HandsOn 26 - Liesegang Experiment

I. Pre-Lab

There are clear similarities between the Liesegang experiment and the diffusion tube experiment. In both experiments:

• the reaction is dependent on the diffusion of the reactants,

• the reaction is between two different molecules and results in the precipitation of a salt, and

• in both reactions the chemical reactants diffuse along a cylinder.

There are, however, some differences between the two diffusion experiments:

• one of the reactants, the potassium chromate, is initially uniformly distributed along the cylinder in the Liesegang experiment,

• the diffusion takes place in a gel as opposed to air, and of course

• multiple disks appear instead of a single disk.
                          
  

Q6.16: Can you account for the appearance of multiple parallel disks in Figures 6.6 and 6.7? Think of what happens as the copper sulfate diffuses into the gel? Will a precipitate of copper chromate appear? If so, what will that do to the local concentration of chromate ion? What will be the consequence for diffusion of chromate ion to or from nearby regions? What happens to the local concentration of the copper ions diffusing through the gel? What happens next?

Q6.17: Can you make a guess as to how long you may have to wait to observe a pattern? Hint: A generalization of the result obtained in Appendix 6-A to cases when randomness of steps is caused by collisions with other particles turns out to be áx2avg = 2Dt, where the proportionality constant D is called the diffusion constant, and its value varies from material to material. The value of the diffusion constant for copper sulfate in the gel is roughly what it is in water, on the order of 10-6 cm2/sec. The preceding equation is usually called the diffusion equation.

II. Experiment

For this experiment, you will need the following materials:

• Glacial acetic acid (17.4 M)

• Potassium Chromate (K₂CrO₄)

• Sodium Silicate solution (commercial water glass) with specific gravity 1.38-1.42 g/ml

• Copper Sulfate crystals

• distilled water

• one 2 dram vial (7.4 ml), actually holds up to 10 ml

• two 50 ml beakers

• 5 ml pipette calibrated in 0.1 ml units

• 0.1 ml pipette calibrated in 0.01 ml units

In the following, it is assumed that the experiment will be done with approximately 8 ml of gel. For other sizes, scale the ingredients proportionately. Leave space at the top of the vial for the copper sulfate.

1. Make 4 ml of sodium silicate solution by adding 0.6 ml of sodium silicate (specific gravity 1.38-1.42) to 3.4 ml of distilled water. This produces a solution with specific gravity 1.06.


2. Make 4 ml of acetic acid solution by adding 0.09 ml of glacial acetic acid to 3.9 ml of distilled water. To this add 0.19 g of potassium chromate. Make this solution in the vial.


3. Pour the sodium silicate solution made in Step 1 into the vial containing the acetic acid solution of Step 2. Put on the cap (quickly) and mix all the reagents by inverting the vial about 10 times. Don't agitate the vial - this will introduce air bubbles into the gel and may distort the pattern.


4. Leave the gel in a safe place where it will not be moved or shaken. The solution will gel in 5 to 20 minutes. Let it harden for several hours if possible.


5. After hardening, add between 0.5 g and 1.0 g of copper sulfate crystals to the top of the gel. Try to cover the surface uniformly.


6. You will start seeing patterns after roughly 24 hours. The pattern will continue forming for one and a half weeks to two weeks.


7. Observe the pattern formed. Record the time it takes for portions of the pattern to develop. Measure the distances between portions of the pattern.

Q6.18: Compare your predicted pattern with what you observed. Is the symmetry what you expected? Refine your speculation about the source of the multiple rings.

Q6.19: Did the pattern take as long to develop as you expected? Does the diffusion equation above provide an approximate relation between the time t for a single ring to form?

Previous: 6.2 - Periodic Precipitation: Liesegang Rings