How do they make disodium phosphate?

January 12, 2026 •

3 min read

Industrially, disodium phosphate is commonly produced via the controlled neutralization of phosphoric acid with a sodium alkali, an exothermic reaction that produces the salt and water. The method of how they make disodium phosphate is a fundamental process in industrial chemistry, essential for creating this versatile compound used across food, pharmaceutical, and water treatment sectors.

Quick Summary

Disodium phosphate is produced by neutralizing phosphoric acid with a sodium-based compound like sodium hydroxide or sodium carbonate. Raw materials often originate from phosphate rock, and the process involves purification and crystallization steps to ensure high purity for commercial applications.

Key Points

Basic Reaction: The core chemical reaction for producing disodium phosphate involves neutralizing phosphoric acid ($H_3PO_4$) with a sodium alkali, like sodium hydroxide ($NaOH$) or sodium carbonate ($Na_2CO_3$).
Two Neutralization Steps: Some industrial processes involve a two-step neutralization, first creating monosodium phosphate ($NaH_2PO_4$) before adding more sodium alkali to form disodium phosphate ($Na_2HPO_4$).
Raw Material Origin: For large-scale industrial production, the process typically begins with treating phosphate rock with sulfuric acid to extract the necessary phosphoric acid.
Purity is Key: Extensive purification steps are required during manufacturing, especially when starting with raw phosphate rock, to remove impurities such as calcium, iron, and aluminum.
pH Control: Careful monitoring and control of the reaction mixture's pH, often in the range of 8.0 to 9.5, is essential to ensure the correct sodium phosphate product is formed.
Crystallization and Drying: The final stage of manufacturing involves controlled crystallization to form pure hydrated disodium phosphate crystals, followed by drying to produce the desired solid or powder form.

Disodium phosphate, or disodium hydrogen phosphate ($Na_2HPO_4$), is a vital inorganic salt known for its applications as an emulsifier, buffer, and anti-caking agent in food, pharmaceuticals, and water treatment. Its production relies on controlled chemical reactions and purification techniques to meet the high-purity standards required for consumer products.

The Direct Neutralization Method

One of the most straightforward methods for producing disodium phosphate involves the neutralization of phosphoric acid ($H_3PO_4$) with a sodium-containing base. This is the process typically used for laboratory synthesis and often represents the core chemical reaction in larger industrial operations.

Using Sodium Hydroxide

The reaction between phosphoric acid and sodium hydroxide ($NaOH$) is a classic acid-base neutralization. To form disodium phosphate specifically, two moles of sodium hydroxide are required for every one mole of phosphoric acid. The balanced chemical equation is:

$H_3PO_4 + 2NaOH \rightarrow Na_2HPO_4 + 2H_2O$

This reaction is exothermic, meaning it releases heat. In a laboratory setting, careful addition and temperature control are necessary to manage the reaction and ensure safety.

Using Sodium Carbonate

Alternatively, sodium carbonate ($Na_2CO_3$) can also be used as the alkaline reactant. This method is also a neutralization reaction, but it produces carbon dioxide gas in addition to disodium phosphate and water. The equation is:

$H_3PO_4 + Na_2CO_3 \rightarrow Na_2HPO_4 + CO_2 + H_2O$

This reaction can be beneficial in certain industrial contexts, but the release of carbon dioxide must be managed. Both the hydroxide and carbonate methods are effective, with the choice depending on factors like cost, desired reaction speed, and ease of handling.

Industrial Production from Phosphate Rock

For large-scale industrial manufacturing, the process begins with naturally occurring phosphate rock. This method is more complex and involves multiple stages to purify the product and remove impurities like calcium, iron, and aluminum.

