LOW-T TOPIC 4: SPECIATION, SOLUBILITY, Ksp & SI

peciation definition

distribution of an element among different aqueous chemical forms

Why speciation matters

toxicity and mobility depend on species not total concentration

Speciation controls

pH Eh ligands temperature pressure

Why assumptions are made

equilibrium assumed to make complex systems mathematically tractable

Speciation diagrams purpose

identify dominant species not exclusive species

Solubility definition

maximum dissolved amount under given conditions

Solubility product Ksp

equilibrium constant for mineral dissolution

Why activities used

ionic interactions reduce effective concentration

Activity coefficient meaning

γ corrects for non-ideal behaviour

Ionic strength effect

increasing ionic strength lowers activity coefficients

Saturation index definition

SI = log(IAP / Ksp)

SI interpretation

SI < 0 dissolution SI > 0 precipitation

Carbonate system importance

CO₂ controls pH and solubility

Fun exam fact

many waters are supersaturated yet do not precipitate

Diagram card – Speciation

draw pH vs species dominance diagram

Calculation card – SI

plug values into SI equation and interpret sign