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Magazine Chapter 15: Problem 4
Why is it impossible to titrate all three protons of phosphoric acid in aqueous solution?
Short Answer
Expert verified
The third proton's low dissociation constant makes it impractical to titrate in an aqueous solution.
Step by step solution
01
Chemical Structure of Phosphoric Acid
Phosphoric acid (H鈧働O鈧�) is a triprotic acid, meaning it has three hydrogen ions (protons) that can dissociate. Each dissociation happens in a stepwise manner, forming H鈧侾O鈧勨伝, HPO鈧劼测伝, and PO鈧劼斥伝 respectively.
02
Understanding Equilibrium Constants
Each proton dissociation has its own equilibrium constant: - First dissociation: \( K_{a1} = 7.5 imes 10^{-3} \) for the reaction \( H_3PO_4
ightarrow H^+ + H_2PO_4^- \)- Second dissociation: \( K_{a2} = 6.2 imes 10^{-8} \) for the reaction \( H_2PO_4^-
ightarrow H^+ + HPO_4^{2-} \)- Third dissociation: \( K_{a3} = 4.8 imes 10^{-13} \) for the reaction \( HPO_4^{2-}
ightarrow H^+ + PO_4^{3-} \).
03
Comparing Equilibrium Constants
The equilibrium constants \( K_{a1}, K_{a2}, \) and \( K_{a3} \) show massively decreasing values. The first dissociation has a significantly stronger tendency to release protons compared to the third. This difference makes it progressively more difficult to dissociate the second and third protons under normal conditions.
04
Role of pH in Titration
Aqueous solutions have a pH range of practical use. The third dissociation constant \( K_{a3} \) is so low, it requires a pH far above the natural range of aqueous solutions to deprotonate the third proton effectively. This makes complete titration of phosphoric acid's third proton impractical.
05
Conclusion
Due to the very low value of \( K_{a3} \), the third dissociation happens at a pH level that is difficult to achieve using standard aqueous titration techniques. Thus, we cannot fully titrate all three protons of phosphoric acid in an aqueous solution.
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
Equilibrium Constants
Equilibrium constants are vital in understanding the behavior of acids in solution. They measure the extent of dissociation of an acid into its ions at equilibrium. For each step of dissociation, a unique equilibrium constant (usually denoted as \( K_a \)) is defined. This constant indicates how readily that dissociation occurs.
- A larger \( K_a \) value suggests a stronger tendency for dissociation, meaning the acid is stronger at that dissociation step.
- Conversely, a smaller \( K_a \) value indicates weaker dissociation.
- First dissociation: \( K_{a1} = 7.5 \times 10^{-3} \)
- Second dissociation: \( K_{a2} = 6.2 \times 10^{-8} \)
- Third dissociation: \( K_{a3} = 4.8 \times 10^{-13} \)
Phosphoric Acid
Phosphoric acid, chemically known as H鈧働O鈧�, is a common triprotic acid. It has three hydrogen atoms that can each be released as protons. This makes it interesting in studies of acid behavior and titration.
- Upon its first dissociation, H鈧働O鈧� loses one proton to form dihydrogen phosphate (H鈧侾O鈧勨伝).
- In the second dissociation, it becomes hydrogen phosphate (HPO鈧劼测伝).
- Finally, it can lose a third proton to form phosphate (PO鈧劼斥伝).
Triprotic Acid
A triprotic acid, like phosphoric acid, can release three protons, each in separate steps. This characteristic gives rise to a sequence of equilibria, each governed by its equilibrium constant. Understanding triprotic acids is essential when considering their behavior during titration.
Some features of triprotic acids include:
Some features of triprotic acids include:
- Three potential dissociation stages, each with decreasing favorability.
- A complex titration curve with multiple equivalence points.
- Different pH values at each endpoint, with the last being exceptionally high.
