Silver Plating Batch That Fails the Adhesion Test
A small plating unit in Gaya takes an order for 200 steel spoons. The operator sets up a bath of 0.5 M AgNO3 with a pure silver plate as one electrode, hangs the spoons from the other electrode bar, and runs 1.5 A for 20 minutes per batch. In the first batch the spoons come out with a dull, patchy deposit that rubs off on handling; one spoon is visibly pitted and blackened on its face. The helper explains that he connected the spoons to the silver plate 'because both are metals, so either way works'. The owner wants three things settled before the next batch: how much silver is actually being consumed per batch, whether a 20-minute batch can ever give a serviceable coating, and what must be changed. [Ag = 108 g mol⁻¹; F = 96500 C mol⁻¹]
1. Calculate the mass of silver deposited per batch at 100% current efficiency, write both electrode reactions, and comment on whether the coating is adequate for 200 spoons.
Read the worked answer
Charge passed: Q = I × t = 1.5 × (20 × 60) = 1800 C. Moles of electrons = 1800/96500 = 1.865 × 10⁻² mol.
Cathode (spoons, negative terminal): Ag⁺(aq) + e⁻ → Ag(s). One mole of electrons deposits one mole of silver, so mass = 1.865 × 10⁻² × 108 = 2.01 g of silver per batch. Anode (silver plate, positive terminal): Ag(s) → Ag⁺(aq) + e⁻. The anode dissolves at the same rate, so the silver-ion concentration of the bath stays roughly constant - that is the point of using a sacrificial silver anode rather than an inert one.
Judgement on adequacy: 2.01 g spread over 200 spoons is about 0.010 g per spoon. Taking the density of silver as 10.5 g cm⁻³ and a spoon surface of roughly 30 cm², the coating is only of the order of 0.3 µm - far thinner than the 5–20 µm a decorative plate needs, so it will appear patchy and wear through at edges. The batch time or current must go up substantially, and the true current efficiency is below 100% because some charge is spent on hydrogen evolution, so the real figure is even smaller. The calculation explains the symptom, it is not merely a number.
2. Explain the pitting and blackening of one spoon, and state the practical correction.
Read the worked answer
The pitted spoon was the one tied to the silver plate, i.e. it became the anode of the cell. At an anode the electrode itself is oxidised, so the steel was attacked rather than plated:
Fe(s) → Fe²⁺(aq) + 2e⁻, with competing 2H₂O → O₂ + 4H⁺ + 4e⁻.
The blackened patch is oxide/roughened metal left where iron dissolved, and the acid generated at that electrode accelerates the damage. No silver can deposit on a spoon that is losing metal - plating requires the workpiece to be the cathode.
The correction follows directly from the diagnosis: confirm polarity before every batch (a simple cell or multimeter check between the bars), colour-code the cathode bar, and wire all spoons in parallel to the negative bar so each sees the same potential. Two further practical points are worth adding as judgement: direct silver on steel adheres poorly because of the galvanic mismatch, so a thin nickel or copper 'strike' undercoat is normally applied first; and the steel must be degreased and pickled, since any grease film produces the dull, rubbable deposit seen on the rest of the batch.
3. Commercial silver baths use K[Ag(CN)2] rather than AgNO3. Justify this choice chemically and note the drawback.
Read the worked answer
In AgNO3 the free Ag⁺ concentration is high (0.5 M in this bath), so deposition is fast and uncontrolled: ions discharge at protruding points and edges before the recesses are covered, giving dendritic, powdery, poorly adherent deposits with bad 'throwing power'. Displacement also occurs - silver deposits on steel simply by contact exchange, without any current, and such immersion deposits are spongy.
In K[Ag(CN)2] the equilibrium Ag⁺ + 2CN⁻ ⇌ [Ag(CN)₂]⁻ has a very large formation constant, so free [Ag⁺] is lowered by many orders of magnitude. The deposition potential is shifted to a more negative value, discharge becomes slow and orderly, and the deposit is fine-grained, dense and adherent even in recesses. The complex also buffers the silver-ion activity, so the bath composition drifts less during a long run, and cyanide keeps the anode dissolving cleanly.
Drawback: cyanide baths are acutely toxic and generate cyanide-bearing rinse water and sludge, so they need closed handling, fume extraction and oxidative effluent treatment (alkaline chlorination). This is exactly why modern replacement baths based on thiosulphate or methanesulphonate have been developed. A good answer states both the electrochemical reason and the environmental cost, rather than simply naming cyanide as the 'correct' reagent.