045 Micro-Water Purification

Expt 045 -- Micro-Water Purification

This experiment is a micro-scale adaptation of the ChemCom Foul Water Lab I. Here modified pipets are used as separatory funnels, sand-filtration funnels, and charcoal absorption columns. 3-4 mL of foul water are converted into about 1-2 mL of clean water, and the students take care not to spill a single drop!

Safety

Use scissors to cut plastic pipets.
Use ordinary laboratory safety procedures.

Procedure

Obtain a test tube containing a 3-4 mL foul water sample. Shake it up to completely disperse the contaminants and note the appearance and odor of the sample. Note the volume of the sample.

Use a thin stem pipet to withdraw the water sample from the test tube. Inject this sample into a cut-off graduated pipet. Place the pipet stem side down on the dirty test tube from which the sample came. Allow the layers to separate (oil to float and sediments to sink) in the pipet.

!!!Click here to See Picture.
While the layers are separating, prepare a sand filter. Use scissors to cut the top of the bulb off of a Jumbo pipet. Be sure to leave most of the pipet bulb to act as a funnel reservoir. Sift sand through a sieve to separate any large pieces. Put a few of the large pieces into the pipet funnel (they should get caught in the tapered tip to act as plug). Transfer the fine sand to a paper, and pour it into the pipet. Tap gently.

!!!Click here to See Movie.
Separate the layers of the foul water sample. Squeeze the bottom layer of settled sediments into the already dirty test tube -- this is the discard test tube. (Do not release the squeeze). Transfer the pipet to the top of a clean test tube. Resume squeezing to expel the next layer into the test tube. Stop just before the oily layer is about to be expelled, then transfer the pipet back to the top of the discard test tube to squeeze out the remaining oily layer into this test tube which already contains the sediment layer.

!!!Click here to See Movie.
Note the clarity and odor of the water separated from the middle layer.
!!!Click here to See Picture.
Place the sand filter column in the next clean test tube. Pour the water into the funnel and allow it to filter.

!!!Click here to See Movie.
Note the clarity and odor of the filtered water.
!!!Click here to See Picture.
Cut off the top of a second Jumbo plastic pipet. Wad a small piece of cotton or tissue into a ball. Place the wad in the pipet, and push it to the tip of the pipet with a stiff piece of wire. Pour the charcoal onto a paper, and transfer it into the second pipet funnel. Tap gently.

!!!Click here to See Movie.
Place the charcoal filter in the next test tube. Pour the water through the charcoal filter.

!!!Click here to See Picture.
The water might drain slowly without pressure. To accelerate the process, hold your thumb tightly on the top of the filter, and press on the sides to exert pressure on the water.

!!!Click here to See Movie.
Continue forcing the water through the column until you have filtered the entire sample. Note the clarity and odor of the filtered water.

!!!Click here to See Movie.
Compare the water quality at each step.

!!!Click here to See Picture.
Note the volume of water after purification.

Questions

How does the appearance and odor change after step 3, the sedimentation separation?
How does the appearance and odor change after step 4, the sand filtration?
How does the appearance and odor change after step 6, the charcoal adsorption?
What two forces caused the separation to occur during step 3?
Which impurity particles are larger, the ones responsible for causing the water to be cloudy or the ones causing the water to smell? Justify your answer.
Since charcoal did the best job of cleaning the water, why waste time with the first two procedures (sedimentation separation and sand filtration); why not just pour the foul water directly into the charcoal column?
Estimate the percentage of water lost during purification. Where is the water that was lost?
State some impurities that would remain after the purification used here.

Handout Makeup

Name ___________________________ Class _______

Teacher __________________________

BeckerDemos 045 Micro-Water Purification

Watch the movies and answer the questions.

Curriculum-

This laboratory fits into discussions of mixtures and solutions. Use it to introduce separations of substances.

Activity-

Experiment - Laboratory Only

Safety-

Use scissors to cut plastic pipets.
Use ordinary laboratory safety procedures.

Time-

Teacher Preparation: 10 minutes

Class Time: 40 minutes

Materials-

(Per group)

2 g activated charcoal (available at pet stores in the aquarium supply aisle)
4 g Sand
4 mL Foul water sample (Mix coffee grounds, crushed garlic, vegetable oil, and water.)
1 thin-stem plastic pipet
1 graduated plastic pipet
2 Jumbo (wide stem) plastic pipets (for filtering columns)
1 20-cm length of stiff wire (coat hanger wire works well)
some cotton or tissue paper
4 small (13x100) test tubes
test tube rack

Disposal-

Dispose of charcoal and sand with ordinary trash.

