Chapter 5: Ionic Equilibrium and Solubility
Introduction
In this Chapter, we will discuss how precipitation and solubility equilibrium involve the delicate balance between dissolved ions in solution and their solid counterparts. When the product of ion concentrations exceeds the solubility product constant K sp , a precipitate forms.
Kidney stones are a painful result of solubility equilibrium gone awry. They form when the concentration of ions like calcium (Ca 2 + ) and oxalate (C 2 O 4 2 - ) in urine surpasses the proportions given from the equilibrium constant Ksp, which leads to the precipitation of solid calcium oxalate in the bladder:
Ca 2 + (aq) + C 2 O 4 2 - (aq) ⇌ CaC2O 4 (s)
Figure 5.1: Kidney stone production in the bladder (left, CC BY 3.0; 2014, Bruce Blaus via Medical Gallery of Blausen Medical , WikiJournal of Medicine). Kidney stones are an incredibly painful ailment due to the crystalline structure of calcium oxalate (right). The jagged, sharp edges of the crystal damage the sensitive tissues of the urinary tract, causing inflammation, bleeding, and other side effects. (CC BY 3.0; 2012, Kempf EK via WikiMedia Commons)
Understanding solubility equilibrium allows doctors to recommend dietary adjustments, such as reducing oxalate intake (e.g., spinach and nuts) or increasing water consumption to dilute urine and reduce ion concentration.
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- 5.1: Solubility as a Physical Property
- this article discusses ionic compounds that are difficult to dissolve; they are considered "slightly soluble" or "almost insoluble." Solubility product constants ( Ksq ) are given to those solutes, and these constants can be used to find the molar solubility of the compounds that make the solute. This relationship also facilitates finding the Ksq of a slightly soluble solute from its solubility.
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- 5.2: Precipitation
- A mixture of metal ions in a solution can be separated by precipitation with select anions. When a metal ion or a group of metal ions form insoluble salts with a particular anion, they can be separated from others by precipitation. We can also separate the anions by precipitating them with appropriate metal ions.