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Cells and Electrodes
Mary McHale
This work is produced by The Connexions Project and licensed under the Creative Commons Attribution License †
Protocol adapted from 'Chemistry in the Laboratory'
1 ElectroGels - Metal Corrosion, Anodic Protection and the Golden Penny
1.1 Objective
The goal of this experiment is:
- Observe the deposition of zinc metal on a penny in contact with metallic zinc and to develop a hy- pothesis to explain how this happens.
- Observe how iron may be corroded or protected from corrosion depending on whether it is connected to copper or to zinc.
- Observe the plating of a penny with zinc and subsequent color changes.
1.2 Grading
You will be assessed on:
- completion of the report form.
- TA evaluation of lab procedure.
1.3 Introduction
1.3.1 Electrochemistry is Everywhere
In general, whenever two condensed phases (solid or liquid) are brought into contact, a potential (or voltage) dierence develops across the interface. Because the interface region is very thin, even transfer of a small amount of charge across the interface can create a very large electric eld. For example, transferring about one picomole ( 10−^12 mole) of electron charge per square centimeter of area will typically create a potential dierence of approximately 1 volt across an interface layer about one nanometer thick. The electric eld in this interface region would be about 109 volts/meter. Electric elds this large can cause the transfer of elec- trons across an interface layer or the transfer of ions between the inside and outside of ells in living organisms. Because contacts between condensed phases are very common in nature, electrochemical phenomena are very common, even though we are often unaware of them. At the cellular level, electrochemical phenomena are crucial to the propagation of nerve impulses, the timing of muscular contractions of the heart, and activity in your brain cells.
∗Version 1.2: Apr 1, 2008 12:25 pm GMT- †http://creativecommons.org/licenses/by/2.0/
Most of the electrical technology created by humans involves the simplest kind of chemical change; electron transfer across an interface. Often, the interface is between a good electron conductor, called an electrode, and a solution containing molecules or ions. The electrode might be a solid (like platinum or copper metal or graphite), or it could be liquid (like mercury metal). When electrons are transferred from the electrode to a molecule, we say the molecule has been reduced. Electron transfer in the opposite sense (from molecule to electrode) is called oxidation. There are two parts to this lab the rst, ANODIC PROTECTION, you will perform in pairs. By coating steel (which is mostly iron metal) with a more active metal like zinc, a process called galvanizing, the steel's corrosion can be retarded or entirely prevented. Often, simply making a good electrical connection between a piece of iron and a piece of zinc is su cient to keep the iron from corroding. We will study the acceleration and the prevention of iron corrosion by connecting it to various metals. The second part of this lab, THE GOLDEN PENNY EXPERIMENT will be set up for you by your TA, so that you can make observations. The golden penny experiment involves the plating of a penny with zinc metal. First, the penny is immersed in a solution containing 1 M NaOH and granular zinc. Subsequent heating of the penny for a few seconds on a hot plate causes the silver color of the penny to turn a bright golden yellow. Explaining the details of the process presents a challenge.
1.4 EXPERIMENTAL PROCEDURE
1.4.1 Special Supplies:
Part 1: Nominal 100 x 15 mm disposable polystyrene. Petri dishes (three per group); ne 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 ber square (available from Flinn Scienti c 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.
1.4.2 Chemicals:
Part 1: Agar (powder), 1% phenolphthalein indicator, 0.1 M potassium ferricyanide [hexacyanoferrate)III], K 3 Fe (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 nishing nails per group (before use, clean by soaking brie y in 3 M H 2 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 (NO 3 ) 2 50:50 mix solution. ! SAFETY PRECAUTIONS WEAR EYE PROTECTION AT ALL TIMES. Sodium hydroxide is cor- rosive. 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 nishing nails (which should have been previously cleaned by immersion in 3 M H 2 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.
in those dishes containing iron nails (with an added drop of 0.1 M potassium ferricyanide, K 3 Fe [CN] 6 , indicating oxidation of Fe to form Fe2+.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 dierence. Note the polarity. Which metal is nearest the positive (+) end? Any voltage dierence indicates an electric eld 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 eld, what ions do you think might be responsible fore the presence of the electric eld? 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 eld 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 dierence 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.
1.5 REACTIONS THAT MIGHT FORM OH- ION.
When a pink color develops around a metal in a gel containing phenolphthalein indicator, it means that the solution next to the metal is basic. In an aqueous gel, the pink color means that some hydroxide ions have been formed. Although any electrons given up when a reactive metal is oxidized might react at the spot where the oxidation occurs, they can also readily travel to any other spot on the surface of the two joined pieces of metal. That means, it is possible that the point where metals atoms are oxidized could be some distance from the point where hydroxide ions are produced. Now let's think about what might be most likely to accept these available electrons. Metal atoms typically don't accept electrons to form negatively charged metal ions. Rather, metal ions tend to give up electrons to form positive ions. Things that are easy to reduce have the most positive standard reduction potentials, like halogens, but we don't have any halogens in our system. The gel surrounding the metal consists mainly of water with about one percent of agar. Although water is not easy to reduce, because water has a negative standard reduction potential in basic solution, this substance can be reduced when the reaction is coupled to the oxidation of Zn metal in basic solution, as shown by the following standard reduction potentials: Zn (OH)^24 − + 2e−^ → Zn (s) + 4OH− ( E ◦in 1 MOH−^ = − 1 .28volts
2 H 2 O + 2e−^ → H 2 (g) + 2OH− ( E ◦in 1 MOH−^ = − 0 .80volts
Agar is a polysaccharide (like starch), and polysaccharides are not easy to reduce. Finally we must not forget that the Petri dishes are open to the air, so the agar gel also contains dissolved oxygen, a good acceptor of electrons. At least two reactions involving oxygen deserve serious consideration: O 2 (g) + H 2 O + 2e−^ → H 2 − + OH− ( E ◦in 1 MOH−^ = − 0 .065volts
O 2 (g) + 2H 2 O + 4e−^ → 4 OH− ( E ◦in 1 MOH−^ = − 0 .40volts
The E ◦^ for reduction of oxygen in basic solution is considerably more positive than for the reduction of water. So we de nitely must consider the possibility that oxygen might be the species that could most easily be reduced, with OH−^ (and possibly hydrogen peroxide) being the reduction product. The reduction of either water or oxygen produces hydroxide ions, but the formation of a pink color with phenolphthalein does not tell us which reaction might be responsible. Thermodynamics (as measured by the standard reduction potentials) favors reduction of oxygen over reduction of water. However, the reduction of oxygen on many metals is known to have a large activation energy, which usually causes the reaction to be slow. Thus, kinetics may favor the reduction of water, particularly because the concentration of water is much greater than the concentration of oxygen in the agar gel. Can you think of an experiment that might allow you to distinguish if water or oxygen is the major species being reduced? 20-25min.
