Thus the appearance of the dark blue colour signals that all of the thiosulfate (and a corresponding amount of H2O2) has been used up. (The acetate buffer keeps the hydrogen ion concentration nearly constant.)
The amount of thiosulfate is intentionally made small, so that no more than 10% of the H2O2 reacts. Thus, during the course of the reaction the concentrations of H2O2, I, and H remain nearly constant, approximately equal to their initial values. This allows us to study the effects of concentration by varying the initial concentration of one or more reactants and measuring the time required for a fixed amount of thiosulfate to react. The faster the reaction, the shorter the reaction time.