet in. This helps absorb combustion byproducts.  

5a)                          Cut a 10-cm length of ignition wire and weigh it as accurately as possible.

5b) Now carefully attach the weighed ignition wire to the two electrodes, sliding the caps down to ensure good contact.

5c)     Detail of the wire, bent so as to touch the pellet.

5d)    Place the ignition cup with pellet in the holder and bend the ignition wire down so that it just touches the surface of the pellet.

6)                          Carefully insert the cap with the electrodes, wire, and cup into the body of the bomb.

7)                          Now screw the cap on tight - the screw threads for the ring are generally not visible after tightening.

                Do not use a wrench! Be careful not to tip the bomb and spill the pellet!

8)         Now fill the bomb with oxygen gas, using the tank in lab and the Parr regulator. The adaptor must be placed securely over the inlet valve before the oxygen is filled. In the photo, note the vent valve directly above the oxygen inlet valve, and the two terminals for the ignition unit between them.

8a)           Control the flow rate of oxygen gas with the (round) black valve on the regulator.

8b)           Make sure to vent the regulator (by pulling down on the black knob) before removing the adaptor (otherwise a sharp bang will be heard when the adaptor is removed and the O-rings inside it will have to replaced).

8c)           Do not fill above 40 atmospheres! Usually 30 atm is sufficient, but if the inside of the bomb is sooty after ignition, insufficient oxygen was present. If this happens, do not use this data, but repeat the trial and use more oxygen.  

9a)                       The bomb calorimeter: a) the inlet valves for hot and cold water; b) the stirrer motor, attached to two stirrers by belts; c) the lever to lift the lid of the calorimeter (pictured in the down position); d) the handle for opening the calorimeter lid; e) the thermometers with the bucket on the left and jacket on the right.

9b)          Opening the calorimeter lid. The lever to lift the lid has been raised (left hand), while the right hand is pulling the handle to pivot the lid open.

9c)     The calorimeter lid pivots open. The hinge is beneath the back left corner of the lid.

9d)         Now place the bucket in the calorimeter.

9e)           There is a layer of air between the bucket and jacket for insulation.

9f)            Next carefully place the bomb in the bucket...

9g)           ...and insert the ignition wires into the electrodes on the cap.

9h)           Do not have the ignition unit plugged in at this time! (Note that the ignition wires are not attached to the bomb in the photo).

10a)                  Now pour 2.00 L of distilled water (use the same volumetric flask each time) into the bucket, covering the bomb. Allow the same "drip time" each trial to make sure the volume of water used is constant. Note wires avoid the circular hole where they could be cut when the lid closes.

10b)         Observe the bomb underwater. If you see a steady streams of bubbles, this indicates a leak and the bomb must be removed from the bucket, vented in a hood, the pellet reweighed and the bomb refilled with oxygen gas. Frequently, a few bubbles will appear right away from air trapped on the surface of the bomb - look for a steady stream or series of bubbles to indicate a leak.

11)                      The calorimeter lid is on a pivot - swing the lid over and close it. Make sure the ignition wires come out of the calorimeter and that the lid drops into place and seals well. Be careful not to cut the ignition wires (avoid the metal circle).

12)                      Now insert the thermometers into the calorimeter. One reads the temperature of the bucket water, the other the temperature of the jacket water (they are labeled). Both are supported on a metal rod which slides up and down - always make sure the rod and thermometers are up (and out of the way) before moving the calorimeter lid - otherwise you will break the thermometers. Left photo: pull out the knob to slide the thermometers up or down (the up position is shown). Right photo: the thermometers in the down position.

13)                      Make sure the stirrer motor is connected to the stirrer drive shaft with a black rubber O-ring. This helps the water in the bucket to circulate and produces better temperature readings. Don’t forget to turn the stirrer motor on before your trial starts.

14)                      The calorimeter jacket should be hooked up to a source of hot and cold water. This allows adjustments to the temperature of the jacket to minimize radiative heat loss. Make sure the water is circulating. At the start of the run, ideally the temperatures in the jacket and bucket should be nearly identical (ideally within 0.1 °C, in practice within 0.2 °C is good enough), and both should be slightly below room temperature. This is difficult to get right - start the hot water flowing (in the sink) early to let it warm up.

15)                      Once the temperatures are close and adjustments are done, begin recording the temperature of both thermometers at least once a minute or every 30 seconds if possible. Use a stopwatch to time this (measure elapsed time from the start). Record at least 5 minutes of such readings to determine any drift in temperatures due to heat gain or loss in the calorimeter. Make sure to relate all timed temperature measurements to the initial time (even though there will be a minute or two when the bomb is fired when you will not be making any temperature measurements – this will be a gap in your data and graph).

16)                      Once the system is stable and several minutes of temperatures have been recorded, the bomb ignition unit may be plugged in and the bomb ignited. Dr. Mahler must be present for this step. Place the safety shield between the calorimeter and the ignition unit and have the person pressing the button as far away from the calorimeter as possible. All other experimenters should be at least as far away, and preferably out of the room. The ignition unit has a small red light, which should momentarily light as current flows (a soft buzzing sound can also be heard at this time), then the light goes out as the wire burns up in the bomb (and the buzzing stops).

17)                      Wait at least 30 seconds past ignition before approaching the bomb after ignition, then record temperature changes at 30 second or 1 minute intervals, as practical. Record data for at least 5 minutes beyond the maximum temperature. This again allows correction for heat loss. All data should be related to the initial time, so zero through five minutes will be temperatures before firing, then minutes six (or seven) on will be after firing the bomb. Your graph for ΔT will not work unless you time the temperature readings in this way.

18)                      Unplug the ignition unit after ignition and returning to the calorimeter. After all the measurements for a trial are done, lift the thermometers and open the calorimeter. Carefully remove the bucket and take out the bomb. Drain and dry the bucket for the next trial. Remove the ignition wires from the electrodes and place the bomb in the hood.

19)                      In the hood, carefully and slowly vent the bomb through the vent valve. Only open the valve slightly - do not vent the bomb too quickly (this can damage the integrity of the valve and its seals).

20)                      Unscrew the ring on the bomb and remove the cap. Carefully remove the unburnt wire and weigh it as accurately as possible. There is a correction for the amount (5858 J/g) of wire burnt in the bomb - ignore "globules" which are usually iron oxides. If the bomb is sooty, not enough oxygen was used and the trial must be repeated. Clean and dry the bomb for the next trial. Inspect all seals and O-rings for damage, and replace as needed.

21)                      Do two or three good trials for determination of the calorimeter constant and at least as many good trials for each unknown sample tested.