Target Concept: Solubility Product Constant (Ksp), Precipitation reactions, Solutions vs. Solid Mixtures.
Welcome to the Flint Water Treatment Plant, April 2014. To save money, the city has just switched its municipal water supply from Lake Huron to the Flint River. However, the Flint River water is highly corrosive due to high chloride levels. Historically, water treatment plants add a chemical called "orthophosphate" to the water.
This acts as a corrosion inhibitor by reacting with the aging lead pipes to form a solid, insoluble mineral barrier (a passivation layer of lead(II) phosphate) on the inside of the pipes. Because the city failed to add this inhibitor, the protective scale has dissolved. Neurotoxic lead is currently leaching directly into the aqueous drinking water supply.
Inquiry Challenge: Act as a municipal water chemist. Calculate and dose the exact minimum molarity of orthophosphate needed to trigger a precipitation reaction, forming the protective solid scale and dropping the aqueous lead concentration to safe parts-per-billion (ppb) levels (≤ 15 ppb) without wasting the budget.
Controls total injected orthophosphate [PO₄]total
Affects the ratio of active [PO₄³⁻] ions
Pipe Cross-Section
Particle Level (Pipe Wall)