 Dilution function for a melting saline Stefan front

For stationary liquid below a salt-free solid and an upward melting interface $a(t)=2A\sqrt{Dt}$, the complementary error function salt profile and flux balance $D{C}_z(a)=-\dot{a}{C}_i$ give the displayed concentration ratio. For $A>0$ it lies below one. At large $A$ it behaves as $e^{-A^2}/(2\sqrt{\pi }A)$.