0.11 From cells and electrodes to golden pennies  (Page 2/4)

Experimental procedure

 Special supplies:

Part 1: Nominal 100 x 15 mm disposable polystyrene. Petri dishes (three per group); fine steel wool; approximately one soldering kit for every six students consisting of 140-watt soldering iron, rosin-core solder, and one 6 x 6 inch ceramic fiber square (available from Flinn Scientific Inc.); digital voltmeters with alligator clip leads and 2 short lengths (3 cm) of Pt wire to use as voltage probes.

 Part 2: Digital voltmeters with alligator clip leads, 6 pennies per group (preferably clean and bright), hot plates, stainless steel forceps; approximately one soldering kit (see the description in Part 1) for every six students.

Chemicals:

Part 1: Agar (powder), 1% phenolphthalein indicator, 0.1 M potassium ferricyanide [hexacyanoferrate)III], $K_3\text{Fe}(\text{CN}{)}_6$ ; two zinc metal strips, 6 x 40 mm, cut from 0.01-inch thick zinc foil; two copper metal strips 6 x 40 mm, cut from 0.01- (or 0.005)-inch thick copper foil; 2 ungalvanized finishing nails per group (before use, clean by soaking briefly in 3 M $H_2{\text{SO}}_4$ acid, rinsing with deionized water, and drying in an oven).

 Part 2: 30-mesh zinc metal; zinc metal powder, 6 x 100 mm strips of zinc metal (one per group) cut from 0.01-inch thick zinc foil; 20 gauge copper wire; 1 M NaOH, 1 M HCL in dropper bottles, and a 1 M NaOH/Zn $({\text{NO}}_3{)}_2$ 50:50 mix solution.

! SAFETY PRECAUTIONS WEAR EYE PROTECTION AT ALL TIMES. Sodium hydroxide is corrosive. You may want to provide latex rubber gloves for handling pennies that have been in contact with 1 M NaOH.

WASTE COLLECTION: Your instructor may direct you to waste containers for NaOH solutions used in this experiment. These substances can be disposed of down the drain only if they are neutralized by sodium bicarbonate.

 5-10 min.

METAL CORROSION AND ANODIC PROTECTION.

