Chemical Kinetics of the Persulfate-Iodide Clock Reaction

The simple bimolecular rate law r = k·[S₂O₈²⁻]·[I⁻] assumes that ionic strength is held constant and that reactant concentrations are low enough (< 0.1 M) for Debye-Hückel screening to apply. At very high ionic strength (I > 1.0 M), specific ion interaction parameters and ion-pairing (e.g., K⁺·S₂O₈²⁻) produce non-linear deviations from the Brønsted-Bjerrum equation. The physical simulation model computes the persulfate-iodide oxidation rate at 25.00 °C and ionic strength I = 0.20 M governed by r₀ = −d[S₂O₈²⁻]/dt = k·[S₂O₈²⁻]₀·[I⁻]₀, where the true second-order rate constant is k = 0.02200 L·mol⁻¹·s⁻¹. Fixed experimental conditions: excess iodide [I⁻]₀ = 0.0800 M and limiting thiosulfate Δ[S₂O₃²⁻]₀ = 0.00100 M. Under constant iodide, the reaction behaves with pseudo-first-order kinetics: r₀ = k'·[S₂O₈²⁻]₀ with k' = k·[I⁻]₀ = 0.02200 × 0.0800 = 0.001760 s⁻¹. The clock time to the sharp starch-triiodide flash transition satisfies Δt = Δ[S₂O₃²⁻]₀ / (2·r₀) = 0.000500 / r₀. For initial persulfate concentrations [S₂O₈²⁻]₀ from 10.0 mM to 60.0 mM (0.010 M to 0.060 M), initial rates span 1.760 × 10⁻⁵ M/s (Δt = 28.409 s) to 1.056 × 10⁻⁴ M/s (Δt = 4.735 s). Linear regression of r₀ × 10⁵ against [S₂O₈²⁻]₀ (in mM) yields slope m = 0.1760 (10⁻² s⁻¹), isolating k' = 0.001760 s⁻¹ and k = k' / [I⁻]₀ = 0.0220 L·mol⁻¹·s⁻¹ with R² = 1.00000; the multi-point initial rates formulation completely eliminates product inhibition and reverse reaction biases.