Some metals can be protected by electroplating a less reactive metal onto their surfaces. Chromium metal is electroplated onto many of the steel parts of an automobile. This not only provides protection, it can add to the appearance of a car. If the electroplating process, ions of the metal to be plated are reduced and depositied on the surface of the object to be coated. The object itself acts as the cathode during the process, and gradually, an ever-thickening layer of new metal is built up.
Aluminum can be protected by deliberately, and in a controlled fashion, forming an oxide coating on its surface. This means that the surface of the aluminum must be oxidized, a process that occurs at the anode, not the cathode. This process is called anodizing, and can be considered the opposite of electroplating.
An anodized surface has the additional advantage of easily being coloured by organic dyes. In this activity, you will anodize a piece of aluminum and patially colour it with an organic dye. By taking quantitative measurement of the current flow, you will be able calculate the thickness of the anodized layer.
| 1. | Obtain a clean 400 mL beaker, and line it with aluminum foil. Also have ready several paper towels and a 100 mL beaker half filled with acetone. |
| 2. | Obtain a piece of aluminum metal to be anodized. Buff it will with steel wook. Wipe it clean, first with a dry paper towel, and then one wet with acetone. Only a small section of the paper towel should be wet with the acetone. Handle the aluminum piece by its edges only; do not touch its surface with your fingers. After cleaning with acetone, lay the piece down on a clean, dry paper towel. |
| 3. | Throw away the paper towel wet with acetone, discrd the acetone from the beaker and rinse the beaker. Be sure your work area is cleared of all acetone before continuiong. |
| 4. | Add 250 mL of 3.3 sulphuric acid to your alumunim lined beaker. Make sure about 2 cm of foil are above the surface of the sulphuric acid. The aluminum foil will act as a cathode and is to be connected to the negative side of your power source. Set up the power supply and ammeter as shown in the illustration above. DO NOT connect the negative terminal to the aluminum foil at this time. The piece of aluminum to be anodized is to be suspended in the sulphuric acid solution in the center of the beaker. Attach a buret clamp to a ring stand above the beaker. The lead from the positive terminal of the power supply should be tied or wrapped around the burrette clamp so that your piece of aluminum will be suspended perfectly in the beaker held by the alligator clamp. |
| 5. | Have your teacher check your set up before you make the final connections, When ready, note the time to the nearest second at which you connect the negative lead to the aluminum foil. Record this time in the data table. |
| 6. | Let the cell operate for 20 minutes. At the start of the cell operation, note and record the current as indicated by the ammeter. Have ready a second 400 mL beaker three-quarters filled with distilled water to use to rinse the piece of aluminum when it has been anodized. |
| 7. | While the cell is operating, we will prepare the dye bath. In a 100 mL beaker, add 10 mL of either red, green, or yellow Rit fabric dye. Add 40 mL of distilled water to this. Set the beaker on a piece of wire gauze on a ring , on a ring stand, and begin heating with your bunsen burner. When the solution begins to boil, turn down the heat to just maintain boiling. |
| 8. | After the aluminum piece has been anodized for 20 minutes, again note and record the ammeter reading. Then, disconnnect the negative lead, noting the exact time when you do so. Record this value. |
| 9. | Use the crucible tongs to remove the piece of aluminum from the sulphuric acid solution. Disconnect the alligator clip. Quickly rinse the aluminum in the beaker of distilled water, and then immediately place it in the hot dye bath. Let it remain in the dye bath for 10 minutes, keeping the dye solution just at or near the boiling point. |
| 10. | After 10 minutes have elapsed, remove the aluminum from the hot dye with your crucible tongs, rinse it well, and dry it with a paper towel. In the observation section, write a complete description of the piece of anodized aluminum. |
| 11. | With a metric ruler, measure the length and width of the anodized portion of your aluminum piece. Record these values. From your data and the instructions in the Calculaitons section, calculate the thickness of the anodized layer. |
Beginning time: __________ Ending time: ___________
Beginning Current: ___________ Ending Current: _________
Anodized aluinum: Length: _______ Width: _______
Observations: Describe the anodized piece of alminum:
Calculatons:
In order to calculate the thickness of the anodized layer, we first
need to determine the amount of aluminum oxide that fromed. The half-cell
reaction that occurred at the anode is given by:
Al(s) ----> Al+3 +
3 e-1
The equaiton tells us that for every mole of electrons that flowed,
one-third of a mole of aluminum was oxidized. A mole of electrons carries
a charge of 96,500 coulombs. A coulomb is a unit of electrical charge.
The flow of one coulomb per second is called an ampere. Therefore,
we can find the number of coulombs your cell used while you were anodizing
the aluminum by multiplying the current in amperes times the time in seconds.
| From your beginning and ending readings of current taken from the ammeter calculate the average value of the current: | current:(average):_________ |
| From your beginning and ending times, calculate the number of seconds that the current was flowing: | time for current: __________ |
| Now multiply the amperes times the seconds: This will give you coulombs: | coulombs: ______________ |
| Divide this value by 96,500 coulombs/mole to find moles of electrons | moles of e-1: _____________ |
| Divide this by 3 to find moles of aluminum that were oxidized: | moles of Al reacted: ______ |
| Aluminum oxide has the formula Al2O3. Since it takes two moles of Al to form one mole of Al2O3, divide the moles of Al reacted by 2 | moles of Al2O3: __________ |
| Mutiply this by 102.0 g/mole, the molecular mass of aluminum oxide. | grams of Al2O3:___________ |
| The density of Al2O3 is 3.97 g/cm3. Divide the grams of aluminum oxide by its density in order to calculate the volume of oxide formed: | volume of Al2O3:__________ |
| Now, from your data, calculate the area of the aluminum piece that was anodized: | area anodized: ____________ |
| If the volume of aluminum oxide is divided by the area anodized, this will yield the thickness of the anodized layer. | thickness of anozidzed layer: ________ |