Principles of Phosphorous Acid Production from Sodium Phosphite

Phosphorous acid production processes vary widely, and depending on the raw materials, the purity, odor, and color of the finished product can vary significantly. Generally speaking, sodium phosphite electrolysis is rarely used in large-scale production, but its production principles offer valuable insights for the purification and production of semiconductor-grade and electronic-grade phosphorous acid. 99% low-iron H3PO3 is typically produced by the hydrolysis of phosphorus trichloride. Using sodium phosphite as a raw material, electrodialysis to produce phosphorous acid can be considered the reverse of an acid-base neutralization reaction. When direct current is applied to the electrolytic cell, the positive and negative electrode compartments react.

The main function of the cathode chamber is to electrolyze water into oxygen (O2) and hydrogen ions (H+).

H2O----H++OH

4OH----O2+2H2O+4e

The generated H+ enters the product chamber through the cation exchange membrane, where it reacts with HPO3- ions entering the cathode chamber to produce high-purity phosphorous acid, which is virtually odorless and free of impurities.

2H+HPO3----H3PO3

In the cathode chamber, the main reaction occurs: water electrolysis produces H2 and OH-.

H2O----H++OH

2H+2e----H2

The resulting hydroxide ions combine with Na+ entering the cathode chamber to produce NaOH.

OH-+Na+----NaOH

Thus, high-purity 99.5% phosphorous acid and the byproduct sodium hydroxide (NaOH) can be produced in the product chamber and the cathode chamber, respectively. While the above reaction appears simple, large-scale production presents significant challenges in terms of cost, equipment, and process.

Although phosphorous acid prepared from sodium phosphite has high purity and few impurities, its production cost is very high and it is difficult to industrialize on a large scale. H3PO3 prepared by hydrolysis of PCl3 has good whiteness, low odor, and low iron chloride ion content, making it suitable for industries such as semiconductors, pesticides, biopharmaceuticals, and surface treatment.