Phosphoric acid is a weak acid, and its basicity is __________.

Phosphoric acid (H₃PO₄) is an oxyacid of phosphorus and is classified as a weak acid because it does not completely dissociate in water. The term basicity of an acid refers to the number of hydrogen ions (H⁺) that can be donated by one molecule of the acid during complete ionization.

In the structure of phosphoric acid, there are three hydrogen atoms attached to oxygen atoms as hydroxyl groups (–OH). Each of these hydrogens is capable of being released as a proton under suitable conditions, which makes phosphoric acid tribasic, with a basicity of 3.

The ionization of phosphoric acid occurs in three steps:

1. 
   

   First ionization:
   H₃PO₄ ⇌ H⁺ + H₂PO₄⁻
   This is the strongest dissociation among the three.

   
   


2. 
   

   Second ionization:
   H₂PO₄⁻ ⇌ H⁺ + HPO₄²⁻

   
   


3. 
   

   Third ionization:
   HPO₄²⁻ ⇌ H⁺ + PO₄³⁻

   
   

Each step becomes progressively weaker because it is more difficult to remove a positively charged proton from an increasingly negative ion. This is why phosphoric acid is weak overall, even though it can release three protons.

The three stages of ionization are reflected in its three acid dissociation constants (Ka₁, Ka₂, Ka₃), which decrease in value sequentially, confirming that the acid’s strength reduces at each step.

Phosphoric acid is widely used in fertilizers, rust removal, and the food industry (particularly in soft drinks) due to its mild acidity and non-volatile nature.

Thus, the basicity of phosphoric acid is 3, corresponding to its three replaceable hydrogen atoms, each capable of producing one hydrogen ion in solution.