Chapter 15: Solubility, Precipitation, and Complexation

The solubility product constant, or Ksp, represents the equilibrium between a solid ionic salt and its dissolved ions in solution, and serves as the quantitative foundation for predicting whether a compound will dissolve or precipitate under given conditions. Understanding the distinction between Ksp as a temperature-dependent constant and solubility as the actual concentration of dissolved material is essential, particularly when comparing the relative solubility of different salts with identical stoichiometric ratios. The chapter explores multiple factors that disrupt solubility equilibria and shift the position of dissolution-precipitation balance. The common ion effect demonstrates how introducing an ion already present in the equilibrium suppresses further dissolution of the solid phase. The pH-dependence of salts containing basic anions, such as carbonates or hydroxides, reveals that increased acidity drives dissolution by converting the basic anion into its conjugate weak acid form and removing it from the equilibrium expression. Complex ion formation provides another mechanism for increasing solubility when Lewis bases coordinate to metal cations, with the formation constant quantifying the stability of these complexes. Precipitation reactions are predicted using the reaction quotient Q, where Q greater than Ksp indicates a supersaturated solution that will precipitate spontaneously to reach equilibrium, while Q less than Ksp indicates an unsaturated solution. Selective precipitation exploits differences in Ksp values to separate metal cations from mixtures by controlling the timing of precipitation. The chapter applies these foundational concepts to ocean acidification, where rising atmospheric carbon dioxide dissolves in seawater to form carbonic acid and shift the carbonate equilibrium toward higher hydrogen ion concentrations and lower carbonate ion concentrations. This shift threatens calcifying marine organisms such as corals and mollusks that depend on maintaining supersaturation conditions with respect to calcium carbonate minerals to construct their shells and skeletal structures.