Preparing a platinum plating bath

• Platinising (platinum black): a dilute chloroplatinic acid solution with a trace of lead acetate, run at high current density for a few minutes. It gives a matte black, very high-area deposit on platinum, gold or titanium for conductivity cells, hydrogen electrodes, fuel-cell test electrodes and counter electrodes. It is the easiest bath to prepare from a bottle of chloroplatinic acid.
• Bright platinum, thin (0.1–2 µm): the diamminedinitritoplatinum(II) or P-salt bath, run hot and near neutral, is the traditional decorative and electronics bath. The deposit is bright and hard; cathode efficiency is low (10–20%) and thickness is limited by stress.
• Bright platinum, thick (2–25 µm): the dinitrosulfatoplatinous acid (DNS) bath, strongly acidic, and the alkaline hexahydroxyplatinate bath, which runs at near 100% efficiency with low stress. These are used for platinised titanium anodes, medical components and thick coatings.
• Hexachloroplatinate baths (chloroplatinic acid in hydrochloric acid) are the simplest of all but give dull, stressed, poorly adherent deposits; use them only as a strike or for platinising.

The standard laboratory recipe is 1–3% (w/v) chloroplatinic acid with 0.005–0.03% lead(II) acetate, in water or 0.1 M hydrochloric acid. A convenient bath for electrode work is:

• 1.0 g of chloroplatinic acid hexahydrate (about 0.37 g of platinum) in 100 mL of 0.1 M hydrochloric acid, with 10 mg of lead(II) acetate trihydrate.

1. Clean the electrode: degrease in acetone, then dip in warm aqua regia for 10–20 seconds (platinum) or in 10% nitric acid (gold), and rinse with deionised water.
2. Electrolyse with the work as cathode and a platinum wire or foil as anode at about 30 mA/cm² for 5–10 minutes, reversing the polarity every 30 seconds. A velvety black deposit forms within the first minute; longer plating gives a thicker, more fragile layer.
3. Rinse, then cathodise the electrode for a few minutes in 0.5 M sulfuric acid at the same current density to desorb chlorine, and rinse again. Store platinised electrodes in deionised water; never let them dry.

Lead-free platinising for electrochemistry where lead would be a contaminant uses 3–5% chloroplatinic acid without additive at 10–20 mA/cm² for a longer time; the deposit is greyer and less adherent. The solution is reusable until its colour fades; top it up with chloroplatinic acid rather than discarding it.

P-salt is diamminedinitritoplatinum(II), Pt(NH₃)₂(NO₂)₂, a cream-coloured solid prepared from potassium tetrachloroplatinate(II). The bath, developed by Keitel and Zschiegner in the 1930s, is still used for thin bright deposits.

Preparing P-salt from potassium tetrachloroplatinate (about 2 g scale): dissolve 4.15 g of K₂PtCl₄ (10 mmol) in 60 mL of water, add 4.2 g of sodium nitrite (60 mmol) and heat at 80–90 °C with stirring until the deep red solution turns pale yellow (about one hour; nitrogen oxides are evolved, so work in a hood). Add 10 mL of 10% ammonia solution, heat for a further 30 minutes, cool in ice and filter the cream precipitate; wash with cold water and dry at 60 °C. Yield about 2.5–3 g (75–90%).

Bath composition and operation:

• P-salt: 10–20 g/L (about 6–12 g/L platinum)
• Ammonium nitrate: 100 g/L
• Sodium nitrite: 10 g/L
• Ammonia (25%): 50 mL/L, to pH 7.5–9
• Temperature 90–95 °C; cathode current density 0.5–1.5 A/dm² (5–15 mA/cm²); platinised titanium anodes; moderate air or mechanical agitation

Expect a cathode efficiency of 10–20%, so a 1 µm deposit takes 20–40 minutes at 1 A/dm². Deposits above about 2 µm crack from internal stress. Ammonia evaporates continuously at this temperature; check pH hourly and replenish.

The DNS bath uses dinitrosulfatoplatinous acid, H₂Pt(NO₂)₂SO₄, at 5–20 g/L platinum with sulfuric acid to pH 1.5–2, run at 60–80 °C and 1–2 A/dm². It is made by converting tetrachloroplatinate to the tetranitrito complex with sodium nitrite (as in the P-salt preparation) and then treating with sulfuric acid; it is more usual to buy the concentrate. Cathode efficiency is 40–60% and deposits of 10 µm and more are routine, which is why it is the bath for platinised titanium anodes.

The alkaline bath uses potassium or sodium hexahydroxyplatinate(IV), K₂Pt(OH)₆, at 10–20 g/L platinum with 15 g/L potassium hydroxide (pH about 13), at 65–80 °C and 0.5–1 A/dm². Efficiency approaches 100%, the deposits are ductile and almost unstressed, and the bath tolerates impurities well. The salt is made from hexachloroplatinate: boiling potassium hexachloroplatinate or chloroplatinic acid with excess potassium hydroxide hydrolyses the chloride ligands, and the hexahydroxyplatinate is recovered as the potassium salt. It is the bath to choose for thick engineering coatings if you can source or make the salt.

• Platinum is deposited directly onto gold, platinum, palladium, nickel and nickel alloys. Copper and brass need a nickel or gold strike first; steel needs nickel. Titanium needs an etch in hot hydrochloric or oxalic acid and immediate transfer to the bath, and only the DNS and alkaline baths give good adhesion to it.
• Clean as for any precious-metal plating: soak clean, electroclean, acid dip, rinse. Water should sheet evenly off the part.
• Anodes are platinised titanium or platinum. Platinum does not dissolve anodically in these baths, so metal is replenished by adding salt, not from the anode. Keep the anode-to-cathode area ratio at least 1:1.
• Analyse platinum content weekly by ICP or by reducing an aliquot to platinum black with zinc and weighing it, and keep a log of ampere-hours. Filter continuously or daily through 1 µm polypropylene.
• A 1 L bath at 10 g/L platinum contains 10 g of platinum: at current spot prices, the metal in a small bath costs several hundred euro, so plan for recovery and avoid drag-out losses with still rinses.

Platinum has a density of 21.45 g/cm³, so 1 µm over 1 dm² weighs 0.2145 g, which is 1.10 mmol. Depositing it from a platinum(II) bath (two electrons per atom) takes 212 coulombs, or 0.059 ampere-hours, at 100% efficiency; from a platinum(IV) bath, twice that. Divide by the cathode efficiency of the bath: 0.059 Ah becomes about 0.39 Ah in a P-salt bath at 15% efficiency, so 1 µm on 1 dm² at 1 A/dm² takes about 23 minutes. The alkaline bath deposits platinum(IV), which needs four electrons per atom (0.118 Ah per µm·dm²), but at near 100% efficiency it still does the job in about 9–10 minutes at 0.75 A/dm². Measure the actual thickness by X-ray fluorescence or by weighing the part before and after, and adjust.

Spent platinum electrolytes, drag-out rinses and filter cake all go to recovery. Acidic chloride solutions are treated with ammonium chloride to precipitate ammonium hexachloroplatinate, which is filtered, washed and calcined to platinum sponge; nitrite and ammine baths are first destroyed by boiling with hydrochloric acid and a little nitric acid in a fume hood (nitrogen oxides), then converted the same way. Zinc dust or sodium formate will drop platinum from almost any bath as black metal if a quicker, cruder recovery is acceptable.