Is Na3PO4 acidic, basic, or neutral?

Sodium phosphate, with the chemical formula Na₃PO₄, is a common inorganic salt that often raises the question: is it acidic or basic? The clear answer is that Na₃PO₄ is a basic salt—and understanding why reveals important principles about acid-base chemistry.

Na₃PO₄ forms when phosphoric acid (H₃PO₄), a weak triprotic acid, reacts completely with sodium hydroxide (NaOH), a strong base. This neutralization reaction produces water and the salt Na₃PO₄. Because the salt comes from a weak acid and a strong base, it doesn’t stay neutral in water—it undergoes hydrolysis, making the solution alkaline (basic).

When Na₃PO₄ dissolves, it breaks into three Na⁺ ions and one PO₄³⁻ ion. The sodium ions (Na⁺) come from a strong base and do not react with water—they’re pH-neutral. But the phosphate ion (PO₄³⁻) is the conjugate base of a weak acid (HPO₄²⁻), so it reacts with water to produce hydroxide ions (OH⁻):

PO₄³⁻ + H₂O ⇌ HPO₄²⁻ + OH⁻

This release of OH⁻ ions raises the pH, typically to around 11–12, confirming the solution is strongly basic. In fact, if you dip red litmus paper into a Na₃PO₄ solution, it turns blue—a classic test for basicity.

To put this in context, compare Na₃PO₄ with other salts:

• KCl (from HCl + KOH): strong acid + strong base → neutral
• NH₄NO₃ (from HNO₃ + NH₃): strong acid + weak base → acidic
• NaCN (from HCN + NaOH): weak acid + strong base → basic
• Na₃PO₄ follows the same pattern as NaCN—weak acid + strong base = basic salt

It’s also worth noting that phosphoric acid has three protons, so it can form different salts: NaH₂PO₄ (acidic), Na₂HPO₄ (slightly basic), and Na₃PO₄ (strongly basic). Only when all three protons are replaced by sodium do you get the fully deprotonated PO₄³⁻ ion, which is the strongest base among them.

Because of its alkalinity, Na₃PO₄ is used in cleaning products, water softening, and as a food additive (E339) to control acidity. However, due to its high pH, it can be irritating to skin and eyes—so it’s handled with care in industrial settings.

In summary, Na₃PO₄ is not an acid—it’s a basic salt. Its alkaline behavior stems from the hydrolysis of the phosphate ion in water, a direct result of being formed from a weak acid and a strong base. This principle helps predict the pH of many salts, making it a cornerstone concept in general and analytical chemistry.