phosphoric acid strong or weak

2 min read 26-10-2024

Phosphoric Acid: Strong or Weak? Unmasking the Truth

Phosphoric acid (H₃PO₄) is a ubiquitous chemical found in everything from fertilizers to soft drinks. But when it comes to its strength, there's a common misconception: is it a strong acid or a weak one?

The answer, as with many things in chemistry, isn't black and white. It depends on how we define "strength".

Understanding Acid Strength

In chemistry, an acid's "strength" refers to its ability to donate protons (H+). Strong acids, like hydrochloric acid (HCl), completely dissociate in water, meaning they release all their protons, leading to a high concentration of H+ ions. This results in a low pH (indicating high acidity).

Weak acids, on the other hand, partially dissociate in water. They only release a portion of their protons, leading to a lower concentration of H+ ions and a higher pH compared to strong acids.

Phosphoric Acid: A Multifaceted Case

Now, let's get back to phosphoric acid. According to a study published in the Journal of Chemical Education by R. A. Day Jr. and J. A. Selbin, phosphoric acid is classified as a weak acid. This is because it doesn't completely dissociate in water. However, the story doesn't end there.

Phosphoric acid is a triprotic acid, meaning it has three acidic protons that can be donated. These protons are released in a stepwise manner, with each step having its own dissociation constant (Ka).

First Dissociation: H₃PO₄ ↔ H⁺ + H₂PO₄⁻ (Ka₁ = 7.5 × 10⁻³)
Second Dissociation: H₂PO₄⁻ ↔ H⁺ + HPO₄²⁻ (Ka₂ = 6.2 × 10⁻⁸)
Third Dissociation: HPO₄²⁻ ↔ H⁺ + PO₄³⁻ (Ka₃ = 4.8 × 10⁻¹³)

The decreasing values of Ka indicate that each subsequent dissociation becomes progressively less likely. This means that the first proton is most easily donated, making the first dissociation relatively strong. However, the subsequent dissociations are weaker.

In essence, phosphoric acid exhibits both weak and strong acidic characteristics, depending on which proton is being considered.

Real-world Implications

This multi-faceted nature of phosphoric acid has real-world implications. For instance, in fertilizers, the first dissociation is crucial for providing essential phosphate ions to plants. In soft drinks, the lower acidity of phosphoric acid compared to stronger acids like citric acid helps prevent damage to teeth.

Conclusion

Phosphoric acid is a weak acid overall, but its triprotic nature leads to a more complex picture. Understanding the stepwise dissociation of phosphoric acid and the relative strengths of each step is crucial for applications in various fields, from agriculture to food science.

References

Day, R. A. Jr., & Selbin, J. A. (1969). Theoretical Inorganic Chemistry. Journal of Chemical Education, 46(4), 235.

phosphoric acid strong or weak