The national standard for phosphorous acid is HG/T 2520-2006. It is generally classified into three grades: 97%-99%. Its chemical formula is H3PO3. It is a diprotic inorganic acid with good water solubility, strong reducing power, and stable properties. Phosphorous acid can be used to produce potassium phosphite, calcium phosphite, aluminum phosphite, ammonium phosphite, etc. In agriculture, phosphorous acid has bactericidal properties and can be used in the production of fungicides, insecticides, glyphosate, etc.

I. Basic Information

English Name: Phosphorous acid

CAS Number: 13598-36-2

Molecular Formula: H3PO3

Molecular Weight: 82

Density: 1.65 g/cm3

UN Number: 2834

EC NO: 237-066-7

Appearance: White crystals

Melting Point: 73.6℃

Boiling Point: 200℃ (decomposes)

Solubility: Easily soluble in water and alcohol; highly hygroscopic.

Density: 1.65 g/cm³

Hazardous Characteristics: Corrosive; decomposes upon heating to produce the particularly harmful gas H₃P.

Hazard Code: 81502

Hazard Class: 8

II. Specifications

H₃PO₃ ≥ 99.0%

PO₄ ≤ 0.1%

Cl ≤ 0.005%

SO₄ ≤ 0.0001%

Pb ≤ 0.0002%

Fe ≤ 0.001%

III. Uses

1. Used in the synthesis of inorganic chemical fertilizers such as potassium phosphite, potassium dihydrogen phosphite, calcium hydrogen phosphite, and ammonium phosphite. These fertilizers have a certain bactericidal effect.

2. Phosphorous acid is the second strongest reducing agent after hypophosphite; it is more stable and easier to use.

3. It is a raw material used in the manufacture of plastic stabilizers and flame retardants.

4. Used in pesticides, nylon whitening, and the synthetic fiber industry.

IV. Production Process 

There are currently about 7-8 production processes for phosphorous acid, most of which utilize byproduct recovery. The byproduct-derived H3PO3 is inexpensive and cost-effective, but its disadvantages include low content, strong odor, and high impurity content. It is also susceptible to interference in demanding synthesis processes, making it unusable. High-quality 99% phosphorous acid produced by PCl3 hydrolysis meets export standards. The production process for this superior product is described below. It mainly uses the PCl3 hydrolysis method. The specific process is as follows: In the hydrolysis method, PCl3 is slowly added dropwise with water under stirring to produce phosphorous acid. After purification, cooling crystallization, and decolorization, the finished H3PO3 product is obtained. PCl3 + 3H2O → H3PO3 + 3HCl. Hydrogen chloride generated during the production process can be recovered and used to produce hydrochloric acid.

V. Precautions

1. Wear protective gloves/protective clothing/protective goggles/protective face mask.

2. Thoroughly wash skin after handling.

3. Store only in the original container. Do not inhale dust or fumes.

4. Do not eat, drink, or smoke while using this product.

VI. Storage and Transportation

25 kg/bag, polyethylene woven bag. Store in a well-ventilated, dry warehouse. Do not store with alkaline or oxidizing chemicals. Transport as Class 8.1 chemicals.

The content of phosphorous acid varies from 70-99%. Currently, high-end phosphorous acid on the market has a content >99%, low iron ion content, little odor, and a clear color. While phosphorous acid produced as a byproduct is cheaper, its quality cannot be guaranteed; even with the same production process, the quality can vary significantly from batch to batch.