Why H3PO3 Is Not a Tribasic Acid

Phosphorous acid (H3PO3) is a well-known substance in chemical research and industrial applications. It is commonly referred to as dihydrogen phosphite. Despite its chemical formula suggesting the possibility of multiple hydrogen ion donations, H3PO3 is not classified as a tribasic acid. This article delves into the structural and chemical properties of H3PO3 to explain why it behaves as a dibasic acid.

Structure and Chemical Formula

H3PO3 has a distinct chemical structure. Its formula can be simplified as HPO(OH)2. In this structure, one hydrogen atom is bonded to the phosphorus atom, while the other four hydrogens are part of hydroxyl (–OH) groups. This unique arrangement plays a crucial role in its acidic behavior.

Dissociation Process

When H3PO3 is dissolved in water, it donates protons through a series of dissociation steps, but not up to three as in a tribasic acid:

First Dissociation:
H3PO3 rightleftharpoons H^ H2PO3^-

Second Dissociation:
H2PO3^- rightleftharpoons H^ HPO3^{2-}

No Further Dissociation:
HPO3^{2-} is not dissociable.

These reactions illustrate that H3PO3 can donate only two protons, making it a dibasic acid.

Comparison with Phosphoric Acid (H3PO4)

Phosphoric acid, H3PO4, serves as a clear example of a tribasic acid. Unlike H3PO3, it contains three carboxylic acid groups (–OH) that can donate protons through successive dissociations:

First Dissociation:
H3PO4 rightleftharpoons H^ H2PO4^-

Second Dissociation:
H2PO4^- rightleftharpoons H^ HPO4^{2-}

Third Dissociation:
HPO4^{2-} rightleftharpoons H^ PO4^{3-}

This spectrum of behavior demonstrates the difference in basicity between H3PO3 and H3PO4.

Why Can't We Judged by OH Groups Alone?

The classification of an acid as tribasic or dibasic cannot be determined merely by the number of hydrogen atoms in the molecule. It is essential to examine the chemical structure and the ease with which hydrogen atoms can be donated. For H3PO3, the stability of the phosphorus-hydrogen bond (P-H) compared to the hydrogen-oxygen (H-O) bonds plays a significant role:

The bond between phosphorus and hydrogen is very stable.

The bond between oxygen and hydrogen is more unstable, making it easier to break and release hydrogen ions (H ).

Therefore, when energy is applied, H3PO3 can only release two hydrogen ions, confirming its status as a dibasic acid rather than a tribasic acid.

Conclusion

From the above discussion, it is clear that H3PO3 is classified as a dibasic acid because it can donate only two protons, not three. This characteristic is determined by the specific chemical structure and the energy required to break the bonds within the molecule. Understanding these principles is crucial for accurate classification and application of acids in various chemical processes.