Incongruent dissolution

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.