Phosphoric Acid and Sodium Hydroxide Reaction Explained

This article walks through the reaction of phosphoric acid and sodium hydroxide, how they react at their molecular structures, and their applications in the current industry.

Different but also Same:

Phosphoric acid H₃PO₄ and sodium hydroxide NaOH both have distinctly different molecular structures, but they do share some common features that make them suitable for acid-base reactions. The structural and chemical properties of each contribute significantly to their reactivity between them.

First, phosphoric acid is a triprotic acid (can donate three protons) in an aqueous solution. Its molecule consists of a central phosphorus atom, which is further bonded to four oxygen atoms, making a tetrahedral geometry. Out of these four oxygens, three oxygens are bonded to hydrogen atoms.

This bonding makes the molecule acidic by providing three ionizable hydrogen atoms. However, the one oxygen in this structure forms a double bond with the phosphorus atom. This stabilizes the structure and adds resonance stability.

This tetrahedral arrangement around phosphorus allows for polarization and stabilization of the negatively charged oxygen atoms when the phosphoric acid dissociates around it. When this happens, it undergoes stepwise ionization, first into dihydrogen phosphate H₂PO₄^- then to hydrogen phosphate HPO₄^2- and finally into PO₄^3- ions making it a moderate acid.

Next is the sodium hydroxide (a.k.a lye), which has a much simpler structure as compared to its counterpart. Lye is composed of sodium Na⁺ and hydroxide OH^- ions which are strongly held together by strong ionic bonds. When introduced in an aqueous solution, it completely into these ions making it a strong base.

NaOH →  Na⁺ + OH^-

The complete dissociation of Na⁺ and OH^- makes them highly reactive and readily available to accept protons from acidic substances. Both molecules of phosphoric acid and sodium hydroxide are highly polar; phosphorus acid has a polar structure due to the highly electronegative oxygen atoms around the central phosphorus atom, and sodium hydroxide has an ionic nature. Due to their complementary structures, their reaction in water happens quickly.

H₃PO₄ + OH^- → H₂PO₄^- + H₂O

The first step is the donation of protons from phosphoric acid; when phosphoric acid encounters sodium hydroxide, the OH^- from NaOH quickly accepts these protons from phosphoric acid. This neutralizes the acid in an acid-base fashion, during which each proton donation corresponds to forming a different phosphate anion and a molecule of water.

Further NaOH molecules keep on entering with H₂PO₄^- to ultimately form the trivalent phosphate anion PO₄^3-. This happens pretty quickly due to the excess of sodium hydroxide ions with a strong ionic bond nature which ensures that the OH^- ions remain highly reactive with this acid. This gradual removal of protons leads to the formation of successively less acidic anions. The final reaction between phosphoric acid and sodium hydroxide is shown below:

H₃PO₄ + NaOH → NaH₂PO₄ + H₂O

Applications of the Reaction:

The reaction between phosphoric acid and sodium hydroxide also has strong industrial applications with the final product of Sodium phosphate, a few of which are as follows:

1. Sodium phosphate is a powerful builder in detergents and cleaning agents where it significantly enhances the performance of surfactants used in the process. 

2. It is used in water treatment to soften water, reducing scale formation, and is also a key nutrient for plants so is used as a supplement in fertilizers.

3. Also used as phosphate buffers, which are essential in biochemistry and molecular biology for maintaining pH levels in solutions. Common use cases include maintaining pH for enzyme activity and biological stability. 

4. The phosphates generated here also serve as an acidity regulator, stabilizer, and emulsifier in various food products, hence important to the food industry.

5. This salt is also used in lab reagents and dental treatments and as a component in intravenous fluids to maintain electrolyte balance.

The use of sodium phosphate is very significant in the common industrial landscape, and if your business is also involved in similar chemicals, then we recommend getting yourself acquainted with the Echemi marketplace, a global platform for the procurement of such chemicals. Certified suppliers and direct communication for a smooth procurement experience are what make Echemi special in the crowd.

Conclusion:

The reaction of phosphoric acid and sodium hydroxide happens in a classic acid-base nebulization reaction, and their salt is quite important in the current industry.