HandsOn 24 - Diffusion Chamber

Now carry out the diffusion experiment described below and compare the outcomes with the theory so far developed.

This experiment works best when carried out by three people. One person acts as timekeeper/data collector, while the other two handle the apparatus and read the measurements from the glass tube during the experiment. Materials for each group of three students:

• Glass tubing, 2 to 3 mm internal diameter, one length of 20 centimeters and one length of 30 centimeters (supplied in the laboratory kit).

• Four 3.7 milliliter vials with caps.

• Six squares of parafilm, 3 centimeter by 3 centimeter.

• Hydrochloric acid (HCl) with specific gravity 1.19. It should be fresh, not left over from last year.

• Ammonium Hydroxide (NH₃OH) with specific gravity 0.90. This also should be fresh.

• Two graduated cylinders, 10 ml or more

• One stopwatch

1. Add a scale and parafilm "skirts'' to the tube. With a marker and a ruler, label the tubing with dashes every half centimeter, starting at one end of the tube, continuing around the bend, along the main length of tube, and around the second bend to the other end of the tube. This is done in order to keep track of position. Cut a tiny slit in the centers of two sheets of parafilm. Place the end of the tubing through the slit sliding it about 1.5 centimeters up the side to create a "skirt'' of parafilm. Be sure that the film does not stretch to cover the end of the tube. Pinch the parafilm around the tube with a slight twisting motion, so that nothing can pass between the parafilm and the tube, while leaving the "skirt'' spread out. Repeat this procedure for the opposite side.


2. Prepare the solutions by first labeling the graduated cylinders and the vials as containing HCl or NH₃OH solutions (one for each container). Pour 3.5 mL of HCl from the stock solution into a10 ml labeled graduated cylinder and immediately cover this with parafilm. Repeat this procedure with the NH₃OH and set aside. Pour the HCl solution from the graduated cylinder into the vial labeled HCl and immediately cap to minimize the loss of vapor. Repeat this with NH₃OH. Each of the vials should be almost full of solution. Set everything else aside, except for the two labeled vials and the 20 mm bent glass tube.


3. Carry out the experiment. Start by completely loosening the caps on the vials but do not remove them. The goal in what follows is to insert the ends of the tubes in the vials as smoothly as possible, without jolts. One person holds each end of the tube and simultaneously they remove the caps from the vials and smoothly place the two ends of the tubing into the vials so that the tubes dip below the level of the liquid at each end. Then fold the parafilm around the mouth of each vial to create an airtight seal. As the tubes enter the vials, the timekeeper begins timing the experiment, thereafter calling out ten second intervals.


4. Look for a precipitation disk composed of solid ammonium chloride NH₄Cl, in the form of dust, due to the reaction of the NH₃OH and HCl vapors. You may have to wait up to a minute before the appearance of this disk. The initial position of the disk and the time of its appearance are your major experimental results. You may be interested in continuing to time for 6-10 minutes as you record the position of the precipitation disk while it moves along the tube. In which direction does this move: toward the HCl end or toward the NH₃ end? Later you can try to explain this effect.


5. Before moving the tube and vials, examine each end of the immersed tube and record the position of the liquid level inside the tube. This is the zero point from which you will measure the distance to the initial position of the disk.


6. Compare the location of the initial position of the disk (the ratio of distances from each end) with that predicted by the theory so far developed in this unit. Compare the time of appearance of the disk with that predicted by the theory.


7. Now prepare to carry out the experiment once more, this time with a tube of length 30 centimeters. Predict what results you expect for the location of the first appearance of the disk and the time for this first appearance. Carry out the experiment and compare your results to your predictions.