Precipitation of Ions

If the solutions of two ionic compounds are mixed, such as sodium iodide and lead(II) nitrate, the cations from one solution can combine with the anions from the other.

As one of the cross products—sodium nitrate— is water soluble, sodium and nitrate ions continue to remain in solution, while lead and iodide ions form a lead iodide precipitate.

The reaction quotient, Q, of the dissolution reaction is equal to the product of the concentrations of lead and iodide ions.

Unlike the solubility product K_sp, the reaction quotient involves ion concentrations at any stage, not just at equilibrium.

The values of Q and K_sp can be compared to predict if a precipitation reaction will occur.

Consider a dropwise addition of sodium iodide solution to the lead nitrate solution.

In the beginning, Q is less than K_sp, and both lead and iodide ions are in solution with the sodium and nitrate ions. This is an unsaturated solution.

As more sodium iodide is added, the concentration of iodide ions continues to increase. The reaction has reached equilibrium when Q = K_sp. At this stage, a small amount of solid lead iodide is in dynamic equilibrium with the ions, forming a saturated solution.

Further addition of sodium iodide makes Q > K_sp, and the reaction shifts towards the precipitate. This is a supersaturated solution, where precipitation continues until the ion concentrations are lowered to their equilibrium values.

For instance, suppose the mixing of sodium iodide and lead(II) nitrate solutions results in a solution that contains 1.6 × 10⁻⁴ M lead ions and 4.0 × 10⁻⁴ M iodide ions.

Here, Q = 2.6 × 10⁻¹¹, while K_sp for lead iodide is 1.4 × 10⁻⁸. Because Q < K_sp, lead iodide will not precipitate.

Predicting precipitation reactions can be very useful during the separation of ionic compounds.

Consider a solution with two metal ions— lead(II) and copper(II). If hydrochloric acid is added to this solution, lead(II) chloride precipitates because it has a small K^sp, while copper remains in solution as copper(II) chloride is highly soluble in water.

This technique is called selective precipitation.