• Obtain two 6 x 40 mm strips of zinc foil, two 6 x 40 mm strips of copper foil, and two 4-penny (40 mm long) ungalvanized iron finishing nails (which should have been previously cleaned by immersion in 3 M $H_2{\text{SO}}_4$ , then rinsed with deionized water, and dried in an oven).
• Clean the zinc and copper strips with steel wool to produce a clean, shiny surface.
• Use detergent (1% alconox located by each sink)to remove the film of oil on the strips, rinse them with deionized water, and dry them with a tissue.15 min. experienced, 30 min. novice
• General Soldering Instructions. Go to a soldering station where you will find a ceramic fiber square, soldering iron, and rosin core solder. When soldering, place the zinc strip on a ceramic fiber square and position the copper strip so the ends of the two strips overlap by about 4-5 mm. Ask a partner to apply pressure to the copper strip, holding it in place while you are soldering the joint. (Your partner may use almost any tool for this except a bare finger, because the strip will get very hot- a rubber stopper is recommended.) In the combination using a iron nail, have the nail ontop of the other metal, heat the nail in order to make a good seal.
• Plug the soldering iron in and let it heat up for a couple minutes. Then tin the tip of the iron with solder, wiping off all excess with a damp sponge or damp paper towels. Place the freshly tinned tip on the copper strip next to the zinc strip, angling the iron to get good thermal contact. Let the copper strip heat up for a good 30 seconds.
• Feed the solder into the area between the zinc strip and the tip of the soldering iron. When the copper strip is hot enough, the solder will flow into the joint. Don't dab at the joint with the tip of iron while soldering. The tip must be kept in continuous good thermal contact with the joint so that the copper strip heats up. Using steel wool, remove any rosin remaining from the soldering operation; then rinse the soldered joints, and dry the metal strips with a tissue.
• If the metal strips no longer have a clean, shiny surface due to excessive touching and handling, use detergent to remove the oily film as before and rinse them thoroughly with deionized water. WARNING!- be very gentle cleaning your soldered metal strips to avoid breaking the joint you just made!
• If you are a novice at soldering or have never soldered before, check out the helpful hints at: How to Solder.
• Now you should have a Zn/Cu piece, a Cu/Fe piece, and a Zn/Fe piece. (Once these soldered bimetal pieces have been made, they can be cleaned and reused several times, so don't throw them away at the end of the experiment unless your instructor direct you to.) 10-15 min.
• Add 3 g of agar to 225 mL of boiling deionized water in a 600-mL beaker. At this point it's best to turn down the heat and stir with a large magnetic stir bar until the agar is dissolved. Be patient, the agar tends to clump and complete dissolution can take 10 min. Be careful not to heat the agar so much that it scorches or boils over. While the agar suspension is hot, add 2 mL of 1% phenolphthalein indicator with a calibrated transfer pipette. Continue stirring for a couple minutes.5-10min.
• Get three nominal 100 x 15 mm polystyrene Petri dishes. Put each of the three soldered bimetal pieces in the bottom (taller) half or a Petri dish. Protecting your hands from the beaker containing the hot agar solution with "hot hands" or paper towels, pour the agar over the metal pieces in the Petri dishes. Cover the metal pieces, but do not fill the dish beyond half its depth. There should be no voids or bubbles underneath the metal strips.
• After the agar cools and gels (approximately 20 min.), use a fine-tipped transfer pipette to place one small (0.02 mL) drop of 0.1 M potassium ferricyanide $K_3\text{Fe}[\text{III}][\text{CN}{]}_6$ along side each iron nail, about 1 cm away from the middle. The ferricyanide salt will diffuse radially outward. If the ferricyanide ions encounter any ${\text{Fe}}^{2+}$ formed by oxidation of Fe, they will react with the Fe(II) ions to form a dark blue compound formulated as $\text{KFe}(\text{III})\text{Fe}(\text{II})(\text{CN}{)}_6$ . (It's reported that you get the same produce by mixing ${\text{Fe}}^{3+}$ with $K_4\text{Fe}[\text{II}][\text{CN}{]}_6$ ). Evidently in the final product the iron atoms have exchanged an electron so that the ${\text{Fe}}^{2+}$ ion is oxidized to Fe(III), and the Fe(III) originally in the ferricyanide ion is reduced to Fe(II). WARNING!- the drop of potassium ferricyanide will not immediately "soak" into the agar gel. It will remain on top like a bead of water. Be careful not to jar or move the Petri dish too much after placing the drop.
• During the course of the afternoon, make periodic observations of the Petri dishes. Look for evidence of formation of hydroxide ion, which will turn the phenolphthalein pink; the formation of insoluble metal ion-hydroxide salts, which will appear as a cloudy band; or the formation of a blue compound in those dishes containing iron nails (with an added drop of 0.1 M potassium ferricyanide, $K_3\text{Fe}[\text{CN}{]}_6$ , indicating oxidation of Fe to form ${\text{Fe}}^{2+}$ .20-25min.
• Obtain two short lengths (about 3 cm long) of platinum wire and a digital voltmeter with leads connected to alligator clips to hold short lengths of wire that will be used as voltage probes. Adjust the voltmeter to its most sensitive voltage range (200 millivolts). First, carefully clamp the alligator clips to the platinum voltage probes and immerse the probes in the agar, one probe midway alongside one metal and the other probe midway along the other jointed metal. (Support the probes at all times with your hands and keep the probes upright, perpendicular to the Petri dish. Make sure the probes do not touch the metal strips.) Note whether there is any voltage difference. Note the polarity. Which metal is nearest the positive (+) end? Any voltage difference indicates an electric field between the two points in the agar created by the formation of positive and negative ions in the two regions. Considering the polarity of the measured field, what ions do you think might be responsible fore the presence of the electric field? Write plausible reactions for the formation of positive ions (metal atom oxidation) and negative ions (reduction of water or oxygen).
• Next, touch the probes directly to the two metals at their midpoints and note any voltage reading. (The voltage reading is expected to be zero volts because metals are such good electronic conductors that only a tiny electric field can exist in the two metals together.) The metals soldered together are said to form an equipotential surface (a surface where the potential is constant, so that the voltage difference between any two points on the metal is zero.)
• Put the top cover on your Petri dish, and tape the top cover in place with two or three short strips of tape. Write your initials or other identifying marks on the tape.
• Continue visual observations in your next lab period, looking for evidence of formation of any pink color or any visible precipitates. Make sketches, and write verbal descriptions of the changes you observe.