An iodine clock is the basis for studying the kinetics of the fast reaction between iodine and ascorbic acid in aqueous solution. UV–visible absorbance, where the molar absorptivity of the triiodide ion is high, is equated with the total concentration of iodine, through the equilibrium, I2 + I− ⇌ I3−, established when the iodine clock is concluding its steady-state titration of ascorbic acid. Persulfate ion is the primary oxidant catalyzed by the iodide ion, which produces the iodine titrant and is recycled. A method testing the second-order rate equation, R1 = k[iodine][ascorbic acid], uses the coefficient of variation, CV. This locates equivalence, enabling [ascorbic acid] to be measured and contributes to a procedure where rate of reaction, R1, at different concentrations of iodine and ascorbic acid determines an average value of the overall rate constant, k. In combination with results from another source, the rate constants of the two individual forms of iodine reacting with ascorbic acid also are determined and show that the triiodide ion is five times more active than I2 at an ambient temperature of 20 °C.