Which is a form of (24) expressed  as a function of volumetric quantities. 

Determination of \(S(t,t')\)

To be revised
Paraphrasing Neuman 1969. Suppose that a tracer stream is injected at the reactor inlet starting at time \(t_{in}\).  For \(t\ >t_{in}\), let the concentration at the inlet be \(C_0\). The concentration within the reactor is given by: