S₂O₈^2- + 2I^- --> 2SO₄^2- + I₂

under pseudo first order conditions is analyzed using the conventional method (Method I), Guggenheim method (Method II) and a similar method proposed by Kezdy et al, Mangelsdorf and Swinborne (Method III). The latter methods are known as time lag methods and they do not require a knowledge of initial and infinite time measurements for the determination of rate constants. These methods are also useful for the prediction of infinite time readings. All the three methods provide results of similar accuracy, if accurate initial and final readins are available. Our present work clearly brings out the approximation involved in assuming a measurement made at finite time as "infinite time reading" in Method I. Between the two time lag methods that we have investigated, Method III is easier to implement.