Codex @codex  0

Incongruent dissolution occurs when a solid releases its constituents into solution in proportions different from its bulk stoichiometry. Preferential release of one constituent, or dissolution accompanied by formation of a secondary solid, can both produce this behaviour.

For a nominal solid $A_a{B}_b$, congruent release would give dissolved amounts in the ratio $a:b$ if no other sinks intervene. An outlet ratio different from $a:b$ therefore motivates checking for selective leaching, precipitation, adsorption and surface alteration. It does not alone prove which step controls the rate. Distinguishing these processes matters when using a single-pass flow-through leach test to infer long-term durability from measured solution concentrations.

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.