1.6 THE GOLDEN PENNY EXPERIMENT.
- Your TA will set this up for you by putting 8 g of 30 mesh zinc in the bottom of a 400-mL beaker. It is best to weigh-out the zinc in a watch glass and pour the zinc into a tilted 400-mL beaker so as to keep the zinc on one side of the beaker. Use a spatula and tilt and tap the beaker on the bench top in order to get all the granular zinc to cover about half the bottom of the beaker (one-half not covered: see Figure 5). Carefully pour 200 mL of 1 NaOH down the side of the beaker, being careful not to disturb the distribution of zinc. Use a stirring rod or spatula to clear any remaining granules so that half of the beaker bottom is completely free of zinc granules. Place the beaker on a hot plate in the fume hood and turn the hot plate to medium heat. The solution should be heated to about 80-90 ◦C; if it is heated to boiling the distribution of zinc granules will be disturbed. Continually monitor and check the temperature to keep it in this range. 20-25min.
- While waiting for the solution to heat, bu six copper pennies with steel wool until they are shiny. Wash them with deionized water and dry. Solder 10-cm lengths of 20-gauge copper wire to two of the pennies, overlapping the wire and penny about 2 to 3 mm from the edge. Solder the free end of one of the copper wires to a 5 x 100 mm strip of zinc metal, as shown in Figure 5. Clean any rosin o the soldered joints with steel wool, and rinse with water.
to a strip of zinc metal in contact with 30-mesh zinc on the bottom of the beaker. (D) A penny soldered to copper wire is immersed in solution. The solution in the beaker is 1 M NaOH. Review the General Soldering Instructions in Part I. Place the freshly tinned tip of the penny next to the wire angling the iron to get good thermal contact. Don't dab at the joint with the tip of the iron while soldering.
- When the solution has warmed, use forceps to place two copper pennies on top of the granular zinc metal and two pennies in the area that is free of granular zinc (make sure that the pennies on the uncovered side do not contact even one grain of zinc). Bend a small "foot" on the penny in the beaker as shown in Figure 5. The "foot" of the zinc strip should rest on the granular size so that both are in direct contact. The penny should be completely immersed in solution but should not contact any granular zinc metal on the bottom of the beaker. Finally, hang the last penny (the one with only copper wire soldered to it) over the edge of the beaker so that the penny is completely immersed in solution. See Figure 5. HINT- use an empty 400-mL beaker to bend and shape your soldered metal pieces to match what is pictured in Figure 5. Do this before attempting to put them in the 400-mL beaker containing your warm NaOH solution. 30min.
- Leave the pennies in the beaker until some of them turn a silvery color. This may take anywhere from 5 to 30 min., depending on the temperature of the solution. (Some of the pennies will never turn silver even after waiting an hour or more.)
- Which pennies turn a silvery color? Is it the three pennies that are in contact with the solution and with zinc, either directly or through the copper wire? Or is it the three pennies in contact with the solution but not indirect or indirect contact with zinc metal? 5-10min.
- Using a pair of forceps, remove the pennies that have turned a uniform silvery color, rinse them with water, and put the pennies on a hot plate for a few seconds. Watch what happens to the silver-colored pennies as they heat on the hot plate. Keep the solution warm in the beaker in case you need to repeat some part of the experiment or try some new experiment, as described below. 5-10min.
- Further Experiments. Solder another length of copper wire to a shiny clean penny. Actually, you can use the lone penny soldered to the copper wire from the rst part of the experiment, just clean it with steel wool and deionized water. Connect one lead of a voltmeter to a zinc strip and the other lead to the copper wire soldered to the penny. Using the same solution you prepared earlier, immerse the zinc strip and penny in the solution. Is there a voltage dierence between the zinc strip and the copper wire soldered to the copper penny? Which metal is the electron source (the negative terminal of this electrochemical cell)? 5-10min.
- Do you think a current ows in the copper wire connecting the zinc strip and copper penny when both are immersed in the solution? If so, which direction will electrons ow, and what are the anode and cathode reactions? Put the digital voltmeter into its current measuring mode on its most sensitive (microampere) scale. Then see if any current is owing when you connect the meter in series between the copper wire soldered to the penny and to the zinc strip -both the penny and the zinc strip should be immersed in the hot 1 M NaOH solution. The series connections should look like this: Penny/copper wire/(+) ammeter(-)/zinc strip. How large a current ows?. Is the current (charge ow) from penny to zinc strip or vice versa?
- In wires the charge carriers are electrons. Current (de ned as a ow of positive charge) is opposite to the ow of electrons. In which direction are electrons owing: from penny to zinc strip or vice versa?
