The indicator reaction will be slow in comparison to the non-observable reaction if t s 2 t s 1 . Consequently we take δ S 1 in the slow regime. Since t s 1 is now fast relative to t s 2 , we rescale the indicator reaction mass action ). The solid black curve (barely visible) is the numerical solution to the mass action equations ( ), and the dashed/dotted red curve is the numerical solution to the composite solution . In panel (a), the initial conditions (without units) are: equations with respect to T :