Mass transfer of an analyte begins when a solid, with the analyte
bounded to it, is suddenly immersed in a solvent. This phenomenon
can be described well by mass transfer kinetics. The
mass transfer of analytes from the core of the solid into the bulk
of the solution occurs through two main steps. The first step is penetration
of solvent into the solid for dissolution of extractable substances,
and the second one is a diffusion transfer of extractable
substances from inside the solid into the bulk of liquid extract.
The second step is usually much slower and regarded as the rate
limiting step for most solid–liquid extraction systems (Cheung,
Siu, & Wu, 2012). Based on these assumptions, the rate of mass
transfer can be approximately predicted by appropriate mathematical
solutions of the Fick’s equation for simplified unsteady state
second-order diffusion (Cacace & Mazza, 2003). From Fick’s second
law, Crank (1975) derived the applied diffusion model for solid–
liquid extraction