Accounting for pH

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Page ID: 70861

Thomas Wenzel
Analytical Sciences Digital Library

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b) The solution is buffered at a pH of 3 and you will need to consider the protonation of phosphate that can occur.

What will happen to the solubility of lead(II)phosphate at this pH?

Allow the students a few minutes to consider this question. They should realize that as phosphate is protonated, the equilibrium will shift towards dissociated ions and thus the solubility is increased.

This is a chance to take some time to discuss the impact that acid rain has on metals in the environment. A good example is the increased solubility of aluminum leading to the precipitation of aluminum(III)hydroxide in fish gills.

What two expressions equate the species in solution to the solubility of the solid?

Groups usually realize that we will need an expression in terms of lead ion and phosphate species.

Where did every mole of phosphate (protonated and deprotonated) come from? What is the S expression for the phosphate ion?

Students should realize that every mole of phosphate must have come from the original lead(II)phosphate and that the S expression must be modified to include the total concentration of phosphate species present. They also realize that since we know the pH, and since we are considering total phosphate species, that an α-value is likely used somehow to solve the problem.

How would the α-value for PO₄³^- help? What is the modified K_sp expression that includes the α-value?

Allow students a few minutes to consider how to factor the α-value into the S and Ksp expression. Summarize the final Ksp expression that includes the α-value at the board.

What is the value of αPO₄³^-?

Have students write the correct expression for αPO₄³^-. It may take them about five minutes to all have the correct expression. In order to save time, once they have written the correct expression give them the exact value of αPO₄³^- instead of having them evaluate it.

What is the value of S?

Students should all be able to calculate S. Discuss how the magnitude of S compares to that of part (a). What is the concentration of free lead(II)?