Step-by-step Industrial Synthesis

Phosphate Extraction: Phosphate rock (primarily calcium phosphate) is first treated with sulfuric acid ($H_2SO_4$) to produce a phosphoric acid solution. This step also precipitates calcium sulfate (gypsum), which can then be separated.
Impurity Removal: The crude phosphoric acid solution contains various impurities that must be removed. Purification techniques, such as further reactions and filtration, are employed to remove metal impurities like iron and aluminum, which would otherwise contaminate the final product.
Intermediate Neutralization: In some processes, the purified phosphoric acid is partially neutralized to form monosodium phosphate ($NaH_2PO_4$) before proceeding. A common method is reacting with a sodium compound like sodium bisulfate.
Final Neutralization: The monosodium phosphate solution is then further neutralized with another sodium alkali (like sodium hydroxide) to achieve the target pH range, typically between 8.0 and 9.5, which produces the disodium phosphate.
Crystallization and Drying: The final step involves controlled cooling of the disodium phosphate solution, which causes pure, hydrated crystals to form. These crystals are then separated, washed, and dried to obtain the final product, which can be anhydrous or various hydrated forms.

Comparison of Production Methods


Feature	Direct Neutralization (Lab/Small-Scale)	Industrial Production (Phosphate Rock)
Raw Materials	High-purity phosphoric acid, sodium hydroxide or carbonate	Phosphate rock, sulfuric acid, sodium alkali
Purity Control	Relatively simple, depending on reagent purity.	Complex multi-stage purification required to remove impurities.
Scale	Small batches for lab or specific applications.	Large-scale, continuous production for commercial use.
Steps	One main reaction step, followed by crystallization.	Multiple steps: acid extraction, purification, neutralization, crystallization.
Cost	Reagent cost can be higher for pure raw materials.	Lower raw material cost (phosphate rock), but higher capital investment for purification equipment.
Byproducts	Water, or water and carbon dioxide.	Gypsum, purified impurities, water, etc.

Conclusion

Whether through the direct neutralization of purified reagents or the more complex industrial method beginning with phosphate rock, the fundamental process of how they make disodium phosphate relies on careful control of pH and reaction conditions. The choice of manufacturing process is dictated by the desired scale, purity requirements, and cost considerations. Both pathways ultimately yield a highly versatile and commercially valuable product, essential for countless applications in modern industry. For further reading on its supply chain, consult documents like those from the U.S. Environmental Protection Agency.

Uses Beyond Food

While well-known as a food additive, disodium phosphate's production also serves other critical industries. Its ability to buffer pH makes it valuable in water treatment for corrosion control, and in pharmaceuticals for maintaining the stability of solutions. It is also found in some detergents and personal care products.

Frequently Asked Questions

The primary chemical reaction involves neutralizing phosphoric acid ($H_3PO_4$) with a sodium-containing base. For example, using sodium hydroxide ($NaOH$) the reaction is $H_3PO_4 + 2NaOH \rightarrow Na_2HPO_4 + 2H_2O$.

In large-scale industrial production, the raw materials are derived from phosphate rock. This rock is treated with sulfuric acid to produce the phosphoric acid needed for the reaction with a sodium source.

High purity is achieved through several steps, including initial purification of the phosphoric acid to remove metal impurities, careful control of pH during neutralization, and controlled crystallization and washing of the final product.

Controlling the pH is crucial during the neutralization reaction. For disodium phosphate, the pH is typically maintained in a moderately alkaline range (8.0–9.5) to ensure that two of the three acidic hydrogen atoms in phosphoric acid are replaced by sodium ions.

After the neutralization reaction, the disodium phosphate solution is evaporated, concentrated, and cooled to induce crystallization. The resulting hydrated crystals are then dried to produce a solid, anhydrous, or powder form.

Yes, aside from sodium hydroxide, sodium carbonate ($Na_2CO_3$) is another common sodium alkali used in the manufacturing process. It produces disodium phosphate, water, and carbon dioxide.

Disodium phosphate is highly versatile due to its properties as a buffering agent, emulsifier, and sequestrant. This makes it valuable in food for regulating pH and stabilizing dairy, in water treatment for corrosion control, and in pharmaceuticals.