As a closing remark, we note that it is not possible for the completion of the indicator reaction to occur before the completion of the non-observable reaction. Thus, when t s2 t s1 , the timescale t s1 gives a reasonable approximation to the completion time of both the indicator reaction and non-observable reaction. Although t s2 has no exact physical interpretation in this situation, we will demonstrate that it is useful in a brute-force scaling analysis, and that more meaningful and physically relevant timescales can be obtained by considering how the kinetics change when t s2 t s1 or t s2 t s1 . This concept will be made clear in the upcoming sections.