🔬 Definition

Chloroplatinic acid is a strong complex acid of platinum, obtained when platinum metal dissolves in aqua regia (3 HCl : 1 HNO₃). It is an orange-red / reddish-brown crystalline, hygroscopic solid. It is the most important water-soluble compound of platinum.

🧪 Formula

H₂PtCl₆

⚗️ Composition (Ions)

It ionises in water as:

H₂PtCl₆ → 2H⁺ + [PtCl₆]²⁻

• 2H⁺ — acidic hydrogens (hence “acid”)
• [PtCl₆]²⁻ — hexachloroplatinate(IV) complex ion (octahedral: Pt at centre, 6 Cl⁻ ligands)

🔢 Oxidation State of Pt = +4

Quick calculation using the complex ion:

• Charge of [PtCl₆]²⁻ = −2
• Each Cl = −1, so 6 Cl = −6
• Let Pt = x → x + (−6) = −2 → x = +4 ✅

That’s why the IUPAC name is Hexachloroplatinic(IV) acid — the “(IV)” tells you the oxidation state directly.

🧠 Memory Trick

“2 H guests, 6 Cl guards, Pt is the +4 king in the castle.”

Or remember the pattern: H₂ + Pt + Cl₆ → just write 2, 1, 6 (H : Pt : Cl). The complex always carries 2− charge, so Pt must balance 6 chlorides: −6 + 4 = −2. If you remember “six Cl, two H, Pt(IV)”, you can never forget the formula.

📝 Exam Point (most asked)

• Platinum dissolves only in aqua regia, forming chloroplatinic acid: 3Pt + 4HNO₃ + 18HCl → 3H₂PtCl₆ + 4NO + 8H₂O
• Test for K⁺ / NH₄⁺: Chloroplatinic acid gives a yellow precipitate of K₂PtCl₆ (potassium hexachloroplatinate) with K⁺ salts — a classic qualitative analysis question.
• It is used to prepare platinum catalysts (e.g., platinized asbestos, Adam’s catalyst) and in platinum refining.

📝 Final Exam Point Summary (Revision in 60 Seconds)

1. Formula: H₂PtCl₆ — 2H⁺ + [PtCl₆]²⁻
2. Oxidation state of Pt: +4
3. IUPAC name: Hexachloroplatinic(IV) acid
4. Preparation: Pt dissolves only in aqua regia (3HCl + HNO₃) → 3Pt + 4HNO₃ + 18HCl → 3H₂PtCl₆ + 4NO + 8H₂O
5. Test: Yellow precipitate with K⁺/NH₄⁺ salts (K₂PtCl₆ / (NH₄)₂PtCl₆)
6. Geometry of [PtCl₆]²⁻: Octahedral, coordination number 6, d²sp³ hybridisation

Common Mistakes to Avoid

• ❌ Writing the formula as PtCl₆ or H₂PtCl₄ — the correct one is H₂PtCl₆.
• ❌ Confusing chloroplatinate [PtCl₆]²⁻ (Pt = +4) with chloroplatinite [PtCl₄]²⁻ (Pt = +2) — different complexes, different oxidation states.
• ❌ Saying “Pt reacts with HCl” — it needs aqua regia, not HCl alone.
• ❌ Forgetting the charge on the complex ion: it is [PtCl₆]²⁻, not 3⁻ or 1⁻.