Over what pH range do you expect Ca3(PO4)2 to have its minimum solubility?

Name: Problem 17, Chapter 6: Modern analytical chemistry
Uploaded: 2020-04-08T11:30:05-08:00
Duration: 2 min 24 s
Channel: Aadit Sharma
Description: Over what pH range do you expect Ca3(PO4)2 to have its minimum solubility?

step 1 For the calculation of concentration of iron in saturated solution of calcium hydroxide, we have

Over what pH range do you expect Ca3(PO4)2 to have its...

Key Concepts

Solubility Product (Ksp)

The solubility product constant is a numerical value that reflects the equilibrium between a sparingly soluble salt and its ions in solution. In the context of precipitation reactions, the Ksp determines the degree to which a salt, such as a calcium phosphate compound, will dissolve under given conditions. When the ion product in a solution exceeds the Ksp of the salt, precipitation occurs. This equilibrium parameter is essential for understanding when and how a salt will become less soluble.

Acid–Base Equilibria

Acid–base equilibria describe the equilibrium between different protonated forms of an acid or its conjugate base in aqueous solution. This is particularly important for salts that involve polyprotic acids, where the pH of the solution alters the speciation of the anion. Understanding how pH shifts the equilibrium between various protonated states helps in predicting the solubility behavior of salts whose anions can accept or donate protons.

Phosphate Speciation

Phosphate is a polyvalent ion that can exist in multiple protonated forms depending on the pH of the solution. The distribution among these forms (such as H3PO4, H2PO4–, HPO4^2–, and PO4^3–) is dictated by its stepwise acid dissociation constants (pKa values). The solubility of a phosphate salt is most minimal when the form of the phosphate ion that pairs most favorably with the cation (typically the fully deprotonated form) predominates in solution.

pKa and pH Dependence

The pKa values of a polyprotic acid are critical in determining the pH ranges over which various species dominate in solution. For phosphate, the pKa values demarcate the transitions between the different protonation states. The pH range that favors the predominance of the anionic species that forms the insoluble salt is key for achieving minimum solubility. This concept is broadly applicable in controlling precipitation reactions where manipulating the pH can alter the ion speciation and, consequently, the solubility.

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