Electrolysis – Electroplating of Metals
In a galvanic or voltaic electrochemical cell, the spontaneous reaction occurs and electrons flow from the anode (oxidation) to the cathode (reduction). In an electrolytic cell, a non-spontaneous reaction occurs using energy supplied by an external source. Electrons are forced from the external circuit at cathode to produce the reduction. The cathode is the negative electrode and is the black connection on the power supply. Electrons are drawn into the external circuit at the anode to produce the oxidation. The anode is the positive electrode and is the red connection on the power supply. Any electrochemical cell can be galvanic or electrolytic depending on the direction in which the reaction is proceeding. For instance, a car battery is a galvanic cell when it is discharging and providing energy and is an electrolytic cell when it is recharging and using energy. Electrolytic cells are commonly used in industry to produce aluminum, silver-plate dinnerware, gold-plate jewelry, and chrome-plate car bumpers.
In an electrolytic cell used for electroplating, the object to be plated is used as the cathode. Metal cations are reduced at the cathode and a layer of metal is deposited on the surface of the object. The anode is usually made of the same metal as the one being plated at the cathode. A concentrated solution of a very water-soluble ionic compound of metal being plated is used in the cell compartment as the electrolyte. This makes the solution very conductive and allows the plating to occur relatively quickly. The rate of plating and the amount of metal plated depend on the electrical current, in amps, and the time. A cell for plating copper is shown below,
1. What happens to the mass of the anode as the electroplating process occurs?
2. What happens to the mass of the cathode as the electroplating process occurs?
3. What happens to the [Cu+2] in the solution as the electroplating process occurs?
4. Write the anode and cathode reactions if a zinc anode and ZnSO4 solution are used.
In order to get good adhesion between the object being plated and the metal being deposited on it, the surface of the object must be very clean and free of any metal oxide coating and other materials such as grease or oil. This may be accomplished by buffing the object with steel wool or by dipping the object briefly in an acid solution followed by rinsing with water. After cleaning, the object should not be handled with your fingers since the oil on your fingers may interfere with the plating process.
In this experiment, a copper ring will be copper plated and a copper pin will be zinc plated.
Safety glasses must be worn during this experiment.
1. Fill a 50x100mm crystallizing dish with the copper plating solution to within an 1/8inch from the top (@210mL). This solution contains sulfuric acid, is corrosive, and must be handled carefully. Clean up any spills immediately
2. Bend a piece of copper sheeting so that it will hang over the edge of the dish and extend into the solution. This will be the anode and does not have to be cleaned.
2. Carefully dip the ring into a beaker containing 6M HCl until it looks like the oxide coating has been removed (about 30sec).
3. Remove the ring with tweezers and rinse the ring with water. Handle the ring with tweezers or with paper towels to avoid getting oil on the surface.
4. Attach the black alligator clip to the ring. The clip should just hold onto the edge of the backside of the ring near the solder joint. Attach the red alligator clip to the copper anode. The power should be turned on before the ring is lowered into the solution.
5. Lower ring into the solution. The ring should be completely under the surface of the solution and held close to the edge of the dish. Plate the ring for about ten minutes or until and even coating has been created. This may take less than 10min. Turn the ring around during the plating process so the copper plates evenly on the outer surface of the ring.
6. Remove the ring from the solution and rinse with water. Carefully dry the ring. Follow your instructor’s directions about coating the ring with a clear acrylic spray.
1. Fill a 50x100mm crystallizing dish with 1.0M ZnSO4 solution to within an 1/8inch from the top (@210mL).
2. Bend a piece of zinc sheeting so that it will hang over the edge of the dish and extend into the solution. This will be the anode and does not have to be cleaned.
2. Carefully dip the pin into a beaker containing 6M HCl until it looks like the oxide coating has been removed (about 30sec).
3. Remove the pin with tweezers and rinse the pin with water. Handle the pin with tweezers or with paper towels to avoid getting oil on the surface.
4. Attach the black alligator clip to the pin. The clip should just hold onto the edge of the pin. Use a point on the ring that will not interfere with the design on the ring. The bottom edge may be the best point. Attach the red alligator clip to the zinc anode. The power should be turned on before the pin is lowered into the solution.
