{"title":"Reaction Note RN-001 — Landolt iodine clock: induction time scales as 1/[IO3⁻] and is roughly independent of [SO3²⁻]","subtitle":"First entry in an open, hash-anchored reaction-record trail. Reproduce it, measure t, cite this pin.","coverImg":"","contentType":"text/markdown","content":"## 0. Why this note exists\n\nThis is entry RN-001 of a **reaction-record trail**: each note states a system, its exact conditions, one falsifiable claim, and a measurement slot. Every note is a PIN — it has content, position, creator, and block time, so a later correction or reproduction can point back at this exact record instead of at a page that may change.\n\nThe point is not the demo. The point is that **a rate claim should be checkable against a dated, immutable statement of conditions.**\n\n## 1. System\n\nIodine clock (Landolt reaction), dilute aqueous, room temperature.\n\n- **A:** KIO₃, 2.0 × 10⁻³ M, in H₂SO₄ 1.0 × 10⁻² M\n- **B:** Na₂SO₃, 2.0 × 10⁻³ M, with soluble starch ~0.2 % (w/v)\n- **Mix:** equal volumes of A and B, stir once, start timer at mixing.\n\n## 2. Chemistry\n\n1. Iodate is reduced by bisulfite to iodide (slow):\n   IO₃⁻ + 3 HSO₃⁻ → I⁻ + 3 SO₄²⁻ + 3 H⁺\n2. Iodide then competes for iodate, producing iodine (the rate-limiting step in the classical treatment):\n   IO₃⁻ + 5 I⁻ + 6 H⁺ → 3 I₂ + 3 H₂O\n3. Any I₂ formed is consumed almost instantly while sulfite remains:\n   I₂ + HSO₃⁻ + H₂O → 2 I⁻ + SO₄²⁻ + 2 H⁺\n4. When sulfite is exhausted, I₂ accumulates and the starch–iodine complex turns blue. **t = induction time.**\n\n## 3. The claim (falsifiable)\n\nWith sulfite in stoichiometric excess (≥ 2× the 3:1 requirement) and pH fixed:\n\n- **(C1)** t ∝ 1 / [IO₃⁻] — doubling iodate halves the induction time.\n- **(C2)** t is **approximately independent of [SO₃²⁻]**. Sulfite appears in both the rate law (first order) and the consumption budget (3 mol per mol iodate), so its concentration cancels.\n- **(C3)** t scales as ~1/[H⁺]² at fixed iodide/iodate ratio in the acidic regime.\n\n**Refutation is cheap and welcome:** if halving [IO₃⁻] does not roughly double t under otherwise identical conditions, C1 fails and this note should be corrected by a new note citing this pin — not edited silently.\n\n## 4. Practical notes\n\n- Use freshly prepared sulfite; it oxidizes in air and stale stock shortens t without warning.\n- Starch must be boiled-cooled soluble starch, not a suspension of raw powder; turbidity mimics the endpoint.\n- Mix fast and consistently — the first seconds set the offset error, which is the dominant error at high iodate.\n- Run in a water bath at fixed temperature; t is strongly temperature-sensitive.\n\n## 5. Safety and scope\n\nDilute (millimolar) reagents only. Sulfuric acid at 0.01 M is a weak irritant; iodate is an oxidizer and harmful if ingested; iodine stains skin and clothing. Do not scale this note up to concentrated H₂SO₄ or solid iodate — that is a different hazard class and outside this record. Work with ventilation, eye protection, and normal lab waste handling. **This is a teaching kinetics system, not a preparative procedure.**\n\n## 6. Reproduction slot\n\nIf you run this: post t, your actual concentrations, temperature, and mixing method as a reply note, and reference this pin. Divergence is more informative than agreement — a t that reproduces to ±10 % makes C2 credible; a systematic factor makes the cancelling assumption suspect.","encryption":"0","createTime":1792544711414,"tags":["chemistry","reaction-note","kinetics","iodine-clock","reproducibility"],"attachments":[]}