Understanding the Basic Nature of Na2HPO4 Solutions

The aqueous solution of sodium hydrogen phosphate (Na2HPO4) is inherently basic due to the presence of the hydrogen phosphate ion (HPO42-). This article delves into the detailed reasons why this occurs, including the principles of dissociation in water, the basic nature of the hydrogen phosphate ion, an increase in OH- concentration, and the buffering action of Na2HPO4.

Dissociation in Water

When Na2HPO4 dissolves in water, it dissociates into sodium ions (Na ) and hydrogen phosphate ions (HPO42-).

Na2HPO4 rarr; 2 Na HPO42-

Basic Nature of HPO42-

The hydrogen phosphate ion (HPO42-) can act as a weak base. This ion has the ability to accept protons (H ) from water, leading to the formation of phosphate ions (PO43-) and hydroxide ions (OH-). This reaction can be represented as follows:

HPO42- H2O leftrightarrow; PO43- H OH-

The presence of hydroxide ions (OH-) increases the pH of the solution, making it basic.

Increase in OH- Concentration

The reaction of the HPO42- ion with water not only leads to the formation of hydroxide ions but also contributes to the overall basic nature of the solution. The increase in OH- concentration is a key factor in the basicity of Na2HPO4 solutions.

Buffering Action

Na2HPO4 also exhibits a buffering effect, which helps to maintain a stable pH in a solution. This buffering action can involve the reaction of Na2HPO4 with acids or bases to resist changes in pH. However, its primary effect is to render the solution basic, primarily due to the generation of hydroxide ions.

Nature of the Salt Solution

The salt Na2HPO4 is composed of a strong base (NaOH) and a weak acid (H3PO4). The conjugate base of a strong base (Na ) does not undergo hydrolysis, while the conjugate base of a weak acid (HPO42-) is strong and undergoes anionic hydrolysis, liberating OH- ions. This hydrolysis makes the solution basic.

Abstracting Protons from Water Molecules

A compound can act as a base in an aqueous solution if it has the ability to attract protons by having a negative charge or a relatively low-energy site on the molecule that can serve as an attachment site for protons from water molecules.

Na2HPO4 fits this criterion because the sodium ions fully dissociate in dilute or moderately concentrated solutions, leaving a doubly negatively charged anion with exposed oxygen atoms that can serve as attachment sites for protons.

Keywords: aqueous solution, sodium hydrogen phosphate, pH buffering