5. Lower pin into the solution. The pin should be completely under the surface of the solution and held close to the edge of the dish. Plate the pin for about ten minutes or until and even coating has been created. This may take less than 10min. Keep the front surface pointed towards the anode during the plating process so the zinc plates mainly on that surface of the pin. Some zinc will plate on the back surface but this will be minimal if the pin is held close to the side of the dish.
6. Remove the pin from the solution and rinse with water. Carefully dry the pin. Follow your instructor’s directions about coating the pin with a clear acrylic spray.
This experiment is modified from Activity 6.4 Electroplating a Copper Ring from Art in Chemistry; Chemistry inArt,by Greenberg and Patterson, Teacher Ideas Press, 1998, pp 165-167.
Electrolysis – Electroplating of Metals
5 50x100mm glass crystallizing dishes labeled copper plating
5 50x100mm glass crystallizing dishes labeled zinc plating
2 10amp variable transformers
2 6-outlet power strips with circuit breaker and reset features
10 6/12 volt trickle charge motorcycle battery rechargers
10 Pairs of wire leads (one black and one red) with alligator clips at both ends
10 Tweezers (metal)
4 100mL beakers labeled HCl
4 100mL beakers labeled Waste
4 Wash bottles
Copper plating solution (2 liters)
1.0M ZnSO4 solution (2 liters)
6M HCl (500mL)
5 Copper strips (1/2 x 4 inch) for anodes
5 Zinc strips (1/2 x 4 inch) for anodes
Electrolysis – Electroplating of Metals
1. The ring had already been sized to the individual student and the joint soldered in a previous activity. The copper for the ring and the pin was ordered from a jewelry supply house and cut on a metal brake. This avoids sharp and ragged edges on the ring and pin. The edges may need a little filing if they are still sharp. The ring was cut from copper strips. The pin was 1x 1.5inch. The pin was cut from copper plate.
2. The metal for the anodes can be metal sheeting from a lab supply source and was cut with metal shears.
3. The copper plating solution came from a commercial plating company and included brighteners that gave a good shiny coating. The ZnSO4 came from a lab supply company.
4. The HCl solution will turn yellow from the dissolved copper ions from the oxide coating forming a complex ion with chloride ion. This solution should be saved and reused for this lab even if it is very yellow.
5. It will save considerable time if a class plating system is set up ahead of time. One transformer is used for the copper plating and one for the zinc plating. A power strip in plugged into the transformer. Five motorcycle rechargers are plugged into the power strip and spaced far enough apart so the students have sufficient space to work. The black leads from the recharger are attached to the black alligator wire leads and the red leads from the recharger are attached to the red alligator wire leads. The crystallizing dishes have the solutions and the anodes in them already and the anode is attached to the red leads. Cleaning stations with beakers containing HCl and waste beakers for rinsing should be set up. The zinc and copper plating stations should be set up in different parts of the room.
6. The transformers are set so the battery chargers produce 2V. The voltage output can be checked using a hand-held volt meter. All the chargers on the same strip should be producing the same voltage once the transformer is set.
7. Better plating is achieved if the power is turned on before the object to be plated in placed in the plating solution. You want a relatively low voltage and current. A high voltage and current will cause rapid but uneven plating with metal whiskers coming off the object. You want relatively low current and an even charge distribution across the object to get even plating. The anode and cathode should be kept as far apart as possible. The object will plate more heavily on the surface towards the anode. This is why the ring must be turned around to get even plating and the front surface of the pin is kept towards the anode. The object will not plate well were the clip is attached so the clip should be attached in the least obvious place.
8. Watch the plating carefully. If the object is left in the solution too long, you may build up a layer of metal that is thick enough to obscure the surface design.
9. The ring and pin must be protected from surface oxidation with an acrylic spray.
10. A bas-relief, zinc pin may be created by coating both sides of a 1x1.5 inch zinc plate with wax. The wax is scraped off the areas that will be etched. The pin is placed in 6M HCl. The acid will reacted with the exposed zinc. After the pin has reacted for the desired length of time, the pin is removed and rinsed with water. The wax is removed by dipping the pin in boiling water.