Chloroplatinic Acid

Chloroplatinic acid (hexachloroplatinic acid, H₂PtCl₆·6H₂O; CAS 18497-13-7), also called platinum chloride or hydrogen hexachloroplatinate, is a Pt(IV) hydrogen salt used in some platinum-toning and Pt/Pd printing formulations as an alternative platinum source to potassium chloroplatinite.^[1] Chemically it is the acid form of the hexachloroplatinate(IV) ion, which gives it slightly different solution behaviour and pH than the potassium salt — more useful in toning solutions where acidity shifts the reaction dynamics.

Photographic uses

• Platinum toning of silver prints: Dilute chloroplatinic acid solutions (typically 0.5–1% with acetic acid buffer) gradually replace silver in a silver gelatin print with platinum, producing extraordinary tonal depth and near-infinite permanence.^[2]
• Alternative Pt/Pd sensitizer: Some Pt/Pd formulas use chloroplatinic acid in place of potassium chloroplatinite for particular contrast responses. The acid form self-acidifies the sensitizer; this changes working pH and reactivity.
• Research-grade platinum source: Where precise Pt(IV) chemistry is needed (some photogravure and electrochemistry crossover applications).

Practical notes

Supplied as dark orange-brown to red crystalline solid, extremely hygroscopic — absorbs water rapidly from ambient air. Keep in a tightly sealed container with desiccant. Dissolves readily in water (very soluble) producing a yellow-to-orange acidic solution.

As expensive as potassium chloroplatinite — approximately $100–150 per gram. Buy in small quantities. The hexahydrate form is more common commercially than the anhydrous; formulas generally assume the hexahydrate unless specifically marked "anhydrous".

Solution stability: stored in amber glass, a 10% working solution keeps for a year or more. Freshly-prepared solutions are more reliable for precision toning work.

Related compounds

Potassium chloroplatinite (K₂PtCl₄) is the Pt(II) potassium salt used as the primary sensitizer in Pt/Pd printing. Palladium chloride is the palladium analog at lower cost. Gold chloride shares the noble-metal toning methodology at even higher expense.