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Chemical affinity is the thermodynamic driving tendency of a specified chemical reaction, defined at fixed temperature and pressure aswhere $G$ is Gibbs energy, $\xi$ the extent of reaction, ${\nu }_i$ signed stoichiometric coefficients and ${\mu }_i$ chemical potentials.^[ref]

For the chosen forward direction, $dG=-\mathcal{A}d\xi$: positive chemical affinity favours a small positive advance. At an unconstrained equilibrium, $\mathcal{A}=0$. For a reaction quotient $Q$ and equilibrium constant $K$, $\mathcal{A}=RT\log (K/Q)$ under the usual activity conventions. This is a thermodynamic condition, not a rate prediction; a favourable reaction can remain slow because of an Gibbs energy of activation.

A single-pass flow-through leach test measures the dissolution of a solid while fresh solution continuously enters a reaction cell and the outlet solution is collected for analysis. Unlike a closed vessel, the cell continually exports dissolved constituents. A reported implementation uses a pump, a solution reservoir, a reaction cell and controlled temperature.^[ref]

At steady state, an elemental mass balance gives the normalized release ratewhere $Q$ is the volumetric volumetric flow rate, $c_e$ is the mass concentration of element $e$, $f_e$ is its initial mass fraction in the solid, and $S$ is the exposed surface area. For example, using litres per day, grams per litre and square metres gives grams of equivalent solid per square metre per day. This expression follows by dividing the exported elemental mass per day by $f_{e}S$.

Changing $Q/S$ changes the accumulation of dissolved material and therefore the chemical affinity for dissolution. A rate independent of further increases in $Q/S$ can help identify a dilute-solution regime. Different elemental release rates can also reveal incongruent dissolution or retention in secondary phases, so interpreting a release rate as whole-solid loss requires an explicit material model.