Copper Plating: Checking Thickness Before Increasing Current
A small plating shop is copper-plating steel badges in a sulfate bath. The owner wants a decorative coating of at least 40 μm on each badge of exposed area 25.0 cm². A trial run uses a current of 0.500 A for 2.00 h. The current efficiency is measured as 92%, meaning 92% of the charge actually deposits copper; the rest is lost in side reactions. The density of copper is 8.96 g cm^-3, its molar mass is 63.5 g mol^-1, and one mole of electrons carries 96,500 C. The bath is well stirred and the cathode is flat enough to assume uniform deposition. The operator says the coating must be thick enough after one run, and asks whether increasing current to 1.00 A would halve the time. The workshop has a 2.00 h slot per batch and cannot exceed 1.00 A on the existing power supply. Before changing the procedure, the owner wants a quantitative check and a comment on the anode and bath maintenance.
1. Calculate the mass of copper deposited and the average coating thickness in the trial run. Does the badge meet the 40 μm specification?
Read the worked answer
Charge passed: Q = I t = 0.500 A × 2.00 × 3600 s = 3600 C.
Moles of electrons = 3600 C / 96,500 C mol^-1 = 0.03731 mol.
Cathode reaction: Cu2+ + 2e^- -> Cu, so moles Cu = 0.03731 / 2 = 0.01865 mol.
Ideal mass = 0.01865 × 63.5 = 1.184 g.
At 92% current efficiency, actual mass = 1.184 × 0.92 = 1.089 g.
Volume = mass / density = 1.089 g / 8.96 g cm^-3 = 0.1216 cm³.
Thickness = volume / area = 0.1216 cm³ / 25.0 cm² = 0.00486 cm = 48.6 μm.
This exceeds the 40 μm minimum by about 8.6 μm, so the trial coating meets the specification, assuming uniform deposition.
2. The operator proposes raising the current to 1.00 A and running for 1.00 h instead of 2.00 h. Will this give the same deposit? What practical factors should be checked before accepting the change?
Read the worked answer
The same charge is passed: 1.00 A × 1.00 h = 1.00 A × 3600 s = 3600 C, exactly as in 0.500 A × 2.00 h. If the current efficiency stayed at 92%, the deposited mass and average thickness would be the same, so in principle the time could be halved.
In practice, higher current density can lower efficiency because hydrogen evolution or side reactions become more important. It can also cause poor throwing power, burning, or uneven deposit in regions of high current density. Stirring and mass transport of Cu2+ must be adequate. The shop should run a test coupon, measure efficiency and thickness distribution, and check adhesion and appearance before changing the batch schedule. The full 2.00 h at 1.00 A would double the charge and should not be used unless a thicker coating is wanted.
3. Explain what happens at the copper anode during plating and why the bath needs regular maintenance even though copper is being deposited at the cathode.
Read the worked answer
At the copper anode, copper dissolves: Cu(s) -> Cu2+ + 2e^-. These electrons travel through the external circuit to the cathode, where Cu2+ + 2e^- -> Cu(s) deposits. With a soluble copper anode, the anode replenishes the Cu2+ removed at the cathode, so the copper concentration can remain roughly steady.
Maintenance is still needed because anode impurities can form sludge, which may fall into the bath or become included in the deposit. Anode bags or filtration help remove sludge. The bath also needs control of Cu2+ concentration, pH, temperature, agitation, and additives; otherwise the deposit may become rough, dull, or poorly adherent. If an inert anode were used instead, oxygen evolution would occur at the anode and the Cu2+ concentration would fall, requiring separate replenishment.