The Steady State Mass Transfer Equation states that the mass transfer rate (˙m˙) is proportional to the concentration gradient (dc/dx) and the cross-sectional area ($A$) perpendicular to the direction of mass transfer. The Steady State Mass Transfer Equation describes the rate of mass transfer through a medium under steady-state conditions.  It’s commonly used in chemical engineering, environmental engineering, and other fields to analyze processes involving the transfer of substances such as heat, gases, or solutes. Mathematically, it can be expressed as:

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Where:

• ˙m˙ is the mass transfer rate,
• $D$ is the mass diffusivity or coefficient of diffusion,
• $A$ is the cross-sectional area perpendicular to the direction of mass transfer,
• dc/dx​ is the concentration gradient.