Lab Hints-

Warn students not to put sand and charcoal in the sinks.
Use fine ground coffee, and even then pass them (dry) through a strainer or sifter to remove any large pieces that might clog up the thin-stem pipet in step 1. Likewise with the garlic powder.
An easy recipe for the foul water is to place 400 mL of tap water in a 500 mL flask along with 1 tsp. of sifted coffee grounds, 1 tsp. of sifted garlic powder and 25 mL of vegetable oil. Stopper and shake. This can be dispensed with a plastic pipet into the student's test tubes, but the mixture tends to separate out; it is therefore important to shake the foul water just before drawing up a sample to ensure a more or less consistent mixture sample for each student. One way to keep the mixture well stirred is to place it in a large beaker on a stir plate, and draw out samples while it is stirring.

Observations-

The separation is similar to some large scale water purification methods. The first step involves just allowing the water to sit. As it does, large solid particles sink to the bottom and oil droplets along with some low density solids float to the top and form a scum layer on top. These can be separated by a technique similar to that employed by a separatory funnel. No stopcock is needed on this micro-scale level, because surface tension holds the liquid inside the cut-off pipet. Thus a few drops of sediment layer can be discarded, then the water layer transferred into a clean test tube, then the top oily layer discarded. (Hint: Do not release the squeeze during this process, for the bubbles might stir up the layers again. Instead, just squeeze, hold it, then squeeze some more, and so forth. The water obtained from this technique is considerably "cleaner" than it was to begin with, but it is still somewhat cloudy and smelly!
After the sand filtration, the sample is much clearer, although it might be slightly colored, and it still has the stench of garlic.
After the charcoal filtration, the sample is generally clear, colorless, and more or less odorless.
Each step involves some loss of water, and the final purified sample might only have about one-fourth the volume that the original sample had!
This purification does not remove soluble ionic salts which do not adsorb on charcoal.

Answers-

Q1. How does the appearance and odor change after step 3, the sedimentation separation? A1. The large particles and the oily droplets have been removed; the liquid is still a cloudy yellow and contains very small suspended particles. It also still smells.
Q2. How does the appearance and odor change after step 4, the sand filtration? A2. The cloudiness and small particles are gone, but the color and odor are still present.
Q3. How does the appearance and odor change after step 6, the charcoal adsorption? A3. The water appears virtually free of contaminants: clear and colorless. There is still a slight odor.
Q4. What two forces caused the separation to occur during step 3? A4. First, intermolecular forces played a role, specifically, water molecules being more attracted to themselves than to either the oil molecules or the molecules making up the large sediment particles. This makes the substances immiscible; that is, it keeps the oil and the sediments from dissolving in the water. Second, the Earth's gravity caused the specific layering to occur. Since gravitational force is proportional to mass, the denser substances, namely the sediments, tend to be pulled more toward the center of the Earth (that is, they sink) while the less dense oil droplets tend to float.
Q5. Which impurity particles are larger, the ones responsible for causing the water to be cloudy or the ones causing the water to smell? Justify your answer. A5. The particles causing the cloudiness are larger. In fact they are generally larger than the spaces between the small sand grains; thus they got trapped and separated out by the sand filtration. The odor-causing particles must have been too small to be caught by the sand, so they passed straight through.
Q6. Since charcoal did the best job of cleaning the water, why waste time with the first two procedures (sedimentation separation and sand filtration); why not just pour the foul water directly into the charcoal column? A6. Charcoal has a large convoluted surface area upon which absorption of very small impurity particles can occur. Had the foul water been poured directly into the charcoal, this surface would most likely have been coated by the oil, rendering the charcoal useless. Also, since the gaps between the charcoal pieces are so large, none of the sediments, large or small would have been trapped.
Q7. Estimate the percentage of water lost during purification. Where is the water that was lost? A7. About half (50%) was lost during the purification in the movies. The water clings to the filters and to the devices used for the separation.
Q8. State some impurities that would remain after the purification used here. A8. No separation of soluble salts was attempted.

Reference-

The experiment is a micro-scale version of A.2 Laboratory Activity: Foul Water in ChemCom: Chemistry in the Community (p. 8-11) American Chemical Society, 1988.

Key Words 1-

mixture, water purity, water, purification, separation, sedimentation, adsorption